SIE/HOME21 uses an innovative construction system to provide long-term urban flexibility while it builds for Vienna's immediate needs. The project delivers 255 homes, sized from 35m² to 75m² and renting at social housing rates, for the city’s most disadvantaged inhabitants. Satisfying urgent housing needs means rapid project execution: the building system's simple logic allowed SIE/HOME21 to be planned and built in an astonishing 20 months. The intelligent urbanism and open, flexible structure allow for a multitude of uses; the project can be retrofitted as the city's evolving needs require.