How Much Does an Extension Cost?

What does an extension cost in 2026?

If you're planning an extension, the first question on your mind is almost certainly: how much will it cost? It's a fair question, but the honest answer is that there is no single figure. Extension costs vary enormously depending on size, specification, location, ground conditions and the complexity of the structural work involved.

That said, having delivered countless residential extensions across Oxfordshire and Cambridgeshire, we can give you a realistic framework so you can budget with confidence.

Single-storey extensions

A standard single-storey rear extension typically costs between £1,500 and £2,500 per square metre for a good-quality finish. A 25m² extension (a common kitchen-diner size) would therefore land somewhere in the region of £37,000 to £62,000.

Factors that push you toward the higher end include:

• Large glazed sliding or bi-fold doors
• Premium kitchen and flooring specifications
• Underfloor heating
• Complex foundation requirements (e.g. clay soil, tree roots)

Double-storey extensions

A double-storey extension is not double the cost of a single-storey. The foundations and roof are the most expensive elements, and you only pay for them once. Expect to pay around £1,800 to £2,800 per square metre for the total footprint.

A 25m² double-storey extension (50m² of added floor space) typically falls between £45,000 and £70,000.

The hidden costs most builders won't mention

This is where many homeowners get caught out. A quote that looks attractive on paper can balloon once the real work begins. Watch out for:

• Party Wall Agreement fees — if you share a boundary, you'll need a surveyor (£700–£1,200)
• Structural engineer fees — for steel calculations and drawings (£500–£1,500)
• Building control fees — council inspections and sign-off (£500–£1,000)
• Soil investigation / party wall surveys — if ground conditions are uncertain
• Utility diversions — moving drains, gas pipes or manhole covers
• Decoration and finishing — many quotes exclude the final decorate and snagging

How to get an accurate figure

The only way to know what your extension will truly cost is to have detailed plans drawn up and priced by an experienced builder. Generic online calculators can be out by tens of thousands of pounds.

That's exactly what our consultation provides. We review your drawings, assess the structural requirements, and give you an itemised, transparent breakdown — no vague estimates, no hidden extras.

Get in touch using the form on our contact page to start the conversation.