Single-storey rear extension

Single-storey rear extensions

A single-storey rear extension adds one storey of new floor across the back of your house, typically 3 to 6 metres deep, with a flat or pitched roof and wide doors to the garden. It is one of the most common extensions in the UK because it often needs no planning application and turns a dark back room into the main living space.

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Single-storey rear extension with white render, anthracite sliding doors and a roof lantern on a 1930s semi-detached house

What does a single-storey rear extension involve?

We design and build the extension from the ground up: foundations, insulated floor slab, cavity walls finished to match the house, a warm flat or pitched roof, glazing and the opening into the existing house. Most of the work happens outside until the last few weeks, when the back wall comes out and the new room is joined to the old one.

The depth decides almost everything else. Three metres gives a generous kitchen-diner on most semis; four metres takes a dining table and a sofa; six metres, which needs prior approval in England, creates a full open-plan living space but brings daylight further from the glass, so we usually add a roof lantern or rooflights.

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

Our guide is £1,900–£3,100 per square metre including VAT for a standard specification, before the kitchen and professional fees. A 3 m × 4 m extension typically comes to £23,000–£37,000, and a 4 m × 6 m extension to £45,500–£74,500. The final price depends on ground conditions, glazing, the roof and how much of the existing house is opened up.

Do you need planning permission for a single-storey rear extension?

In England a single-storey rear extension can often be built under permitted development: up to 3 metres beyond the original rear wall of an attached house, or 4 metres on a detached house, no higher than 4 metres, with eaves no higher than 3 metres if any part is within 2 metres of a boundary.

England also has the larger home extension route: a single-storey rear extension of up to 6 metres on an attached house or 8 metres on a detached house, after prior approval from the council. The council notifies your neighbours and has 42 days to decide; the fee is £249 (£310 for applications made from 8 December 2026). The route is not available on designated land such as conservation areas.

How long does it take to build?

Most single-storey rear extensions take 10 to 16 weeks on site. Add 2 to 6 weeks for the design and drawings, 8 weeks if planning is needed (or 42 days for prior approval in England), and a few weeks for Building Regulations drawings and structural calculations, some of which can run alongside the planning stage.

Different rules in Wales, Scotland and Northern Ireland. The limits above are for England. Check your own house with our depth checker or read how far you can extend without planning.

What is included?

Measured survey and design drawings
Foundations, drainage and insulated floor
Walls finished to match your house
Flat or pitched roof with rooflights or a lantern
Bifold, sliding or French doors
Steel beam to open the back of the house
First and second fix electrics, plastering and screed
Building control sign-off

Guide prices by region

Per square metre of new floor, including VAT, standard specification.

Region	Per m² inc VAT
London	£2,470–£4,030
South East England	£2,130–£3,470
East of England	£2,010–£3,290
South West England	£1,980–£3,220
West Midlands	£1,900–£3,100
East Midlands	£1,840–£3,010
North West England	£1,790–£2,910
Yorkshire and the Humber	£1,770–£2,880
North East England	£1,710–£2,790
Wales	£1,770–£2,880
Scotland	£1,800–£2,940
Northern Ireland	£1,650–£2,700

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FAQ

Single-storey rear extension questions

How deep can a single-storey rear extension be without planning?

In England, 3 metres on an attached house and 4 metres on a detached house under permitted development, or up to 6 and 8 metres with prior approval. Wales allows 4 metres; Scotland and Northern Ireland have their own limits.

Is a flat roof or a pitched roof better?

A warm flat roof keeps the extension low under first-floor windows and suits a roof lantern; a pitched or lean-to roof gives a vaulted ceiling. Both last for decades when built properly.

Can I stay in the house during the build?

Usually yes. The noisiest, dustiest stage, removing the back wall, is kept to the final weeks and screened off. Most families set up a temporary kitchen elsewhere in the house.

Will the extension make the room behind it dark?

Not if it is designed well. Rooflights or a lantern over the old back room, and a wide opening into it, keep daylight reaching the middle of the house.

Do I need a structural engineer?

Yes for the beam over the new opening and usually for the foundations. Their calculations go to building control with the drawings.

Does it add value to the house?

Nationwide's October 2025 house price research found that a 10% increase in floor area is linked to about 5% higher value on average. It is a guide, not a promise; local agents can tell you what extended homes on your street sell for.

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