Cost guide · Extension sizes · September 2026
3 metre extension cost in 2026
A mid-range single-storey rear extension 3 m × 3 m, measured outside, costs about £19,000 to £23,500 for the building work including VAT (£15,800 to £19,600 before VAT), UK average outside London, September 2026 prices, or about £27,500 to £39,000 with design, fees and a 15% contingency. In England, Wales and Northern Ireland, and in Scotland outside conservation areas, it can often be built without a planning application, if the house keeps its permitted development rights. The new walls take about 0.35 m each, so the room inside is about 2.65 m × 2.3 m (6 m²).
For a room 3 m × 3 m inside (9 m²), allow about £26,000 to £32,000 for the building work, on the same basis. That extension is about 3.35 m deep outside, over the 3 m limit for a terraced or semi-detached house in England.
Work out the cost of your 3 metre extension
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The calculator starts at about 6 m², the floor inside a 3 m × 3 m extension measured outside. For a room 3 m × 3 m inside, enter 9 m². Already have a builder’s price? Check where your quote sits against the typical range.
3 metre extension cost by width
A “3 metre” extension usually means 3 m deep from the back of the house, so the price depends on how wide it is, and on whether 3 m is the outside size or the room inside. The table uses outside sizes, because that is how the planning limit is measured. Small extensions cost more per m² because the site setup and the opening into the house cost much the same whatever the size: £3,200 to £3,900 per m² at 6 m² inside, against £2,500 to £3,100 at 16.5 m², mid-range, including VAT, UK average outside London, September 2026 prices.
| Outside size (deep × wide) | Budget | Mid-range | Premium | Full budget, mid-range, with fees and 15% contingency |
|---|---|---|---|---|
| 3 m × 3 m (about 6 m² inside) | £15,400 to £19,000 | £19,000 to £23,500 | £24,500 to £30,000 | £27,500 to £39,000 |
| 3 m × 4 m (about 8.5 m² inside) | £20,000 to £24,500 | £24,500 to £30,500 | £31,500 to £39,000 | £34,000 to £47,000 |
| 3 m × 5 m (about 11.5 m² inside) | £25,000 to £31,000 | £31,000 to £38,500 | £40,000 to £48,500 | £41,500 to £56,000 |
| 3 m × 6 m (about 14 m² inside) | £29,000 to £36,000 | £36,000 to £44,500 | £46,000 to £56,500 | £47,000 to £63,000 |
| 3 m × 7 m (about 16.5 m² inside) | £33,500 to £41,500 | £41,500 to £51,500 | £53,500 to £65,500 | £53,500 to £71,000 |
These are modelled planning ranges, not quotes. Sizes are measured outside, the way England’s, Wales’s and Scotland’s planning limits are. Prices use the floor area inside, allowing 0.35 m for each of the three new walls (our assumption for a new cavity wall built to current insulation standards; ask your designer for yours). Building work covers foundations, walls, roof, insulation, standard windows and doors, electrics, heating, plastering, flooring and decoration, plus the opening into the house. The full budget adds a measured survey, design drawings, a structural engineer, building control and a 15% contingency. It leaves out a planning application or prior approval, a party wall surveyor and a new kitchen, which you may also need. Each figure is rounded on its own.
The full budget for 3 m × 3 m
What a builder quotes is not the whole cost. For a mid-range extension 3 m × 3 m outside (about 6 m² inside) in an average UK area outside London:
| Item | Cost |
|---|---|
| Building work, mid-range | £19,000 to £23,500 |
| Measured survey | £600 to £1,800 |
| Design drawings (planning and building regulations) | £3,600 to £6,000 |
| Structural engineer | £900 to £3,000 |
| Building control (council, full plans) (10 to 40 m² band) | £740 to £1,000 |
| Contingency (15% of the building work) | £2,900 to £3,500 |
| Total budget | £27,500 to £39,000 |
Each line is rounded on its own, so the total can differ from the sum of the lines by a few hundred pounds. Building control figures: middle half of 91 councils in England and Wales for a 10 to 40 m² extension (full range £587 to £1,876). Most councils in our survey charge less for an extension under 10 m²; see building control fees by council. In Scotland a building warrant replaces building control; the Scotland row in the region table includes it. Not included: a planning application (£548 in England, £585 in Wales, £371 in Scotland and £347 in Northern Ireland) or prior approval (£249, England) if you need one, a party wall surveyor (about £1,100 to £1,800 per neighbour, including VAT) if you need one, and a new kitchen, which you can add in the calculator.
Put a quote on the same basis first. Check whether the builder’s 3 m × 3 m is the outside size or the room, whether the price includes VAT, and what it leaves out, such as a kitchen, wide doors or a longer steel beam. Then compare it with the tables on this page, or check your quote for your area. These are modelled planning ranges, not quotes: a price above the range is a reason to ask what it includes, not proof that it is too high.
3 metre extension cost by region
Building costs vary across the UK. In our model London is about 35% above the UK average outside London, and Northern Ireland about 18% below.
| Region | Building work | Full budget with fees and 15% contingency |
|---|---|---|
| UK average (outside London) | £19,000 to £23,500 | £27,500 to £39,000 |
| London | £25,500 to £32,000 | £35,500 to £48,500 |
| South East | £22,000 to £27,000 | £31,000 to £43,000 |
| East of England | £20,500 to £25,000 | £29,000 to £41,000 |
| South West | £19,800 to £24,500 | £28,500 to £40,000 |
| West Midlands | £18,400 to £23,000 | £27,000 to £38,000 |
| East Midlands | £18,100 to £22,500 | £26,500 to £37,500 |
| Scotland | £18,100 to £22,500 | £26,500 to £37,000 |
| North West | £17,700 to £22,000 | £26,000 to £37,000 |
| Yorkshire and the Humber | £17,500 to £21,500 | £26,000 to £36,500 |
| Wales | £17,100 to £21,000 | £25,500 to £36,000 |
| North East | £16,700 to £20,500 | £25,000 to £35,500 |
| Northern Ireland | £15,600 to £19,300 | £23,000 to £33,000 |
3 m, 4 m, 6 m or 8 m deep: what changes
Going deeper adds floor area at a lower cost per m², but in England it changes how you get permission. These are 4 m wide, measured outside, mid-range:
| Depth outside | Building work | Per m² inside | England: terraced or semi-detached house | England: detached house |
|---|---|---|---|---|
| 3 m deep (about 8.5 m² inside) | £24,500 to £30,500 | £2,900 to £3,600 | Permitted development | Permitted development |
| 4 m deep (about 12 m² inside) | £32,000 to £39,500 | £2,700 to £3,300 | Prior approval | Permitted development |
| 6 m deep (about 18.5 m² inside) | £46,500 to £57,500 | £2,500 to £3,100 | Prior approval | Prior approval |
| 8 m deep (about 25 m² inside) | £63,000 to £78,000 | £2,500 to £3,100 | Planning application | Prior approval |
Permitted development and prior approval only apply to houses that still have those rights, and only within the other limits under planning rules. Prior approval is not available in a conservation area or on other protected land, so there a deeper extension needs a planning application.
A two-storey extension 3 m deep
A two-storey rear extension 3 m deep and 4 m wide, measured outside, adds about 17.5 m² over both floors and costs about £41,500 to £52,000 for the building work, mid-range, including VAT, UK average outside London, September 2026 prices, before fees. In England it can be permitted development if it goes no more than 3 m beyond the back of the original house and is at least 7 m from the rear boundary, outside conservation areas, National Parks, National Landscapes (Areas of Outstanding Natural Beauty), the Broads and World Heritage Sites, with other conditions on the roof pitch and on upper-floor side windows (Planning Portal). Wales (Welsh Government guidance), Scotland (Planning circular 1/2024) and Northern Ireland (GPDO (NI) 2015) set their own limits for two-storey extensions.
Extras that sit outside these prices
The finish columns cover the specification: flooring, lighting, heating and the standard of the windows and doors. They do not cover wide sliding or bifold doors or roof lanterns beyond a standard allowance, a bigger steel beam for a wider opening, or difficult ground, such as deeper foundations near trees or a drain under the extension. Any of these can push a quote above the range at any finish. These are typical supply and fit prices from the Estiquote rate book:
| Item | Excluding VAT | Including VAT |
|---|---|---|
| Aluminium bifold door set, about 3 m wide | £3,000 to £4,700 | £3,600 to £5,600 |
| Aluminium bifold door set, about 4 m wide | £4,300 to £5,700 | £5,200 to £6,800 |
| Roof lantern, about 1.5 m × 2 m | £1,800 to £3,800 | £2,200 to £4,600 |
| Rooflight or roof window, about 0.8 m × 1.2 m | £750 to £1,800 | £900 to £2,100 |
| Steel beam for an opening wider than 4 m | £1,500 to £2,500 | £1,800 to £3,000 |
The building work already includes standard windows and doors and the opening into the house with its beam, so an upgrade adds the difference, not the whole price.
What people get wrong about a 3 metre extension
A room 3 m deep inside can take you over the limit. In England the limit is measured outside. Allowing about 0.35 m for the new wall, a room 3 m deep inside needs an extension about 3.35 m deep outside, which is over the 3 m limit for a terraced or semi-detached house in England. Built to 3 m outside, the room is about 2.65 m deep. Ask your designer for the real wall build-up and the internal size.
An earlier extension usually counts. The limit applies to everything added to the original house, meaning the house as first built, or as it stood on 1 July 1948 if it is older (1 October 1973 in Northern Ireland). If a previous owner added a kitchen extension or a conservatory on the back since then, the new extension plus the old one must stay within the limit.
The building work is not the budget. For a 3 m × 3 m extension, fees and the contingency add about £8,500 to £15,500 to the building work (mid-range, including VAT, UK average outside London, September 2026 prices). A new kitchen, a planning application and a party wall surveyor are extra if you need them.
Going wider keeps you within the limit in England. If you need more room than 3 m deep gives you, a wider extension, up to the full width of the back of the house, stays within the 3 m limit. Going deeper means prior approval or a planning application: see 6 metre extension costs.
How deep can you build without planning permission?
Each UK nation sets its own limits for single-storey rear extensions. The depth counts from the back wall of the original house. The English and Welsh guidance and Scotland’s rules measure it outside, to the outer edge of the new wall (England, Wales); Northern Ireland’s rules do not say, so check with your council.
| Nation | Terraced or semi-detached | Detached | Deeper than that |
|---|---|---|---|
| England | 3 m | 4 m | Prior approval up to 6 m, or 8 m if detached (fee £249, not on protected land). Beyond that, a planning application. |
| Wales | 4 m | 4 m | A planning application |
| Scotland | 3 m if terraced, 4 m if semi-detached, where any part is within 1 m of a boundary | 4 m where any part is within 1 m of a boundary | No set depth if no part is within 1 m of any boundary; the height and garden limits still apply. No permitted development in a conservation area. |
| Northern Ireland | 3 m | 4 m | A planning application |
England. A single-storey rear extension to a house (not a flat or maisonette) needs no planning application if:
- it goes no more than 3 m beyond the back wall of the original house, or 4 m for a detached house
- it is no more than 4 m high and no higher than the existing roof
- its eaves are no higher than the existing eaves, and no higher than 3 m within 2 m of a boundary
- it uses materials similar to the house
- with any other buildings, it covers no more than half the land around the house.
Other conditions apply (Class A of the order). The depth is measured from the base of the original rear wall to the outer edge of the new wall, not counting gutters (technical guidance). “Original” means the house as first built, or as it stood on 1 July 1948 if it is older, so an extension added since then counts towards the limit. These rights do not apply where the council has removed them, for example with an Article 4 direction or a condition on the original planning permission. Extending a listed building needs listed building consent if it would affect its character, which almost any extension will (Planning Portal). Check with the Planning Portal or your council before you rely on these rights.
Deeper than that. Up to 6 m, or 8 m for a detached house, you can use prior approval (fee £249, fee schedule) instead of a planning application if the house is not on protected land: see prior approval for a 6 metre extension.
A planning application for a householder extension costs £548 in England (fee schedule), £585 in Wales (fee document), £371 in Scotland (circular 1/2026) and £347 in Northern Ireland (fees regulations). In England, a lawful development certificate costs £274 if you apply before you build, half the planning fee, or the full £548 for an extension already built. It confirms in writing that the extension is lawful, for example because it is permitted development, which can help when you sell.
Wales. A single-storey rear extension can go up to 4 m beyond the back of the original house, for any type of house. It can be no higher than 4 m or the existing roof, with eaves no higher than 3 m within 2 m of a boundary, and can cover no more than half the land around the house. Other conditions apply, and there is no prior approval route for a deeper extension (Welsh Government guidance, the 2013 order).
Scotland. Where any part of a single-storey rear extension is within 1 m of a boundary, it can project no more than 3 m from a terraced house or 4 m from other houses. Further than 1 m from every boundary there is no set depth. Wherever it is, the eaves can be no higher than 3 m and the extension no higher than 4 m. The house and its extensions can cover no more than twice the ground the original house covered, and the extension, earlier extensions and other buildings can cover no more than half the rear garden, not counting the original house, paving or decking. There are no permitted development rights for these extensions in a conservation area (Class 1A, Planning circular 1/2024). In Scotland you apply for a building warrant rather than building regulations approval: see extension costs in Scotland.
Northern Ireland. A single-storey rear extension can go up to 3 m beyond the back of the original house, or 4 m for a detached house. It can be no higher than 4 m, with eaves no higher than 3 m within 2 m of a boundary, and can cover no more than half the land around the house. Other conditions apply (GPDO (NI) 2015). Here “original” means the house as it stood on 1 October 1973, or as first built if newer.
Everywhere. Permitted development is about planning only. You will usually still need building regulations approval (a building warrant in Scotland). In England and Wales the Party Wall etc. Act 1996 may apply if you build on or at the boundary, work on a shared wall, or dig foundations close to a neighbour’s (GOV.UK).
Questions people ask
How much does a 3 metre extension cost?
A mid-range single-storey rear extension 3 m deep costs about £19,000 to £23,500 for the building work at 3 m wide and about £24,500 to £30,500 at 4 m wide, measured outside (about 6 m² and 8.5 m² inside), including VAT, UK average outside London, September 2026 prices. Fees and a 15% contingency come on top.
How much does a 3m x 3m extension cost?
Measured outside, a 3 m × 3 m extension gives a room of about 6 m² and costs about £19,000 to £23,500 for the building work, mid-range, including VAT, UK average outside London, September 2026 prices; the full budget with fees and a 15% contingency is about £27,500 to £39,000. If 3 m × 3 m is the room inside (9 m²), it is about £26,000 to £32,000, or £35,500 to £48,500 with fees and contingency.
How much does a 3m x 4m extension cost?
Measured outside, a 3 m × 4 m extension gives a room of about 8.5 m² and costs about £24,500 to £30,500 for the building work, mid-range, including VAT, UK average outside London, September 2026 prices; about £34,000 to £47,000 with fees and a 15% contingency. If 3 m × 4 m is the room inside (12 m²), it is about £32,000 to £39,500, or £42,500 to £57,500 with fees and contingency.
How much does a 3 m extension with a new kitchen cost?
For a 3 m × 3 m extension measured outside, the building work and a mid-range new kitchen come to about £28,000 to £39,500, or about £38,000 to £57,500 with fees and a 15% contingency, including VAT, UK average outside London, September 2026 prices. Add the kitchen in the calculator for your own size.
Do I need planning permission for a 3 metre extension?
Often not, for a single-storey rear extension to a house. In England, Wales and Northern Ireland, an extension up to 3 m beyond the back of the original house is permitted development if it meets the height and other limits, the house is not a flat or maisonette and the council has not removed those rights. In Scotland 3 m is within the depth limit for any house, but these rights do not apply in a conservation area. You will usually still need building regulations approval, or a building warrant in Scotland.
Is the 3 metres measured inside or outside?
Outside, in England, Wales and Scotland. The English and Welsh guidance measures from the back wall of the original house to the outer edge of the extension’s wall, not counting gutters, and Scotland’s rules use outside measurements. Northern Ireland’s rules do not say, so check with your council. Allowing about 0.35 m for the new wall, a 3 m extension measured outside gives a room about 2.65 m deep.
Does an old extension count towards the 3 metres?
Usually, yes. The limit applies to the total enlargement beyond the original house, meaning the house as first built, or as it stood on 1 July 1948 if it is older (1 October 1973 in Northern Ireland). A rear extension or conservatory added since then counts towards the 3 m.
Sources
Prices come from the same cost model as the rest of Estiquote; read how Estiquote estimates work. Prices reviewed 25 September 2026. Official sources checked 22 October 2026:
- Planning Portal: extensions, planning permission (England)
- The Town and Country Planning (General Permitted Development) (England) Order 2015, Schedule 2, Part 1, Class A
- Ministry of Housing, Communities and Local Government: permitted development rights for householders, technical guidance
- Planning Portal: Application fees in England from 1 April 2026
- Planning Portal: listed buildings
- Welsh Government: planning permission for extensions
- Welsh Government: permitted development for householders, technical guidance
- The Town and Country Planning (General Permitted Development) (Amendment) (Wales) Order 2013
- Welsh Government: fees for planning applications in Wales from 1 December 2025
- The Town and Country Planning (General Permitted Development) (Scotland) Order 1992, Schedule 1, Class 1A
- Scottish Government: Planning circular 1/2024, householder permitted development rights
- Scottish Government: Planning circular 1/2026, planning fees
- The Planning (General Permitted Development) Order (Northern Ireland) 2015
- Planning (Fees) (Amendment) Regulations (Northern Ireland) 2025
- GOV.UK: party walls and building work