2026 UK extension cost ranges by type
Single-storey rear extensions in 2026 typically run £2,800–£3,800 per m² for mid-specification work. A standard 25 m² rear extension therefore sits between £70,000 and £95,000 before finishes, glazing uplifts and structural complexity adjustments.
Double-storey extensions are more cost-efficient per m² because foundations and roof are amortised over twice the floor area. Typical 2026 rate is £2,400–£3,200 per m². A 40 m² (20 m² over two floors) double-storey extension sits between £96,000 and £128,000.
Wrap-around extensions combine rear and side return. They are the most complex from a planning and structural perspective and typically run £3,200–£4,200 per m² accounting for additional steelwork and corner detailing.
What changes the cost — beyond the floor area
Specification is the biggest variable after size. The same 25 m² shell can absorb £15,000 of standard finishes or £45,000 of high-end joinery, glazing and bathroom fittings. Glazing alone — bi-folds, sliding doors, roof lanterns — can move a project by £8,000–£20,000.
Structural complexity is the next biggest variable. Knock-throughs requiring twin steels and pad foundations cost more than simple lean-to additions. Removing chimney breasts mid-project adds structural cost. Underpinning shallow foundations is rare but expensive when needed.
Site logistics matter more than people expect. Side-access for plant adds time and cost when missing. Skip permits, parking suspensions, scaffold licences and crane lifts on tight London terraces add real money — we cost these honestly at quote stage rather than discover them mid-project.
Regional variation across the UK
Central London (W, NW, SW, EC, WC postcodes): typically 15–25% above the UK average due to labour costs, parking constraints and logistics complexity. Outer London (E, SE, N, S postcodes): 5–15% above the UK average. Home Counties (Surrey, Kent, Essex, Herts): broadly at the UK average. South-East market towns: 5–10% below outer London prices.
The structural shell follows these patterns most strongly. Finishes vary less by region because most specialist fittings and joinery are nationally priced. Professional fees and statutory costs are flat across the country.
Want a real number for your project?
Send us your address, brief and budget — we will come back with an honest assessment within one working day.