Activities related to refurbishment of the building stock as a percentage of all building work have been constantly growing in most central European countries over the last 20 years. The main argument in this paper is the need to improve knowledge about composition of the existing building stock, the dynamics of its transformation and to relate this to the different actors in property professions. A review is presented of traditional and new sectoral research approaches to partial or national building stocks to provide a context for assessing an integrated approach to the German building stock with methodological propositions for improving the synergy between different sectoral approaches. Detailed consideration is given to life cycle analysis, building product modelling, historical building research and new simulation techniques. Sustainability indicators and the integration of building stock, infrastructure and land use are discussed.

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Formulating a unified theory of the built environment may require that the built environment be understood as a complex social-ecological system, where multiple-related metabolisms interact at different scales. From this broad systems perspective, the dividing line between what is considered as nature and what is considered as built environment becomes a cultural attribute that changes with the historical context. Over the past four centuries, notions of environmental accounting and material metabolism have expanded from year-to-year economic and biological exchanges to energy, material, financial, and information flows extended through time and space. At present, the necessary extension of system limits in time and space is best achieved by combining a number of methods, including flow-based models and resource-conservation-based models, and top-down and bottom-up modelling approaches. Artefacts, flows, and actors can be linked over time by means of a common framework for describing the built environment, and by life cycle-oriented product modelling techniques. Despite such advances, existing theory seems incapable of fully integrating spatial and physical relationships, and is especially challenged when dealing with concepts of time. Ecological models provide a useful basis for new timing tools that integrate different time scales, past and future, and that allow for an assessment of adaptive capacity and other aspects of system resiliency. These models can be used to understand better the impact of different managerial and social policies at both the macro-and the micro-level. The management of the long-term evolution of this social-ecological system can only be assured through appropriating ecological concepts of time, and by integrating the history of nature with the history of human culture.Keywords: built environment, conceptual frameworks, ecosphere, ecosystem, metabolism, sustainability, temporal perspectives, theory-building, time, urban systemsFormuler une théorie unifiée du milieu bâ ti peut nécessiter que ce milieu soit compris comme un système socio-écologique complexe où des métabolismes multiples interagissent à différentes échelles. À partir de cette vaste perspective de systèmes, la ligne de partage entre ce qui est considéré comme la nature et ce qui est considéré comme le milieu bâ ti devient un attribut culturel qui change avec le contexte historique. l'impact des différentes politiques de gestion et politiques sociales aux niveaux macro et micro économiques. La gestion de l'évolution dans le long terme de ce système socio-écologique ne peut être assurée que par l'appropriation de concepts écologiques de temps et par l'intégration de l'histoire de la nature dans celle de la culture humaine.