House extension cost in 2026 (UK)

Two extensions of the same floor area can differ by tens of thousands of pounds. The biggest variables are:

• Number of storeys — a double-storey costs roughly 50–70% more than a single but gives you twice the floor space, so it's better value per m².
• Size and shape — simple rectangles are cheapest; L-shapes, curves and glass boxes cost far more.
• Finish level — standard plaster-and-paint versus bi-fold doors, roof lanterns, underfloor heating and a high-spec kitchen.
• Region — London and the South East run 20–40% above the national average.
• Access and groundworks — tight side returns, deep foundations, removing trees or diverting drains all add cost.
• Glazing — large bi-folds, sliding doors and rooflights are a major line item.

A healthy budget has four parts: the build, the fit-out and finishes, the professional fees, and a contingency. Roughly:

• Labour — around 40–45% of the total.
• Materials — a similar share to labour.
• Professional fees — 10–15% (architect/designer, structural engineer, party wall surveyor, building control).
• VAT — 20% on most extension work (some energy-saving measures are 0%).
• Contingency — set aside 10–15% on top for surprises.

Builders typically add 15–25% to labour and materials to cover project management and overheads — that's normal and worth paying for the coordination it buys.

The headline £/m² rate rarely includes the team that gets you to site. Budget for:

• Architect / designer — £1,000–£2,500 for a single-storey, rising to £3,000+ for full building-regs and construction drawings. Architects often charge 5–12% of build cost for a full service.
• Structural engineer — £400–£800 for calculations on a simple job, £1,000–£3,000 for more involved steelwork and foundations.
• Planning fee — the householder application fee in England rose to £548 from April 2026 (a Lawful Development Certificate is around £129).
• Building control — £700–£1,500 depending on project value.
• Party wall surveyor — £800–£1,500 per affected neighbour, where a Party Wall Agreement is needed.

Basic (£1,800–£2,200/m²): standard brick-and-block shell, French doors or a standard window run, plaster and paint, basic radiators and a modest kitchen. Perfectly liveable and the smart choice if budget is tight.

Mid-range (£2,400–£3,200/m²): bi-fold or sliding doors, a roof lantern, better flooring, a quality kitchen and underfloor heating. This is where most family projects land.

Premium / London (£3,500–£4,500/m²): structural glazing, bespoke joinery, high-end appliances, MVHR and architect-led detailing. The build quality and finishes are exceptional but the price reflects it.

The quote you sign rarely covers everything. Watch for:

• Kitchen or bathroom units, worktops and appliances (often £8,000–£25,000+ on their own).
• Knocking through into the existing house and making good.
• Upgrading or relocating the boiler, consumer unit or drainage.
• Deeper foundations if the ground or nearby trees demand it.
• Landscaping and reinstating the garden after the build.
• Asbestos removal in older properties.
• The 10–15% contingency you'll almost certainly dip into.

You can trim a lot without ending up with a poor result:

• Keep the shape simple — a clean rectangle is far cheaper than curves or a frameless glass box.
• Stay within permitted development where possible to skip the planning fee and weeks of waiting.
• Don't move services — keep kitchen and bathroom fittings near existing pipes and drains.
• Phase the fit-out — finish the shell now, upgrade finishes later when funds allow.
• Fix the design before you start — mid-build changes are the most expensive money you'll spend.
• Get at least three detailed quotes and compare line by line, not just the bottom line.

For most homeowners, yes — a well-designed extension adds usable living space and typically adds value, especially when it creates a sought-after open-plan kitchen-diner. It's almost always cheaper than moving once you factor in stamp duty, estate agent and legal fees. Just be realistic: in some areas you can over-improve beyond the ceiling price for the street, so check what extended homes nearby actually sell for before committing to a premium spec. This is not a DIY job — groundworks, structural openings and building regs all require professionals.