UK extension cost calculator
Enter your own build rate in pounds per square metre and this adds the things the headline rate never includes: design and structural fees, building control, the party wall surveyor, a contingency and VAT. It will not run without a rate, because there is no official UK cost per square metre and the published figures disagree by more than two to one.
Why this page does not publish an average cost per square metre
Every other extension cost page opens with a number. This one does not, and the reason is worth two minutes of your time, because that number is where most extension budgets go wrong before a single brick is laid.
There is no official, free, published UK cost per square metre for building work. The industry benchmark is the RICS Building Cost Information Service, which is a paid subscription, and its figures are not ours to republish. What is free is a collection of trade estimates, and they do not agree with each other.
| Published spread, excluding VAT and fees | |
|---|---|
| Standard specification, outside London | £1,500 to £3,200 per m² |
| Standard specification, London | £2,500 to £3,800 per m² |
That is the spread across free trade estimates, not a figure this site endorses. On a 20 square metre extension the bottom and the top of the first row are £30,000 and £64,000 for what is described as the same job. Averaging two numbers that far apart does not produce research, it produces a number with no meaning that is wrong for almost everyone who reads it.
What the cost per square metre leaves out
A rate per square metre is a build rate. It is the shell, the roof, the floor and usually a basic finish. Five things sit outside it and all five are real cheques.
- Design and structural fees. A designer or architect, and a structural engineer for any beam over an opening. Planning application fees sit here too.
- Building control. Either your council or an approved inspector. Councils publish their fee scales, and they vary.
- Party wall surveyors. Zero if the neighbours consent in writing, and two surveyors if they do not.
- VAT. Twenty per cent on top of everything in almost every case. See below for the two exceptions and why they rarely apply.
- Contingency. Drains in the wrong place, foundations deeper than expected, asbestos in a garage roof.
Permitted development: when you do not need planning permission
Most single storey rear extensions on a house are permitted development, which means no planning application and no planning fee. The limits are set out by the Planning Portal and they are specific.
| Limit | Detached house | Semi detached or terraced |
|---|---|---|
| Rear projection, single storey | Up to 4m | Up to 3m |
| Rear projection with prior approval | Up to 8m | Up to 6m |
| Maximum height | 4m | 4m |
| Eaves height within 2m of a boundary | 3m | 3m |
Three more rules catch people out. Extensions and outbuildings together may cover no more than half the land around the original house, and "original" means as first built or as it stood in 1948, not as you bought it, so a previous owner's extension counts against you. A side extension may be no more than half the width of the original house, no more than 4m high and single storey only. And no verandas, balconies or raised platforms are permitted development at all. Materials have to be similar in appearance to the existing house.
The Larger Home Extension route, and its 42 day clock
The 8m and 6m figures in the table are not automatic. They come through the Larger Home Extension prior approval procedure, which is a notification to the council rather than a planning application, and it runs on two clocks.
The council must consult your adjoining neighbours and give them a minimum of 21 days to object. It then has 42 days from receiving a valid application to make its decision. If it has not notified you within those 42 days, you may proceed with the development. That is a real deadline for the council, not a target, and it is one of the few places in the planning system where silence works in your favour.
For programming, treat the prior approval route as six weeks minimum before a spade goes in the ground, and longer if a neighbour objects.
VAT is 20 per cent on almost every extension
This is the single biggest line people leave out of an extension budget, and the advice circulating about avoiding it is mostly wrong.
VAT Notice 708 treats an extension onto an existing dwelling that is lived in as standard rated at 20 per cent. That is the default and it is what the calculator assumes. There are two exceptions and neither is a choice.
- The 5 per cent reduced rate in section 8 applies where the building has not been lived in during the 10 years immediately before you start your work. Ten years, not two, and the clock runs to the start of the work, not to the purchase. Paragraph 7.3 sets a shorter two year threshold for the specific case of a flat above a shop.
- Zero rating reaches an enlargement only where the work creates a new dwelling wholly within the enlarged part, under paragraph 3.2.5. An extra bedroom and a bigger kitchen is not a new dwelling.
The Community Infrastructure Levy and the 100 square metre threshold
CIL is a charge some councils levy on new floor space. The minimum threshold is 100 square metres of new floor area, so most extensions never reach it. If yours does, there is a residential extension exemption, and this is the part that costs people money: the exemption has to be claimed and granted in writing before work starts. Begin first and the exemption is lost and the levy is payable in full.
A residential extension does not need a commencement notice. Annexes and self build exemptions do, and missing it there carries a surcharge of 20 per cent of the levy, capped at £2,500.
Party wall notice periods are law, not courtesy
The Party Wall etc. Act 1996 is not advisory and it has nothing to do with planning permission or building control. If your work falls into one of these categories you must serve written notice, and the periods are fixed by statute.
| Work | Section | Notice period |
|---|---|---|
| Building on the line of junction between two properties | s.1(2) and s.1(5) | At least one month before |
| Cutting into, raising or rebuilding a party structure | s.3(2)(a) | At least two months before work begins |
| Excavating within 3m and lower than the neighbour foundations | s.6(1) and s.6(5) | At least one month before |
| Excavating within 6m where a 45 degree line from their foundations is met | s.6(2) and s.6(5) | At least one month before |
The response clocks matter as much as the notice periods. Under section 1(4) an adjoining owner has 14 days to consent to building on the line of junction. Under section 6(7), if they have not consented within 14 days of a notice for excavation, a dispute shall be deemed to have arisen. There is no penalty clause for silence because none is needed: silence is a dispute, surveyors get appointed, and the building owner pays for them.
For a typical rear extension with foundations near a shared boundary, serve notice two months before you want to start and budget for the possibility of two surveyors. That is why the calculator has a field for it and flags when you have left it empty.
Stop the estimate and the job being two different things
Built Simple keeps the takeoff, the estimate, the programme and the actual costs on one job, so the number you quoted is the one you are tracking against in week six. Free to start.
Start free with Built SimpleFrequently Asked Questions
How much does an extension cost per square metre in the UK? There is no official figure. Free trade estimates put a standard specification somewhere between 1,500 and 3,200 pounds per square metre outside London, and 2,500 to 3,800 in London, which is a spread of more than two to one for the same description. The industry benchmark, RICS BCIS, is a paid subscription. Get a rate from a builder who has seen the site and use the calculator to add fees, building control, contingency and VAT on top.
Do I pay VAT on a house extension? Yes, 20 per cent, in almost every case. VAT Notice 708 treats an extension to a dwelling that is lived in as standard rated. The 5 per cent reduced rate applies only where the building has not been lived in during the 10 years immediately before the work starts, and zero rating only where the enlargement creates a new dwelling in its own right.
How far can I extend without planning permission? A single storey rear extension may project up to 4 metres on a detached house and 3 metres on a semi detached or terraced house under permitted development, to a maximum height of 4 metres. Those double to 8 and 6 metres through the Larger Home Extension prior approval procedure. Extensions and outbuildings together may not cover more than half the land around the original house.
How long does prior approval for a larger extension take? Up to 42 days. The council must give adjoining neighbours a minimum of 21 days to object, and has 42 days from a valid application to decide. If it has not notified you inside 42 days, you may proceed.
Do I have to pay the Community Infrastructure Levy on an extension? Only if the new floor area is 100 square metres or more, and even then a residential extension is exempt. The exemption must be claimed and granted in writing before work starts. Start first and it is lost.
How much notice do I have to give my neighbour for an extension? Two months for cutting into or rebuilding a party structure under section 3(2)(a) of the Party Wall etc. Act 1996, and one month for building on the boundary line or for excavation near their foundations. If they do not respond within 14 days to an excavation notice, a dispute is deemed to have arisen and surveyors are appointed at your cost.
What is not included in a cost per square metre? Design and structural engineering fees, the planning application, building control, party wall surveyors, VAT and contingency. On a small extension those together are routinely a quarter of the total cheque.
Related calculators and guides
Sources
- Planning Portal, house extensions, permitted development limits and the Larger Home Extension prior approval procedure.
- VAT Notice 708: buildings and construction, HM Revenue and Customs. Paragraphs 3.2.5, 7.3 and section 8.
- Community Infrastructure Levy guidance, Ministry of Housing, Communities and Local Government.
- Party Wall etc. Act 1996, legislation.gov.uk. Sections 1, 3 and 6, quoted from the Act itself.
- Party Wall etc. Act 1996: explanatory booklet, HM Government.
- Cost ranges are the spread across free published trade estimates and are attributed as such. The RICS Building Cost Information Service is the industry benchmark and is a paid subscription, so its figures are not reproduced here.
