<p>Building circularly means building with future reuse in mind. Since Pearce and Turner (1995) put forward their ideas on the circular economy, this approach has gained prominence in other disciplines, such as architecture. (McDonough y Braungart, 2005). However, until very recent times architecture has been "linear" if we understand this as the opposite of the aim of reuse: the initial purpose was disconnected from the final one. Thus, to have elements of analysis and comparison, in this text we will approach the way of building in Spain in the 20th century. We will focus on the case of railway social housing, which had an abundant production (around 20,000 dwellings) in the period 1939-1989, within the framework of the existing housing legislation at that time in which there was no purpose of reuse, as the context and needs were different. However, these are dwellings that are still in use where the principles of circular architecture are applied, in this case to improve and adapt to regulations regarding energy saving with the intention of making this continuity of use more sustainable.</p><p>The article is based on the analysis of the thermal envelope carried out based on the projects available, as well as the study and application of the regulations on thermal insulation in the period under study, which specifically did not exist until the 1979 basic standard on thermal conditions (NBE-CT-79). The aim is to simulate, after the study carried out and after the recent update of the basic document on energy saving, (CTE DB-HE 2019) what renovation actions are necessary in these dwellings to ensure compliance with the new and rigorous standards set with the aim of reconciling these dwellings, which are still in use and were built under criteria of minimum cost, with the lowest energy consumption and therefore with greater energy efficiency and environmental sustainability.</p>