Mildmay Community Centre in Islington is a strong example of how an existing community building can be transformed into a high-performing, low-energy facility while remaining firmly rooted in local life. Originally a former tram power station later adapted for community use, the building had become cold, inefficient, and expensive to run. A deep retrofit and extension were commissioned to create a more welcoming, flexible space that could better serve the neighbourhood. The project increased usable floor space by around 35 per cent through careful re-planning and opening up underused areas, allowing the centre to expand its programme of activities and services. Internally, layouts were reconfigured to improve circulation, accessibility, and flexibility of use, while a new foyer and reception created a clearer arrival point and improved wayfinding throughout the building. Additional openings were introduced to bring in more natural light and maximise passive solar gain. Sustainability was central to the scheme. The building was upgraded to full Passivhaus standards, with a continuous insulation layer introduced across walls, roofs, and floors to significantly improve thermal performance. High-performance triple-glazed windows and excellent airtightness dramatically reduced heat loss, creating a stable and comfortable internal environment throughout the year. A mechanical ventilation system with heat recovery provides fresh air while retaining warmth, allowing the centre to operate with minimal conventional heating. Renewable technologies, including photovoltaic panels and solar thermal systems, further reduce operational carbon, while rainwater harvesting supports low-impact water use. Roof gardens planted with native species and wildflowers enhance biodiversity and provide valuable outdoor space for users. Post-completion monitoring has shown energy use reductions of up to 95 per cent compared with pre-refurbishment levels, despite increased occupancy. This fabric-first approach has delivered long-term environmental benefits alongside lower running costs for the community organisation. The project has been widely recognised across the industry. It was named Best Public Building at the 3R (Refurb, Rethink, Retrofit) Awards, won the Leisure category at the Greenbuild Awards, received the Retrofit award at the UK Passivhaus Awards, was highly commended for Design and Innovation at the RICS London Awards, and won for Building Performance at the Constructing Excellence Awards. Mildmay Community Centre now stands as a leading example of how deep retrofit can deliver meaningful social and environmental value, proving that ambitious sustainability targets can go hand in hand with strengthening community infrastructure.