How Much Does a House Extension Cost in the UK?

The essential position

Published 2026 consumer benchmarks place many single-storey extensions around £2,200–£3,300 per m² excluding VAT, but a reliable budget must be adjusted for location, size, access, structure, glazing, specification and how much of the existing house is altered. Small or complex extensions often cost more per square metre.

Key points

• Use cost per square metre only for early feasibility.
• Add VAT, professional fees, authority charges, surveys, kitchen and specialist items consistently.
• Carry a meaningful design-development and construction contingency.
• Obtain comparable contractor tenders from coordinated technical information.

1. What current cost-per-square-metre ranges mean

A 2026 Homebuilding benchmark suggests approximately £2,200–£3,300 per square metre for single-storey extension work excluding VAT, depending on quality and complexity. This is a useful first filter, not a quotation.

The rate can rise for small side returns, constrained access, deep excavation, extensive temporary works, large structural openings or premium glazing. Regional labour markets and contractor availability also matter.

• State whether the rate is net internal or gross external area.
• Confirm whether VAT and fit-out are included.
• Do not compare a shell rate with a fully finished rate.

2. Costs outside the basic building rate

Budgets frequently miss the kitchen, sanitaryware, floor finishes, bespoke joinery, landscaping, utility upgrades and work to the existing house. Professional and statutory costs can include architect, structural engineer, surveys, planning, Building Control and party-wall advice.

An extension that reorganises the whole ground floor may deliver better value but costs more than the new footprint alone suggests.

• Prepare an inclusions and exclusions schedule.
• Allow for temporary accommodation if needed.
• Budget for testing, certificates and completion items.

3. Design decisions that drive cost

Simple spans, repeated openings and rational drainage are normally more economical than multiple transfers, long cantilevers and fragmented roof geometry. Large sliding doors, roof lanterns and flush thresholds need coordinated structure, weathering and drainage.

Value engineering should protect the brief. Reducing floor area, simplifying structure or concentrating premium materials can save more effectively than removing technical performance or cutting coordination.

• Resolve structural grid and openings early.
• Use standard sizes where they support the design.
• Compare whole-life performance, not only purchase cost.

4. Contingency, tendering and cost control

At early stage, a contingency of around 10–15% is commonly prudent, with the appropriate level depending on survey information and project risk. Existing buildings contain unknowns that new-build rates do not fully capture.

Tenders should be based on the same drawing and specification package. A low quotation with broad exclusions is not directly comparable with a complete price.

• Issue a tender return schedule.
• Clarify provisional sums and client-supplied items.
• Keep a change-control log during construction.

Questions homeowners commonly ask

Official guidance and further reading

• Homebuilding: extension costs in 2026
• RIBA: working with an architect