Cost guide · Extension sizes · September 2026

6 metre extension cost in 2026

A mid-range single-storey rear extension 6 m deep and 4 m wide, measured outside, costs about £46,500 to £57,500 for the building work including VAT (£39,000 to £48,000 before VAT), or about £59,500 to £78,000 with design, fees and a 15% contingency, UK average outside London, September 2026 prices. In England you can apply for prior approval (fee £249) instead of a planning application, if the house keeps its permitted development rights and is not in a conservation area or on other protected land. In Wales and Northern Ireland it needs a planning application. In Scotland it can be permitted development only if no part is within 1 m of any boundary and the house is not in a conservation area. The room inside is about 5.65 m × 3.3 m (18.5 m²).

If 6 m × 4 m is the room inside (24 m²), allow about £60,500 to £75,000 for the building work, on the same basis. That extension is about 6.35 m deep outside, beyond the 6 m prior approval limit for a terraced or semi-detached house in England, so it needs a planning application.

By Estiquote · Prices reviewed 25 September 2026, next review December 2026 · Official sources checked 22 October 2026 · How we calculate

Work out the cost of your 6 metre extension

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The calculator starts at about 18.5 m², the floor inside a 6 m × 4 m extension measured outside. For a room 6 m × 4 m inside, enter 24 m². The calculator’s planning option is a householder planning application: £548 in England, or the Welsh, Scottish or Northern Irish fee if you choose one of those regions. Prior approval (£249, England only) is not in it. Already have a builder’s price? Check where your quote sits against the typical range.

6 metre extension cost by width

A “6 metre” extension usually means 6 m deep from the back of the house. At 4 m wide, measured outside, that is a room of about 18.5 m², enough for a kitchen and a dining table. The cost per m² is lower than for a small extension: about £2,500 to £3,100 per m² at 18.5 m² inside, against £3,200 to £3,900 for a 3 m × 3 m extension (about 6 m² inside), mid-range, including VAT, UK average outside London, September 2026 prices, because the fixed costs are spread over more floor.

Single-storey rear extension 6 m deep, measured outside: building work by finish, and the full mid-range budget (UK average outside London, including VAT, September 2026)
Outside size (deep × wide)	Budget	Mid-range	Premium	Full budget, mid-range, with fees and 15% contingency
6 m × 3 m (about 13 m² inside)	£27,500 to £34,000	£34,000 to £42,000	£43,500 to £53,500	£45,000 to £60,000
6 m × 4 m (about 18.5 m² inside)	£37,500 to £46,500	£46,500 to £57,500	£60,000 to £73,500	£59,500 to £78,000
6 m × 5 m (about 24.5 m² inside)	£50,000 to £61,500	£61,500 to £76,500	£79,500 to £97,000	£77,000 to £99,500
6 m × 6 m (about 30 m² inside)	£61,000 to £75,500	£75,500 to £93,500	£97,000 to £119,000	£93,000 to £119,000
6 m × 7 m (about 35.5 m² inside)	£72,500 to £89,500	£89,500 to £111,000	£115,000 to £141,000	£109,000 to £139,000

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 6 m × 4 m

What a builder quotes is not the whole cost. For a mid-range extension 6 m × 4 m outside (about 18.5 m² inside) in an average UK area outside London:

Full budget for a single-storey rear extension 6 m × 4 m outside (about 18.5 m² inside), UK average outside London, mid-range finish, including VAT, September 2026
Item	Cost
Building work, mid-range	£46,500 to £57,500
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)	£740 to £1,000
Contingency (15% of the building work)	£7,000 to £8,700
Total budget	£59,500 to £78,000

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); 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 6 m × 4 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. As an illustration, a quote of £60,000 including VAT for 6 m × 4 m measured outside is above the mid-range £46,500 to £57,500 for an average UK area outside London, and within £53,500 to £66,500 in the South East. For a room 6 m × 4 m inside, it is below £60,500 to £75,000. Either way, ask what it includes. 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.

6 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.

Single-storey rear extension 6 m × 4 m outside (about 18.5 m² inside) by region: mid-range, including VAT (September 2026)
Region	Building work	Full budget with fees and 15% contingency
UK average (outside London)	£46,500 to £57,500	£59,500 to £78,000
London	£63,000 to £78,000	£78,000 to £101,000
South East	£53,500 to £66,500	£67,500 to £88,000
East of England	£50,000 to £62,000	£63,000 to £83,000
South West	£48,500 to £60,000	£61,500 to £81,000
West Midlands	£45,000 to £56,000	£58,000 to £76,000
East Midlands	£44,500 to £55,000	£57,000 to £75,000
Scotland	£44,500 to £55,000	£57,000 to £75,000
North West	£43,500 to £53,500	£55,500 to £73,500
Yorkshire and the Humber	£43,000 to £53,000	£55,000 to £73,000
Wales	£42,000 to £52,000	£54,000 to £71,500
North East	£41,000 to £51,000	£53,000 to £70,000
Northern Ireland	£38,000 to £47,500	£49,000 to £65,500

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:

Single-storey rear extension 4 m wide at different depths, measured outside: building work and the planning route in England (UK average outside London, mid-range finish, including VAT, September 2026)
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.

Prior approval for a 6 m extension in England

Prior approval is a lighter process than a planning application. It covers a single-storey rear extension more than 3 m and up to 6 m deep, or more than 4 m and up to 8 m for a detached house. It is not available in a conservation area, a National Park, a National Landscape (Area of Outstanding Natural Beauty), the Broads, a World Heritage Site, a Site of Special Scientific Interest or other protected land.

You send the council a description of the extension, a plan, its depth beyond the original back wall, and its height at the highest point and at the eaves. If it joins an earlier extension, give the same measurements for the whole enlargement. Add your neighbours’ addresses, your own contact details (paragraph A.4 of the order) and the fee of £249 (fee schedule).

The council writes to your adjoining neighbours and gives them at least 21 days to comment. It checks that the extension meets the permitted development limits, and can refuse if it does not or if you have not given enough information. If a neighbour objects, it also decides whether the effect on all adjoining properties, such as loss of light or privacy, is acceptable. If it has not told you its decision within 42 days of receiving your details, you can start work. If it refuses, you can appeal or make a planning application, which costs £548.

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. A deep, narrow room may also need rooflights or a lantern to bring light into the middle. These are typical supply and fit prices from the Estiquote rate book:

Common extension extras, supplied and fitted (UK average outside London, rate book reviewed 25 September 2026)
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 6 metre extension

6 x 3 can mean two different extensions. 6 m deep and 3 m wide, or 3 m deep and 6 m wide, cost about the same to build (see the questions below), but the planning route is not the same. In England the 3 m deep version can be permitted development, if the house still has those rights and the extension is no wider than the back of the house, while 6 m deep can go through prior approval instead of a planning application, if the house still has those rights and is not on protected land. The wide, shallow version needs a wider opening in the back wall, which usually means a bigger steel beam. Say which way round you mean when you ask for quotes.

A room 6 m deep inside can take you past the prior approval limit. In England the limit is measured outside. Allowing about 0.35 m for the new wall, a room 6 m deep inside needs about 6.35 m outside, beyond the 6 m limit for a terraced or semi-detached house in England, and that needs a planning application. Built to 6 m outside, the room is about 5.65 m deep.

An earlier extension usually counts. Prior approval allows 6 m beyond the original house, meaning the house as first built, or as it stood on 1 July 1948 if it is older. A 3 m extension added since then leaves 3 m to go (5 m for a detached house).

Prior approval only lets you go deeper. Every other limit still applies, including the height, the eaves, similar materials and leaving at least half the land around the house free, and you will usually still need building regulations approval before you build.

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.

How far a single-storey rear extension can go beyond the back of the original house without a planning application
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. Up to 3 m (4 m for a detached house) is permitted development if the house still has those rights and the extension meets the other limits. Deeper, up to 6 m (8 m detached), you can use prior approval if the house is not on protected land.

Scotland. If no part is within 1 m of any boundary there is no set depth, but the height, footprint and garden limits still apply, and there are no permitted development rights in a conservation area.

Wales and Northern Ireland. A 6 m extension needs a planning application.

The full limits for each nation, with sources, are on 3 metre extension costs.

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.

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 6 metre extension cost?

A mid-range single-storey rear extension 6 m deep, measured outside, costs about £34,000 to £42,000 for the building work at 3 m wide (about 13 m² inside), £46,500 to £57,500 at 4 m wide (about 18.5 m²) and £75,500 to £93,500 at 6 m wide (about 30 m²), including VAT, UK average outside London, September 2026 prices. Fees and a 15% contingency come on top.

How much does a 6m x 3m extension cost?

Measured outside, a 6 m × 3 m extension gives a room of about 13 m² and costs about £34,000 to £42,000 for the building work, mid-range, including VAT, UK average outside London, September 2026 prices; about £45,000 to £60,000 with fees and a 15% contingency. If 6 m × 3 m is the room inside (18 m²), it is about £45,500 to £56,000, or £58,000 to £76,500 with fees and contingency.

How much does a 6m x 4m extension cost?

Measured outside, a 6 m × 4 m extension gives a room of about 18.5 m² and costs about £46,500 to £57,500 for the building work, mid-range, including VAT, UK average outside London, September 2026 prices; about £59,500 to £78,000 with fees and a 15% contingency. If 6 m × 4 m is the room inside (24 m²), it is about £60,500 to £75,000, or £75,500 to £98,000 with fees and contingency.

How much does a 6m x 4m kitchen extension cost?

Measured outside, a 6 m × 4 m extension with a mid-range new kitchen costs about £55,500 to £73,500 for the building work and kitchen, or about £70,000 to £96,500 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 6 metre extension?

In England, a single-storey rear extension 6 m deep can go through prior approval (fee £249) instead of a planning application, if the house still has its permitted development rights and is not on protected land. In a conservation area or on other protected land, and in Wales and Northern Ireland, it needs a planning application. In Scotland it can be permitted development only if no part of it is within 1 m of any boundary of the garden, it is not in a conservation area and it meets the other limits.

How long does prior approval take?

The council gives your adjoining neighbours at least 21 days to comment and has 42 days from receiving your details to decide. If it has not told you its decision within 42 days, you can go ahead.

Is a 6x3 extension the same as a 3x6?

They give about the same floor area and cost about the same to build: measured outside, about £34,000 to £42,000 and £36,000 to £44,500 mid-range, including VAT, UK average outside London, September 2026 prices. In England, though, 3 m deep can be permitted development, while 6 m deep can go through prior approval, in both cases if the house still has its permitted development rights, and for prior approval only if it is not in a conservation area or on other protected land.

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: