Proposing an LCA methodology for the assessment of neighbourhood refurbishment measures

Environmental impacts of new construction in the built environment have been determined for considerable time using life cycle assessments (LCAs). However, the significance of the existing building stock is neglected when considering environmental impacts at the level of embodied energy. Today alone, most of the buildings that will remain in place in 2050 are already in existence. For achieving national and international climate protection goals, the LCA of refurbishment measures is crucial. Thus, the link between building LCAs, which are conducted based on EN 15978, and refurbishment measures is established and ultimately transferred to the neighbourhood level. This paper provides a methodology in accordance with applicable standards to make use of a large activation potential in neighbourhoods. An initial focus is on the survey of the area to be investigated. The subdivision and typologisation of the building stock based on established toolboxes within the neighbourhood as well as the description of the implemented measures are besides in the focus of the methodology. Multiple scenarios for existing buildings in the neighbourhood combined with a consistent framework enables LCA to be conducted. The connection of the spatial component by a demarcated neighbourhood and the connection with the structural dimension by buildings enables a holistic view of refurbishment measures in the urban environment. As a link between the individual building and the municipality, the neighbourhood serves as a meso level.


Introduction
In 2019, the European Commission's Green Deal [1] formulated a uniform framework for greenhouse gas (GHG) emission reduction targets at the European level. A reduction of GHG emissions of 55% by the year 2030 compared to the year 1990 is one of the core targets of the regulation. By now, a wide variety of nations have adopted analogous reduction targets and specifications for a green transformation to meet the climate goals of the Paris Agreement of 2015. Underlining the relevance of the effort are global CO 2 budgets to meet the 1.5-and 2.0-degree Celsius warming estimates. To ensure a high probability of meeting the global warming targets, current CO 2 emissions leave 400 Gt to meet the 1.5 degree target and 1.150 Gt to meet the 2.0 degree target [2].
The building sector is responsible for about 40% of energy-and process-related GHG emissions [3]. Moving from being one of the largest contributors to GHG emissions to a potential sphere for sequestration, carbon capture and CO 2 emission reduction particularly requires the effective targeting of transformation actions. Already today, about 80% of the existing buildings will remain standing in 2050 [4]. The inevitable urban transformation thus calls for a sustainable adaptation of the existing building stock. Energetic refurbishment measures are the central element for the reduction of GHG emissions in the building sector. The holistic assessment of energetic refurbishment measures is essential in order to evaluate the embodied emissions caused by the newly installed building materials besides the savings due to the reduction of hot water and heating demand.
• Within the framework of the proposed methodology and according to Özdemir et al [17], the following life cycle modules are applied for the assessment of environmental impacts. Product stage (A1-3): modules A1-3 consist of the initial phase of a construction project in which preliminary work occurs. Processes from the beginning of production until leaving the factory site are covered. The raw material supply, thus the extraction and processing, the transport from the place of raw material extraction to the factory, within the factory and finally to the factory gate as well as the production of the building materials including all auxiliary and operating materials are considered. • Use stage (B2-4): The use stage maintenance, repair, and replacement processes. This includes all measures that contribute to maintaining the technical and functional quality as well as the visual quality of the building. Components that are replaced over the life cycle are accounted by these modules. For example, windows are typically replaced at least once in the assessment period. Accordingly, a new manufacturing and another disposal process are required to assess the impacts during the replacement process. • Operational energy use (B6): Module B6 covers the building's internal energy use for heating, hot water, air conditioning, ventilation systems and lighting. • End of life stage (C1-4): The end-of-life stage includes waste products from the deconstruction, waste processing, and disposal processes of a building. Waste processing is the collection of waste fractions that are destined for reuse, recycling, or energy recovery. The recovery process ends with a product that is no longer considered waste. All transport routes are included until the end of the waste status is reached, also including interim storage.

LCA of refurbishment measures
The methodological framework for LCA of refurbishment measures has not yet been standardised. Studies carried out in the past differ particularly in the definition of the system boundaries and which measures are attributed to the refurbishment measures respectively to the building itself [18]. When considering measures subsequently carried out in an already existing system, the system boundaries define a critical point in the assessment. The European standard DIN EN 15978:2012-10 [11] describes the limits of modernisation measures in module B5, but the systematic approach for the elaboration of an LCA is not sufficiently illustrated.
Hafner and Storck [19] developed a methodology for the LCA of vertical building extensions. An approach that can also be applied to refurbishment measures. Following this approach and still in the review phase, DIN prEN 15978-1:2021 [20] provides the first normative framework in which refurbishment measures are addressed in the LCA context. The methodology centres on extending the life cycle of the existing building and the differentiated assessment of existing materials and newly installed constructions and materials. Dismantling work carried out at the beginning of the new life cycle is likewise considered. Moreover, after the new life cycle, the entire building, i.e. both the new materials and the materials of the existing building, is assessed with Module C for demolition. Replacement cycles of building components installed at different times are additionally examined separately. The methodology was enhanced and the relationship between the building, neighbourhoods and the municipal level established [21].
Zimmermann et al [22] present a similar approach. Likewise, they propose the differentiated consideration of existing buildings and refurbishment measures, in which module-specific impacts are evaluated. However, they include a doubled consideration of module D-after the refurbishment measure respectively after demolition-and after the extended life cycle into their methodology. As an informative module, the potential reusability and recyclability of a building (module D) according to DIN EN 15978:2012-10 [11] consistently has to be specified outside the system boundaries.

LCA of neighbourhoods
Besides the LCA of refurbishment measures (on building level), the scope of this paper will be extended to include the neighbourhood. Initially, the neighbourhood must be defined and LCAs at the neighbourhood level elaborated to enable the LCA of refurbishment measures at this scale to be investigated in the subsequent step.
In the urban context, the neighbourhood forms the meso level between buildings and the city in its entirety as described by Slabik et al [21]. As a linking and structural entity, the neighbourhood not only performs relevant to urban planning, but also to administration and management. As a melting point of social interaction and the infrastructural link between heat and electricity supply systems as well as building structures and the residents who use them, it is difficult to determine the environmental impacts of this complex structure. Thus, different dimensions of the neighbourhood are to be considered and analysed separately. The building and spatial level of the neighbourhood, i.e. the construction structures, form the key focus for LCA of refurbishment measures at the neighbourhood level. The complexity and multidimensional perspective of the neighbourhood, however, also complicates the delimitation and the exact assessment of existing building retrofits. Hence, previous studies of LCA in the neighbourhood context are based on fundamentally different approaches. The approaches for LCA on the neighbourhood level vary from simple data collection to complex simulation methods [23].
In 2015, Lotteau et al [23] compared a total of 21 different studies from 14 publications that address the connection between LCA and the neighbourhood level in a major literature review. Although the normative requirements for the preparation of an LCA are fulfilled by the studies, the approaches diverge considerably. The greatest differences can already be seen in the first phase of the LCA, the goal and scope definition. Fundamentally, the functional unit and the assessment period of the studies are defined differently. Also, the selected impact categories for identifying the environmentally relevant impacts vary. Thus, the studies carried out so far cannot be ultimately compared and are not based on the same methodological approach.
When LCA of refurbishment measures are examined in the context of the neighbourhood concept, the basic assumptions and thus the comparability of the results can diverge even further. A non-existent, generally valid definition of the term 'neighbourhood' makes it difficult to provide an equivalent basis for such studies. Göswein et al [24] specify the functional unit of refurbished wall area and distinguish between bio-based and conventional materials for the use of thermal insulation systems. The neighbourhood studied is comparatively large, covering an area of 442 hectares with approximately 10 000 people inhabiting it. However, the methodology carried out is based only on the consideration of modules B4 and B5.
Perez and Rey [25] also investigate urban renewal scenarios and consider not only buildings but also the 1029 residents of the neighbourhood. Over a period of 60 years, all life cycle modules are considered, and the environmentally relevant impacts of the scenarios are determined. There is no differentiated consideration of emissions caused by subsequent measures or the existing building stock.
A number of further studies [26][27][28] also evaluate the impact of refurbishment or modernisation measures in the wider context of the neighbourhood, revealing that the differences in approaches are substantial. Thus, this paper provides a consistent methodology for LCA of refurbishment measures at the neighbourhood level.

Objectives and structure of the study
This paper proposes a methodology for the uniform approach towards the environmental impact through LCAs of refurbishment measures at the neighbourhood level. Initially, the neighbourhood level is discussed in depth. Focus is on the identification and demarcation of the neighbourhood from other urban structures. In this respect, the most important demarcation criteria are explained. Furthermore, scenarios are introduced to depict the utilisation of existing buildings in the neighbourhood. In addition to refurbishment, demolition and new construction are examined. Consequently, system boundaries for LCA are derived, which combine the neighbourhood as a spatial structure and the building components.
After the neighbourhood has been defined as a spatial and built structure, the typology of the building stock is elaborated. Key characteristics of specific building periods are presented and linked to the use and the year of construction using an well-established building typology [29]. For the preparation of the LCA of refurbishment measures, proxy buildings according to the building typology are used to represent the building stock accordingly. In this context, criteria for selection as well as limitations are discussed. Subsequently, rules for the preparation of building specific LCAs of the proxy buildings based on the building scenarios are presented. The extrapolation of the results is described and a summary of the conclusions which can be derived from the calculations is given. Finally, the methodology is discussed along with an outlook on possible applications and optimisations.

Defining the scope of the neighbourhood
Neighbourhood boundaries constitute the fundamental basis for the further examination of refurbishment measures and the evaluation of environmentally relevant impacts related as such. As mentioned previously, only the structural-technical dimension of the neighbourhood is considered in this methodology. An exact and transparent procedure for identifying the neighbourhood ensures an objective description of the neighbourhood itself-especially because the spatial awareness of neighbourhoods is often subjective and influenced by personal perceptions as well as the social environment [30]. Therefore, the scope of the neighbourhood is defined on three different layers: demarcation of the neighbourhood, building scenarios, LCA boundaries.

Demarcation of the neighbourhood
As an important feature for locating and demarcation neighbourhoods, geographical dividing lines serve initially. According to Sudau [31], these dividing lines represent barriers, obstacles or impediments. Importantly, the dividing lines must be surmountable-large areas of water without bridges delineate neighbourhoods more clearly than, for example, traffic routes. Only in the case of insurmountable structures this criterion can be evaluated as the initial demarcation feature of a neighbourhood. Fundamental factors here are the obstruction of social interaction and, consequently, the formation of a spatially identifiable boundary. However, by looking at development plans, aerial and satellite photographs, these dividing lines become recognisable in the urban landscape.
In addition, land use is another important demarcation characteristic. In urban areas, the same use of land can be an indicator for the formation or separation of neighbourhoods. Residential areas are the central focus in this regard, although scattered settlements and farmyards should also be included. If parks or cemeteries could be assigned to the neighbourhood, it must be decided individually to what extent the structure contributes positively to social networking. To identify uses, for example, land use plans can be consulted.
The final crucial demarcation feature of neighbourhoods is the building and settlement structure. Homogeneous building types and utilisations often indicate a structural connection of the area and ultimately condone or also demarcate a neighbourhood. The age of buildings and structures can serve as a characteristic as well. Within the framework of the European research project [32], other country-specific typologies [33] were also developed and compiled, which can be used simultaneously for the methodology. In the presented methodology, the classification and structural categorisation of the building stock is conducted using the German IWU building typology [29].

Buildings scenarios
When considering the LCA of refurbishment measures, not only the location of the neighbourhood in the urban context is of relevance, but also the building-specific determination of the object examined. Thus, not all buildings located in the defined neighbourhood necessarily are included in the LCA. Accordingly, a division into various scenarios is sensible to ensure that the relevant buildings in the neighbourhood can be correctly identified.
Four different scenarios for buildings in the existing neighbourhood are considered in the proposed methodology: (1) Existing building (2) Energetic refurbishment (3) Demolition and new building (4) New Building Although the existing building (1) is part of the neighbourhood, it is not included in the LCA because no subsequent changes are made during the period under consideration and thus no energy optimisation and reduction of heat and hot water demand is to be expected for the next 50 years. Besides, no additional materials are installed in the building, meaning that the life cycle is not extended either.
The scenario energetic refurbishment (2) describes an existing building that is energetically refurbished within the given assessment period. The extent of the refurbishment is not decisive. Every refurbishment activity that has a direct influence on the energy quality or the heat demand of the building means a classification in scenario 2. The scenario demolition and new building (3) includes all buildings that are demolished during the assessment period and rebuilt with the same function. By contrast, the new building (4) scenario consists of a new construction activity as a redensification measure in the neighbourhood without demolishing or deconstructing an existing building.

LCA boundaries
Finally, essential criteria of LCA are defined, which are standardised in DIN EN ISO 14044:2021-02 [10] and DIN EN 15978:2012-10 [11]. The assessment period of the methodology is 50 years, whereby the starting point should be chosen analogously at the beginning of the whole neighbourhood revitalisation or large-scale refurbishment measures with the goal of GHG emission reduction of a large-scale property owner. In contrast, measures that are only carried out on a few or individual buildings are less applicable for the starting point.
The functional unit, i.e. the reference value of the results, is 1 m 2 gross external area (GEA) of the building. However, the functional unit serves the methodology not only as a reference value for environmentally relevant impacts, but also as an extrapolation value for further calculation steps. This procedure is explained more in detail in section 3.
Within the framework of the proposed methodology, the life cycle modules for production (module A1-3), for the use phase (module B2-4), for the operational energy supply (module B6) and disposal (module C1-4) are considered. Service lives of the building components are considered according to [34] and apply to life cycle periods of 50 years. The recycling potential (module D) is given as an informative and separate figure outside the system boundaries. The global warming potential (GWP) is the focus of the environmentally relevant impacts and thus the main indicator used in the proposed methodology for comparing refurbishment scenarios in the neighbourhood. GWP describes the contribution and the resulting amplification of the anthropogenic greenhouse effect in the impact assessment. Using the carbon dioxide equivalent (kg CO 2 -eqv.), quantifying the GWP puts all convergent emissions in relation to their GHG potential. Calculation of the GWP is carried out using formula (1): Thereby TH describes the considered time frame. The absorption of thermal radiation after a concentration increase of the gas I respectively r by one unit is represented by a i and a r . The variables c i and c r represent the gas concentration at time t after emission [35].
The impact unit GWP is chosen as it is the main indicator referred to in climate debate and additionally on national level GHG reduction budget are defined to comply with Paris Agreement-in Germany i.e. detailed GHG reduction measures are defined for building sector amongst others. The data basis for the assessment of potential environmental impacts is the ÖKOBAUDAT database [36] published by the German Federal Ministry of Housing, Urban Development and Construction. A unified database for the LCA of buildings, it contains data sets for construction, transport, energy, and disposal processes. All impact indicator datasets are provided in compliance with DIN EN 15804:2022-03 [12]. The ÖKOBAUDAT contains association and company specific datasets which are categorized as generic, average, representative or specific datasets. The data sets should be provided with safety margins of about 10%-30% to eliminate uncertainties and to follow a value-conservative approach.
Consistent data availability and quality is almost impossible to guarantee for existing buildings. However, calculations for LCA must be based on planning documents that are sufficiently precise. Below, these characteristics are important decision criteria, according to which the example buildings are selected. The more detailed the existing data and the better the information on planned refurbishment, demolition, or new construction measures, the more accurate the LCA results will generally be.
Along with the requirements for data quality, cut-off criteria should equalise the influence of the smallest mass-related influences. Thus, material flows that account for less than 1% by mass in relation to the respective total mass considered are to be neglected. Hence, it is ensured that the ratio of expenditure and result is economically related and that minor contributions do not have to be accounted for.

Proposed methodology
Along with the initial phase of identifying and demarcating the neighbourhood, as presented in section 2, the proposed methodology for the LCA of refurbishment measures in neighbourhoods is based on a total of five steps. The principal elements of the methodology are illustrated in figure 2.

Typologisation of the building stock in the neighbourhood
Once the assessment area has been defined and the neighbourhood has been demarcated from the rest of the urban structure, the building stock within needs to be analysed in detail. Hence, the established TABULA building typology [33] is used (see table 1) to categorise the building stock according to age classes and shared structural features. This aims to divide the building stock into clusters capable of representing the neighbourhood. Typologisation defines the second phase of the approach, as shown in figure 2. Two main characteristics constitute the building typology used.
On the one hand, the age of the structures is decisive to assign the buildings to an age class based on typical building materials and constructions. The classification by specific building materials and constructions dates to historical and legal factors. Changes in the structural design of buildings are due to laws and regulations intended to promote energy conservation. Table 1 provides an overview of the building periods considered and the characteristic features for Germany.
Secondly, the building type, i.e. the size of the buildings, is essential to typologies the building stock. A particular distinction is made within the TABULA building typology [29] between single-family and multi-family houses, large multi-family houses, terraced houses, and high-rise buildings. Single-family houses are free-standing residential buildings with 1 or 2 units. Multi-family houses are residential buildings with between 3 and 12 units while large multi-family houses contain 13 or more units. Besides, terraced houses are residential buildings with 1 or 2 units as a semi-detached house, a row house or other type of building.  Furthermore, the TABULA building typology [29] provides exemplary refurbishment packages in addition to the typical building constructions for the building envelope and the technical supply. Based on the requirements of the German Energy Saving Regulation of 2014 (EnEV 2014) on the one hand and the Passive House Standard on the other. Accordingly, data gaps on existing buildings and existing construction designs as well as on upcoming refurbishment measures could be closed using the building typology. However, gaps in the data must be considered as a limitation to the high quality of data required for the selection of the proxy building. Therefore, only under special circumstances buildings with data gaps should be assessed further and the building typology should be used to obtain the required data. High accuracy in the modelling of residential buildings must be a top priority.

Determining proxy buildings
After the necessary building clusters have been formed and the entire scope of the neighbourhood has been surveyed, proxy buildings must be identified, each of which represents a cluster and marking the third stage of the methodology. The decisive criteria for determining the proxy buildings are the data depth and data quality of the planning and construction plans. High data quality and depth guarantees the accuracy of the mass balance and thus also the preciseness of the LCA results. However, with existing buildings this is frequently a critical issue, as documents are either no longer to be found as well as the existing documents are already outdated or no longer legible. Therefore, the building that allows the most accurate re-modelling is selected as a proxy. If necessary, data gaps can be filled with the IWU building typology.
The modelling can be done visually, for example with 3D modelling software, or as a mass balance, in which materials and building constructions are accounted and compiled in a structured way. As a minimum, level of detail 200 is to be adhered to model the building geometry and building constructions. Cut-off criteria can be applied already during modelling, thus eliminating insignificant parts of the material accounting. Both options are ultimately the basis for preparing building specific LCAs.

Conduct building specific LCAs
The scenarios presented in section 2.2 provide the foundation for the building specific LCA and define all potential options for managing existing buildings in neighbourhoods. Within the methodology the fourth stage according to figure 2 is reached. The framework for compiling the building LCAs is presented in figure 3. In [21], besides the illustrated methodology, also the framework for LCA of refurbishment measures on building and municipal level has been elaborated. Referring to a starting point of the assessment period, all environmentally relevant impacts of the building corresponding to the life cycle phases A1-3, B2-4 as well as B6, C1-4 and separately D are covered. However, the methodology does not include any influences that occur before the start of the assessment period and therefore only examines impacts that are caused either during operation or by refurbishment measures, demolition, or new construction. Likewise, this means that no demolition, i.e. no evaluation with Module C, is carried out at the end of the assessment period for the whole neighbourhood. On the one hand, the demolition of the entire neighbourhood is not realistic and, on the other hand, influences from the previous life cycle appear that are not considered as part of the construction in the methodology. Thus, neither the construction of the buildings before the assessment period nor the final demolition is included in the methodology. An overlap of two life cycles is to be avoided because this can significantly distort the influences and savings potentials of the refurbishment measures in the neighbourhood. In case biogenic carbon (GWP biogenic) is also to be considered as an indicator, it must be remarked upon that biogenic carbon is not considered in the demolition of existing structures for the same reasons as mentioned above.
In addition, a temporal component is incorporated within the methodology, covering measures that do not commence at the beginning of the period, but rather at any time within the 50 year timeframe. We introduce the terminology 0−x for existing buildings and x−50 for new buildings. The time point x refers to the refurbishment measure as well as the demolition or new construction. Replacement cycles (module B2-4) are considered separately for existing and refurbished respectively new buildings in this context. Eventually, these can proceed simultaneously or detach at point in time x. Figure 3 illustrates the procedure for preparing building specific LCAs in the neighbourhood according to the four different scenarios discussed. In the process, the amount of implemented models depends on the considered buildings and the relevant scenarios of the examined neighbourhood.
Since the Existing building scenario (1) is not necessarily to be included in the LCA, consideration only applies in exceptional cases. Thereby, only the operation (module B) must be accounted for. In particular, the operational energy use with module B6 as well as the replacement cycles of the existing building structures with modules B2-4.
In the scenario Energetic refurbishment (2), the actual refurbishment is carried out at a point in time x. The time of refurbishment may vary between the beginning (year 0) and the end (year 50) of the assessment period. Accordingly, operational energy use before as well as after the refurbishment measure must be differentiated. The respective influences of energy use in operation are displayed by the modules B6 stock, 0-x and B6 refurbished, x-50 . Besides, demolition work frequently arises at the actual time of the refurbishment measure. For instance, before the exterior wall is renewed and in preparation for a composite thermal insulation system, the exterior plaster must be dismantled. Also, the replacement of windows or doors causes demolition and disposal to be accounted for. Respective influences are assessed with the module C x . Alongside demolition and disposal work, further influences arise from newly installed materials and building constructions. Impacts caused by manufacturing and production at time x are included in the module A x . The newly installed building constructions are in turn subject to replacement cycles for the remaining time of the assessment period (year x-50), as accounted within the balance with module B6 refurbished, x-50 . Simultaneously, the replacement cycles from the existing buildings are included with B6 stock, 0-x . Scenario 3 Demolition and New Construction (3) covers the different life cycle modules like the previous scenario. Here, time point x indicates no refurbishment, but rather the demolition and new construction of a building identical in function. Environmental impacts occur at this stage due to demolition (C x ) and the construction of the new building (A x ). Also, operational energy use is considered separately in B6 stock, 0-x and B6 new building, x-50 . The major difference to scenario two concerns the acquisition of the replacement cycles, as the existing buildings are demolished. Thus, the replacement cycles are considered separately also before as well as after demolition and new construction. Modules B2-4 stock and B2-4 new building, x-50 are applied in this context.
Ultimately, the fourth scenario, New building (4), examines the new construction of a residential building as a relevant measure to be included in the neighbourhood LCA. The new building can be constructed at any time x between the beginning (year 0) and the end (year 50) of the assessment period. Emissions to produce the building occur right at this point, which will be accounted in module A x . Likewise, from time point x, the operational energy use will be captured with module B6 new building, x-50 as well as the replacement cycles of the new building with B2-4 new building, x-50 .
Apart from the consideration of emissions caused precisely at the starting point or within the assessment period, no additional influences are to be considered. Double counting or the inclusion of emissions from a previous life cycle are thus avoided. The life cycle of the existing buildings is extended by applying the methodology, which is why only the influences from the extended life cycle are included.

Approximation to the neighbourhood assessment
The fifth and final step for the LCA of refurbishment measures in neighbourhoods is the extrapolation of the results based on the LCAs of the proxy buildings. The methodology focuses on the consideration of environmentally relevant impacts, measuring the efficiency of refurbishment measures using the GWP. Accordingly, the outcomes from the process LCI must be offset against the GWP values and extrapolated according to the clusters. Since the functional unit refers to the GEA, this is also applied to the extrapolation of the results. The building-specific results of the LCA are thus scaled down to 1 m 2 GEA. Subsequently, the areas of the GEAs are determined for each cluster-either by planning documents, information from stakeholders or, if necessary, by estimates based on the IWU building typology. The reference value of the GEA area can then be applied for extrapolation and indicates the impacts of all building scenarios in the given cluster.
However, it should be noted that the clusters may have to be subdivided. For instance, if a cluster has been formed that consists of the same building types, but in which different scenarios are carried out according to the four variants presented, this cluster must be partitioned.

Discussion
The proposed methodology provides a novel framework for LCAs of neighbourhoods as seen in table 2. The traditional LCA at building level was first linked to refurbishment measures and subsequently transferred to the neighbourhood level. The main goal is to reduce GHG emissions on a scale as large as possible to meet national and international climate protection targets. Thus, the summary of the methodological framework can be understood as an overview and explanation, which presents the key elements. Modified life cycle phases are related to the time of the refurbishment measure and are divided according to the time of the By defining the neighbourhood, topologizing and identifying the proxy buildings, the building specific LCAs can be conducted, which reflect the building stock in the neighbourhood in its entirety. An assessment period of 50 years is determined. The functional unit is 1 m 2 GEA, with the entire building, including all installations necessary for the housing functions (e.g. heating, water heating), placed within the system boundaries. High modelling accuracy is achieved by determining the proxy building, as important planning and construction data is often lacking or outdated, especially in the case of existing buildings. Thus, the main objective for the identification of the proxy buildings is a high data quality and depth. The extrapolation and hence the last step of the methodology is conducted using the functional unit. The GEA areas of the building clusters as well as the buildings must be determined accordingly. Multiplying the values related to the functional unit by the corresponding GEA areas of the buildings in the cluster results in a calculation of the total emissions that can be evoked through the measures. When comparing variants of different refurbishment programmes, focus should be put on the GHG saving potential that can be achieved by the respective measures. Here, the embodied emissions are compared with those achieved by the reduced heating demand. The results are specified according to the objective of the study and can be given either in absolute or percentage values.
The methodology of LCAs-particularly in the refurbishment field-is not only relevant at the neighbourhood level and becomes even more important with a view to national or international climate protection goals, for instance the Renovation Wave for Europe [37]. The relationship between the building, neighbourhood and municipality levels has already been investigated by Slabik et al [21]. An overview of the relationship between the levels, also with reference to building constructions and the entire building stock of a state, is provided in figure 4.
On the building level, it is crucial to identify which measures perform better in an environmental context based on the building construction and materials used. Furthermore, demolition and new construction measures can be compared with refurbishment measures if the preservation of existing structures is viewed in general terms. Even the consideration of life cycle costs can be included at the building level and complements the environmental dimension of LCA with economical elements. In the broadest sense, social components can also be considered. The renovation of buildings and the provision of barrier-free housing is just one aspect in this context.
Looking at the neighbourhood level it becomes evident that these potentials can be used on an even larger scale. By considering climate protection targets, an increased activation potential can be used by large stakeholders (e.g. housing companies, housing associations) towards a rapid transformation of the building stock. In Germany, for instance, the goal is to reach a nearly climate-neutral building stock by 2045. To achieve European targets of GHG neutrality by 2050, these measures must be accelerated. Large construction measures over several years require participation and the social acceptance of residents additionally. Increased involvement in decision-making and the empowerment of citizens is an important driver of energy system transitions and needs collaborative mechanisms that link local residents and policymakers more effectively [38]. SLCAs currently provide no uniform information in the building sector due to a lack of data and validated methodologies. Thus, the focus should be on an upstream citizen participation to create acceptance for the measures. In addition, the objective of the proposed methodology is to compare environmental impacts and therefore does not consider the social pillar of sustainability. Likewise, the interaction of buildings and energy infrastructure can be investigated with the proposed LCA methodology. Renewals of heating networks and the reduced heat demand values must be optimally coordinated along the transformation process of the built environment. Thus, the reduced demand must be balanced by the energy system to ensure that the supply is efficient even after refurbishments [39].
Furthermore, the municipal level is composed of different neighbourhoods that are not exclusively used for residential purposes. The data basis and the availability of data may vary here in some cases. However, the results of the studies should be noted in this context. The building stock within a city or municipality cannot feasibly be refurbished at once. Rather, this level can be used to derive potentials that have implications for the environmental efficiency of certain measures or combinations of measures. Thus, this approach provides essential contributions to address and ultimately achieve national climate protection goals.

Conclusion
Within this paper, a coherent approach is proposed, enabling the analysis of the environmental impact of refurbishment measures in a neighbourhood. Various indicators can be compared and, by means of the uniform methodology, can also be compared across the boundaries of individual neighbourhoods. However, particularly the comparison of variants of different refurbishment measure sets ought to be considered as a priority. Both energy efficiency of the buildings on the one hand and the use of different materials on the other hand need to be considered. The validation of the methodology is currently being conducted based on various exemplary neighbourhoods in Germany and promises to be a tool that can be used not only in the scientific field but also by decision-makers in the economy.
As Kuittinen et al [40] have already shown, there is a wide range of possibilities for storing and sequestering carbon in the built environment. Accordingly, in the future it will not only be relevant to consider different renovation measures regarding increasing the efficiency of buildings and the environmental savings potential of different renovation scenarios, but also to think about carbon storage options. Existing measures needs to already be utilised today and can be applied, for example, in the form of biobased materials in refurbishments.
The need for an increased renovation rate for the building stock is essential to achieve an effective reduction of GHG emissions. By extending the lifetime of buildings and thus reducing the need for new materials, this effect is also enhanced. If further circular economy (CE) measures are integrated, the savings potential for GHG emissions can be increased [41]. The consideration of CE in refurbishment measures can thus have great relevance and contribute to climate mitigation targets being achieved. Neighbourhoods that have a uniform ownership structure and thus a large activation potential for the transformation of the building stock can serve for a large-scale implementation. With the help of the proposed methodology, different refurbishment scenarios can be compared on a neighbourhood level and implemented according to their environmental quality.

Data availability statement
No new data were created or analysed in this study.