Adding an extension is one of the best ways to gain extra space, avoid the stress and expense of moving, and add substantial resale value to your property. Whether you are planning a spacious new kitchen diner, an extra bedroom, or a home study, understanding the actual construction costs is essential for successful budgeting.
In this 2026 guide, we outline the average costs of a house extension in the UK per square metre (m²), single vs. double-storey extensions, additional professional fees, and how to hire vetted builders via GetBuilder.
Average house extension costs in 2026
On average, building a standard house extension in the UK costs between £1,800 and £3,300 per square metre for basic construction. This means a typical medium-sized extension of 30 square metres ranges from £54,000 to £90,000 for shell and basic internal fit-out.
Estimated project costs by extension size and specification level (national averages, excluding VAT):
| Project Size & Specs | Average Area | Average Total Cost |
|---|---|---|
| Small Extension (Basic) | 15 m² | £27,000 - £45,000 |
| Medium Extension (Standard) | 30 m² | £54,000 - £90,000 |
| Large Extension (Premium) | 50 m² | £90,000 - £165,000+ |
Single-storey vs. double-storey extension costs
While a double-storey extension is more expensive overall, it is generally much more cost-effective per square metre because foundations and roofing costs are shared across both floors. The cost per m² for different structures breaks down as follows:
- Single-storey extension: £1,800 - £3,300 per m². Standard, flexible structure that is fast to construct and often falls under Permitted Development rights.
- Double-storey extension: £2,000 - £3,200 per m². Adds significant floor area (e.g. kitchen diner below and master bedroom above). Typically requires planning permission and structural reinforcement.
Key cost factors for home extensions
Several factors dictate where your project will fall on the pricing spectrum:
- Location: London and the South East attract a 20% to 40% premium on labor rates and material deliveries compared to northern regions.
- Specification level: Basic finishes (uPVC doors, standard fixtures) cost around £1,200 to £1,900 per m². Mid-range to premium finishes (aluminium bi-fold doors, bespoke glazing, underfloor heating) range from £2,500 to £3,300+ per m².
- Soil & Ground Conditions: If your site requires deep foundations, soil removal, or piling near trees or drains, groundworks can add £3,000 to £10,000 to the initial budget.
Professional fees and additional costs to keep in mind
When budgeting, remember that the building quote rarely covers the entire project scope. Allow an additional 10% to 15% for the following:
- Architectural Drawings: Expect to pay £1,500 to £4,500 for design, planning submission, and construction drawings.
- Structural Engineer Calculations: Essential for foundations and load-bearing steel beams. Typically costs £400 to £2,000.
- Planning & Building Control: Planning permission fees are around £250 - £350, and Building Regulations inspections range from £300 to £800.
- Party Wall Agreement: If you are building close to or on a boundary line, you may need a Party Wall agreement, costing £1,000 to £2,500 per affected neighbor if surveyors are needed.
- VAT: Most building quotes exclude VAT. Don't forget to add 20% to the final bill.
How to hire vetted builders on GetBuilder
Building an extension is a major structural alteration that requires skilled project management. To find a vetted and reliable local builder, follow these steps:
- Post your project plans: Describe your extension size, specifications, and preferences for free on GetBuilder.
- Compare detailed quotes: Ensure builders submit fully itemized quotes detailing groundworks, structural steel, brickwork, roofing, and internal fit-out separately.
- Verify credentials: Check if the builder holds valid Public Liability Insurance (minimum £2M - £5M) and is associated with respected bodies like the Federation of Master Builders (FMB).
