Single-storey rear extension

Why most people build one

Of the homeowners who told our quote tool what they were planning over the last 90 days, roughly two in three wanted a rear extension, and single storey outnumbered two-storey by about three to one. It is usually the cheapest way to add ground-floor space, it rarely needs a planning application, and you keep the upstairs of the house intact while it is built.

What it costs

Indicative UK range: £24,900 – £50,200 built and finished (BestBuilders’ 2026 regional cost model, averaged across 519 UK towns). Where you land inside that depends far more on foundations, the size of the opening into the house and the glazing than on an extra square metre — see what moves the price.

What you are allowed to build

As permitted development: up to 3m deep from the original rear wall on a semi-detached or terraced house, 4m on a detached house, 4m high overall, and eaves no higher than 3m within 2m of a boundary. Deeper — 6m and 8m — is possible through the larger home extension prior approval route, which involves notifying neighbours. Full conditions on the planning permission page.

When it is the wrong extension

• When you need bedrooms. This adds ground-floor space. If the problem is bedroom count, a loft conversion or a two-storey extension answers it and this does not.
• When it eats the garden you use. Going 4m deep on a short garden can leave a dark room and no outside space — two problems instead of one.
• When the light goes. Extending across the full width leaves the original middle room without a window. Rooflights, a partial-width design or a side return solve it, but it has to be designed in from the start.