A Research Methodology for Mitigating Climate Change in the Restoration of Buildings: Rehabilitation Strategies and Low-Impact Prefabrication in the “El Rodezno” Water Mill

New environmental challenges, coupled with the fact that 80% of the residential buildings that will exist in Europe in the year 2050 have already been built, mean that rehabilitation and restoration must be prioritised over new buildings. Construction is one of the largest generators of CO 2 . Using prefabricated and industrialised products and systems can help to mitigate its harmful effects thanks to the greater control and environmental evaluation that can be carried out on these products from their manufacture until the end of their useful life (LCA). In the county of the Sierra de Cádiz (Andalusia, Spain), there are 85 water mills, many of which are derelict and in disuse, which, due to their location, size, and characteristics, are ideal for rehabilitation and restoration for residential use. Taking the “El Rodezno” mill as a case study, this paper proposes rehabilitation strategies using prefabricated industrialised elements that have a low environmental impact. The methodological discussion takes as its starting point the process of design and testing that Alvar Aalto applied in 1940 and from subsequent studies that have confirmed a research structure based on the project design and the built project with the appropriate field of study and confirmation of the applicable strategies and solutions. To this end, this article is written on the basis of the two main phases of Alvar Aalto’s method, using the same terms that the Danish architect defined: Scientific Observation , for the study of preceding works and projects in light prefabrication and for the analysis of certain construction products and systems that, based on other research, have evaluated their LCA, and Construction Period , for the rehabilitation strategies of the “El Rodezno” mill, considering the studies and analyses of Scientific Observation . For the roof solution, we took as an example the rehabilitation of the roof carried out with the same methodology, construction criteria, and prefabricated products analysed in this article and used in the intervention strategies in “El Rodezno”. The paper concludes with the validity of the methodology applied to test the starting hypotheses that lead to intervention strategies that confirm the environmental and economic advantages of industrialised prefabrication, the importance of the design and synergy that results from combining different construction systems, and technologies that improve the acceptance of prefabrication by the inhabitant and boost the circular economy.