Horizontal building extensions: Potential in Finnish blocks of flats

Horizontal extensions of blocks of flats are a promising, but little studied, approach for circular and socially sustainable urban renewal. Based on research-by-design of real cases in Espoo, Finland, this article explores horizontal extension opportunities in suburban housing estates built between the 1960s and the 1980s. The analysis discloses how different modes of extension affect urban block morphology, flat distribution in buildings and qualities of individual dwellings. The results confirm that horizontal extensions provide a substantial and feasible approach in renewing ageing housing estates. As horizontal extensions help to postpone usable buildings' demolition and extend the life of carbon-intensive concrete structures, they can limit the building sector's carbon emissions currently when rapid climate change mitigation is a priority. They offer moderate possibilities to increase estates' density, while preserving the greenery and social spaces that characterise the original neighbourhood unit designs. Furthermore, well-designed horizontal extensions can adapt flat distribution to meet current market demand, improve accessibility for the elderly and the disabled, and enhance the experiential housing quality, especially in gable flats. Owing to standardisation of housing estate planning and production in the studied era, comparable potentials are expected to be found in other cases, both nationally and internationally. PRACTICE RELEVANCE For urban planners and project developers, a practical framework is provided to reveal the potential of buildings' horizontal extension in renewing and retrofitting suburban housing estates, characterised by standardised and serial construction technology. The study establishes buildings' horizontal extension as a complementary approach to vertical extension in urban renewal, providing an alternative to demolition and redevelopment. For architects, a broad set of carefully studied design options and templates is created that fulfils contemporary requirements for accessibility and housing quality. For developers, it offers insights into how much new floor area can be achieved and to what extent the flat distribution can be adapted to fit the current demand. Urban renewal and densification through horizontal extensions foregrounds the preservation of existing building frames, facilitating carbon-neutral retrofitting strategies. Furthermore, the article gives actionable tools for building and area preservation of modern neighbourhood units.