Rear Extension Cost
A rear extension is the plainest way to add a kitchen: one new room across the back of the house, with a flat or low-pitched roof and wide doors to the garden. The price follows floor area closely, so depth and width are the two numbers to settle first.
£32,500to£47,000
For 3 m deep across a 6 m wide semi (18 m²). Building work at a standard to good finish, with VAT. A guide, not a quote.
Rear extension prices by size
Building work only, including VAT, at 2026 national rates. Add the kitchen and fees with the calculator.
| Size | Floor area | Standard to good finish | High finish |
|---|---|---|---|
| 3 m by 4 m | 12 m²Behind part of the house | £24,000 to £31,000 | £33,500 to £36,000 |
| 3 m by 6 m | 18 m²Full width of a typical semi | £32,500 to £47,000 | £47,000 to £54,000 |
| 4 m by 6 m | 24 m²The detached-house depth limit | £43,000 to £62,500 | £62,500 to £72,000 |
| 4 m by 8 m | 32 m²Full width of a detached house | £57,500 to £83,000 | £83,000 to £96,000 |
| 6 m by 8 m | 48 m²Needs prior approval or planning | £86,500 to £125,000 | £125,000 to £144,000 |
Where these figures come from
- Planning Portal: planning permission for extensions
- Scottish Government: householder permitted development rights (Circular 1/2024)
- mygov.scot: building warrants
- HMRC: Buildings and construction (VAT Notice 708), for the 20% rate on extensions
- GOV.UK: Fees for planning applications in England
- GOV.UK: Permitted development rights for householders, technical guidance
- GOV.UK: Building regulations approval
- GOV.UK: The Party Wall etc. Act 1996, explanatory booklet
How far back you can build
Depth is set by planning before it is set by budget. In England, permitted development lets a single-storey rear extension reach 3 m beyond the original back wall of a terraced or semi-detached house and 4 m beyond a detached one, with no planning application. It can be up to 4 m high, or 3 m at the eaves where it stands within 2 m of a boundary, and the extension plus any other outbuildings must leave at least half the garden uncovered.
Want more? The larger home extension scheme doubles those depths to 6 m and 8 m. You apply to the council for prior approval (£249), it writes to the adjoining neighbours, and it has 42 days to decide. The scheme does not apply in conservation areas or national parks, a listed building needs listed building consent whatever the size, and "original" means the house as it stood in 1948, so an old extension counts against your allowance.
Scotland sets its own permitted development limits, and every extension there needs a building warrant from the council before work starts. Wales and Northern Ireland have separate rules again. If you are near any limit, a lawful development certificate (£274 in England) puts the council's agreement in writing before you spend money on foundations.
Depth against width
Two extensions with the same floor area do not cost the same. Going wider lengthens the opening into the house, and the opening is where the steel is. A 3 m gap can be spanned by a single beam resting on the existing walls. Take out the whole back wall of a 6 m house and the engineer will usually specify a steel frame with its own posts and pad foundations, which costs more to make, to lift in and to fire-protect.
Going deeper is cheaper per metre, because you are adding plain wall, floor and roof. The cost arrives later: beyond about 4 m the middle of the old house loses its daylight, and you will want rooflights over the junction to get it back. Budget for them at the start. They are far cheaper to include in the roof than to add once it is built.
What a rear extension price should list
A quote worth comparing sets out foundations and their assumed depth, drainage alterations, walls and insulation, the roof and its covering, structural steel, doors and rooflights by size and maker, electrics by number of points, heating, plastering, floor screed and making good to the garden. If the builder has allowed a provisional sum for anything, it should say so and say how much.
It will usually leave out the kitchen, floor finishes, decorating, and the professional fees: about £1,000 to £2,500 for drawings, £600 to £1,200 for the structural engineer and £700 to £1,100 for building control. None of that is hidden, but it does mean the builder's figure is not your budget.
Where the price climbs
Three things push a rear extension above the table. The first is ground: clay, made-up ground or a tree within a few metres can take the foundations from 1 m deep to 2 m or more, and nobody knows until building control has seen the trench. The second is drains. Many back gardens have a shared sewer running along the rear of the row, and building over or within 3 m of it needs a build-over agreement from the water company and sometimes a new manhole.
The third is region. The rates here are national. Builders in London and the South East usually charge 20 to 40% more for the same work, which takes an 18 m² rear extension from £32,500 to £47,000 to about £39,000 to £66,000.
Rear extension costs, answered
How much does a 3 m rear extension cost?
Across a 6 m wide house that is 18 m², or about £32,500 to £47,000 for the building work with VAT at a standard to good finish. Behind a 4 m section of the house it is 12 m² and about £24,000 to £31,000. Both figures exclude the kitchen and fees.
Is a flat roof cheaper than a pitched roof?
Usually, yes. A warm flat roof uses less structure and less labour than a tiled pitch, and it stays under the height limits more easily. A pitched roof needs enough wall above the extension to sit against, which many houses lack below the first-floor windows.
Do I need a party wall agreement for a rear extension?
If you dig foundations within 3 m of a neighbour's building and deeper than theirs, or build on the boundary line, the Party Wall etc. Act 1996 applies in England and Wales. You serve notice at least a month ahead. If the neighbour consents in writing there is no surveyor to pay; if not, allow £700 to £1,500 per neighbour for one surveyor agreed by both sides, and more if each side appoints its own, since you pay for both.
Other cost guides
- Kitchen extension costThe main guide, the price board and the calculator.
- Side return extension costFilling the alley beside a Victorian or Edwardian kitchen.
- Small kitchen extension costJobs under 15 m², and why they cost more per square metre.
- Kitchen diner extension costKnocking through and extending to make one family room.
Get a quote for your rear extension
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