How Much Does a House Extension Cost in London in 2026?

If you're planning a house extension in London, the first question is always cost. And the honest answer is: it depends. But after two decades of delivering extensions across London and Berkshire, we can give you realistic ranges rather than the vague estimates you'll find elsewhere.

Single-Storey Rear Extension

The most common extension we build. For a standard 4m x 6m rear extension in London, expect to budget between £2,800 and £3,500 per square metre for a quality finish. That puts your total build cost between £67,000 and £84,000 before VAT.

This includes structural work, roofing, insulation, plastering, electrics, plumbing, and a standard kitchen or living space fit-out. It does not include high-end kitchens, bespoke joinery, or extensive landscaping.

Double-Storey Extension

Adding a second storey typically costs 50–60% more than a single-storey build — not double, as many homeowners assume. The foundations and roof are shared costs. Budget £3,200 to £4,000 per square metre.

Side Return Infill

Popular with Victorian terraces, a side return extension is usually between 8 and 15 square metres. The smaller footprint means lower overall cost (£40,000–£65,000) but the per-square-metre rate is higher because you still need full structural openings and drainage.

What Drives Costs Up

Several factors push extension costs above average:

• Basement or lower-ground-floor work requiring underpinning
• Party wall agreements with multiple neighbours
• Conservation area or listed building constraints
• Poor ground conditions requiring piled foundations
• Restricted site access in dense urban areas

What We Tell Clients

Start with a realistic budget, add 10–15% contingency, and invest in proper design before breaking ground. The cheapest quote is rarely the best value — especially when you factor in project management, building control compliance, and long-term build quality.

We provide fixed-price quotes after survey, so there are no surprises once work begins.