The PassivhausSozialPlus subproject of the MOBASY research programme was carried out in Darmstadt from 1 November 2017 to 31 October 2022 under the funding code 03SBE0004C provided by Neue Wohnraumhilfe gGmbH. Its main objective was to demonstrate how operating costs in social housing can be substantially lowered by combining a Passivhaus energy standard with innovative cost‑management concepts such as flat‑rate billing and budgeted allowances for water and household electricity. To this end a former U.S. Army barracks site was used: two‑thirds of the existing 1955 block was extensively retrofitted with Passivhaus components, while the remaining one‑third was demolished and replaced by a new, barrier‑free building. The retrofit included high‑performance insulation, airtight construction, triple‑glazed windows, and a low‑energy heating and hot‑water system that together meet the stringent heating‑load and air‑tightness criteria of the Passivhaus standard. Although the report does not provide explicit numerical performance figures, the chosen design is expected to reduce heating demand to below 15 kWh m⁻² a⁻¹ and achieve a total primary energy consumption of less than 50 kWh m⁻² a⁻¹, in line with typical Passivhaus results. In addition to the building envelope, the project installed smart displays in each apartment that provide real‑time feedback on water and electricity consumption, thereby encouraging tenants to stay within their allocated budgets. The flat‑rate billing scheme covers most ancillary costs, while separate budgets for water and electricity allow tenants to manage their usage actively. This dual approach is intended to make housing costs more predictable for low‑income residents and to reduce the administrative burden on landlords.

The technical work was complemented by a social‑scientific survey that gathered tenants’ experiences with the flat‑rate system, their satisfaction with the new housing, and their perceptions of the budgeted utilities. The collected data will feed into the methodology developed in part A of MOBASY for estimating energy consumption in well‑documented buildings, thereby extending the applicability of the Passivhaus model to social housing contexts. The project also produced a series of publications, conference presentations, and outreach activities such as newspaper articles and guided tours, ensuring that the findings reach both the academic community and the public.

Collaboration was central to the project’s success. Neue Wohnraumhilfe acted as the owner and project sponsor, while the Institute Wohnen und Umwelt conducted a 2018 pre‑study on legal and organisational feasibility with support from the state of Hesse. The construction and retrofit were carried out by specialised contractors experienced in Passivhaus construction. Throughout the five‑year period, the project team coordinated closely with local authorities, tenant associations, and the research partners of MOBASY to align technical solutions with policy requirements and tenant needs. The final report, dated 30 March 2023, consolidates the technical outcomes, the social‑scientific insights, and the lessons learned, providing a comprehensive blueprint for scaling similar cost‑reduction strategies in other social housing developments.