Property investment: Comparing house extensions and garden annexes

Homeowners in the UK are increasingly weighing the benefits of house extensions against the construction of self-contained garden annexes to add space and property value.

According to a 2025 survey of UK architects, a single-storey house extension typically costs between £22,500 and £125,000. A 10m² kitchen extension averages between £80,000 and £100,000 including professional fees and VAT. In comparison, a fully self-contained garden annexe generally costs between £80,000 and £100,000, though bespoke options, such as those from Timberspecs, can start from approximately £99,845.

Value increases vary by method. A Nationwide House Price Index report from October 2025 indicated that adding a large double bedroom and bathroom via extension or loft conversion can increase a three-bedroom house's value by up to 24%. While extensions provide permanent floor area, annexes offer greater flexibility. Industry estimates suggest garden annexes can increase property value by anywhere from 5% to 30%, depending on build quality and location.