How much does a single storey rear extension cost in 2026?

Most projects land between £42,000 and £95,000, with the middle of the market around £62,000. The spread is wide because specification moves the number far more than size does. Two identical footprints can differ by thirty thousand pounds on glazing and finishes alone.

How long does a single storey rear extension take?

Allow 12 to 18 weeks on site once work begins. Before that, design and planning typically take eight to sixteen weeks depending on whether you need permission. The build is the predictable part. The approvals are where projects slip.

Do I need planning permission for a single storey rear extension?

Often not. Many of these fall under permitted development, which means no application at all, provided you stay within the size limits and are not in a conservation area or covered by an Article 4 direction. The Project Calculator will tell you which side of the line you are on in about a minute.

What makes the price go up the most?

Three things, in order. Moving drainage or a soil stack. Structural openings that need steel, particularly wide ones. And glazing, where the jump from standard doors to a slim framed sliding system can add fifteen thousand pounds on its own. Size is a surprisingly small factor by comparison.

Is it cheaper to extend or to move?

Extending almost always wins on pure cost. Stamp duty, agent fees, legal costs and removals on a typical UK move comfortably exceed the cost of a well judged extension, and you keep the house, the street and the school catchment. The exception is when the house cannot take what you need.

Cost guide · Updated 2026

How much does a structural engineer cost?

Steel beam calculations for the opening between old and new. Every extension needs one, and building control will ask for it.

£600£2,500

These are Extnd estimates built from recent UK project costs, not official statistics. Treat them as a planning range rather than a quote.

Axonometric drawing of a single storey rear extension, with the proposed work picked out in green — The beam building control will ask about

Where the money actually goes

What the fee is made of, and which parts you can avoid.

Item	Typical cost	What it covers
Single beam calculation	£350	One steel over one opening. The simplest case
Rear extension structural package	£900	Beams, padstones, foundation design
Loft conversion structural package	£1,200	Floor strengthening, ridge and purlin support
Site visit	£250	Where the existing structure needs checking first
Building control queries	£200	Responding to the inspector during the build
Revisions after design changes	£300	Change the opening size and the beam changes with it

What moves this number

Drainage. If a soil stack or manhole sits where you want to build, budget several thousand before anything else happens.
Structural openings. Wide steels need bigger foundations at both ends. A five metre opening is not twice the price of a two and a half metre one.
Glazing. The single biggest lever. Standard doors to slim framed sliders can be a fifteen thousand pound swing.
Ground conditions. Clay, trees nearby or a high water table all push foundations deeper.
Where you are. London and the South East run roughly twenty to thirty per cent above the national average on labour.

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Questions people ask before they commit

Do I need a structural engineer for a rear extension?

Almost always. Opening up the back wall to join old and new needs a steel beam, and building control will not sign off a beam without calculations from an engineer. It is not optional.

Does the engineer need to visit?

Not always. For a standard extension the design drawings are usually enough. Lofts and older houses often need a visit to check what is holding the roof up before anything is designed.

Is this included in the architectural fee?

Usually not. It is a separate discipline and a separate invoice. We coordinate it so the beams appear on the building regulations drawings, but the engineer bills for the calculations.

What does building control actually check?

That the calculations exist, were done by someone competent, and match what is being built. The inspector will ask to see them before the steel goes in.

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Cost guide

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How much does a structural engineer cost?

Where the money actually goes

What moves this number

Get your cost in 60 seconds.

Questions people ask before they commit

Compare with other projects

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