House Extension Cost Calculator
Work out what a house extension would cost for your project. Choose the type, size, and finish level to get a full cost breakdown with professional fees and optional extras. No email needed - results shown straight away.
Get price updates and money-saving tips
Join our free newsletter — extension pricing updates, cost-saving tips, and the best deals. No spam, unsubscribe any time. Privacy policy.
How this calculator works
Our extension cost calculator estimates the total build cost based on your extension type (single-storey, double-storey, side return, or wrap-around), floor area in square metres, and specification level. It prices the structural build, foundations, roofing, internal fit-out, and external finishes separately so you can see where the money goes.
Professional fees are calculated on top, including architect or designer fees, structural engineer costs, planning application fees, and Building Regulations approval. The tool also flags common extras like party wall surveyor costs, skip hire, and temporary accommodation if needed. All prices are adjusted for your UK region.
Extension costs vary widely depending on ground conditions, access, the age and construction of your existing property, and the level of finish you want. These estimates give you a solid budgeting figure, but actual quotes may differ. We recommend appointing an architect or designer first and then getting at least three builder quotes based on detailed drawings.
Common questions
How much does a house extension cost in the UK in 2026?
A single-storey rear extension typically costs between £30,000 and £60,000 for a 20 sqm space at mid-range spec. Costs vary significantly by region, spec level, and extension type. A double-storey extension offers better value per square metre at around £1,200 to £2,000 per sqm.
Do I need planning permission for a house extension?
Many single-storey rear extensions fall under permitted development and don't need planning permission, provided they don't extend more than 6 metres from the original house (8 metres for detached houses). Double-storey extensions, side extensions, and extensions in conservation areas or on listed buildings usually require a formal planning application. Always check with your local council before starting work.
How long does it take to build a house extension?
A typical single-storey rear extension takes 10 to 14 weeks to build once work starts on site. Double-storey extensions usually take 14 to 20 weeks. Add 8 to 12 weeks before that for design, planning, and Building Regulations approval.
What professional fees should I budget for with an extension?
You should budget for an architect or designer (£1,500 to £3,000), structural engineer (£300 to £800), planning application fee (£258), Building Regulations approval (£400 to £900), and potentially a party wall surveyor (£700 to £1,500) if your extension is near a shared boundary. These fees typically add £3,000 to £6,500 on top of the build cost.