Average cost of a home extension in the UK

Extension costs vary widely by location, specification, and site conditions. Treat any "average" as a planning guide — not a quote.

Indicative ranges (UK, order of magnitude)

Published industry guides often quote roughly:

• Single-storey rear extension — commonly from the low tens of thousands of pounds per square metre of build cost, rising sharply in London and the South East
• Double-storey extension — higher total cost, sometimes better £/m² than single storey because foundations and roof serve more floor area
• Side return / wraparound — can be complex structurally; budget for design and unforeseen ground conditions
• Conservatory or orangery — often lower than a full masonry extension, but still subject to planning and building regulations

Always get multiple written quotes for your drawings. National averages hide local labour rates, access issues, and finish levels.

What drives the price

• Location — London and high-cost regions command higher labour rates
• Ground conditions — poor soil, drainage, or tree roots add cost
• Specification — bifolds, kitchens, underfloor heating, and high-end finishes add up quickly
• Structural complexity — steelwork, party walls, and basement interfaces
• Professional fees — architect, structural engineer, party wall surveyor, building control
• VAT — clarify whether quotes are inclusive

Budget beyond the build

Remember:

• Temporary kitchen / alternative cooking if you lose facilities
• Landscaping and making-good after builders leave
• Contingency (often 10–15% for renovations)
• Furnishings and decoration

Protecting your budget once you hire

Cost overruns often come from vague scopes and unstructured payments. Before work starts:

1. Lock a detailed schedule of works
2. Agree how variations are priced in writing
3. Use staged payments tied to completed work — see milestone payments for homeowners
4. Vet who you hire — UK builder vetting checklist

How Fort helps

Fort keeps milestones, documents, and communication aligned so budget conversations stay tied to visible progress. Explore homeowner features or register your interest to build with clearer financial control.