How Much Will My Extension Cost?

cost of building and extensionWhile every extension is unique and it is impossible to give an accurate price, builders and architects alike will be able to give a pretty good estimate of the cost of an extension just by looking at the initial plans. When I first spoke to a builder about my plans he said “about £25,000” and the final quotation for the work was only about £2000 more. A good builder who really knows his trade does not actually need the building control approval and construction drawings to know what to do.

Often an estimate is based purely on the surface area of the extension. Foundations, brickwork, flooring and roof materials are pretty standard in any build. What often makes the biggest difference in end price are the fixtures, fittings and more luxury items.

See the bottom of the page for how much my extension ended up costing!

Rough Estimate:

  • £2,000 per square metre for ground floor extensions
  • £1,500 per square metre for upper floors extensions, per floor

Our extension is 5m by 4.5m, so 22.5 square metres. A rough building cost would be £45,000. However, in reality our build came up much less, but partly because it is a simple square(ish) extra room. When you start to add kitchens or bi-fold doors the costs skyrocket.

Always allow for an extra 10% over and above your budget – there are always extras, sometimes they are needed, and sometimes you just cannot resist. Also, do not forget to add VAT at 20% on to quotes – most builders quote excluding VAT, so always double check if the quote is including or excluding VAT. A builder may quote £30,000 for your extension, but after VAT is added it may come out at £36,000 – if you forget to budget for this you may find that you do not have cash for that new sofa and flat screen TV!

Breakdown of Costs

First of all, remember that in addition to materials you will need to pay for:

  • Labour costs – 4 builders a day over 6 weeks will add up. Assume around £100 a day each for 30 days work = £12,000 in labour
  • Machinery and skip hire – diggers and other machinery may be needed. Skip hire is around £200 for a large skip and depending on the depth of the foundations 6 to 8 skips may be needed before the building is completed = up to £1600 in skips alone.

For materials the costs mostly are:

  • Cement to fill the foundation trench
  • Bricks
  • Sand and cement
  • Insulation bricks
  • Cavity wall insulation
  • Engineering bricks
  • Steel work for the roof
  • Timber for the roof
  • Insulation panels for the roof and floor
  • Block and beam floor
  • Screed for the floor
  • Electrics
  • Plumbing
  • Fascia and soffit
  • Roof tiles and felt
  • Carpets / wooden floors
  • Windows
  • Internal door
  • Plaster board
  • Skirting boards
  • Paint

This is a rough list of essentials. The costs soon add up.

How much did my extension cost?

Now that it is completed I can tally up the total cost of the extension. I have not included the cost of architect designs, planning permission, building control and inspection (because these will vary and I do not have the figures at hand at the moment!).

New extension build costs

The builders charged £28,250 for the extension. This included the plasterboard and plastering, French doors with side panels, two roof lights, radiator, electric circuit and ceiling light fitting, screeded floor. The original quote for the extension was £26,600 but the additional foundation depth cost an extra £1650.

I had two builders and two labourers. Each builder took about £800 cash a week, and I think the labourers about £500 a week. So the five weeks to build the extension included around £13,000 in labour fees, the remaining £16,900 in materials.

We also paid:

  • Decorating: £320
  • Satellite TV install: £202
  • Electrics testing: £69.6
  • Light bulbs: £34.95
  • Cable covers: £16.79
  • Radiator and outside tap plumbing: £160
  • Ceiling light: £53.94
  • Paint and outside lights £129.85
  • More paint £43.44
  • Karndean floor: £1639

Total for the new living room extension: £30,919.57

Interior alterations – New Kitchen Diner and Utility

The builder charged by job so no breakdown on the kitchen unfortunately.

  • Kitchen units, sink, 900 mm range cooker, extractor hood, splashback, and internal living room doors: £5587
  • Karndean floor (kitchen, front hall, lobby, WC, utility, middle room, under the stairs): £2569
  • Alterations (new doorway, new window, utility wall): £1860
  • Drainage (kitchen sink waste): £960
  • Electrics: £890
  • Plumbing: £1266 (I think we paid too much for this)
  • Replastering ceiling and new walls: £630
  • Decorating (walls, doors, skirting etc.): £850
  • Paint: £62.68
  • Tiles: £157.50
  • Tiling: £360
  • Tile borders: £44.53
  • Shelf and surround: £80
  • Built in bookcase: £500

Total for the new kitchen and alterations: £15,816.71

*************** Total: £46,736.28 ****************

My thoughts on the cost of my extension

The flooring accounted for almost 10% of the build, coming in at £4,208 for Karndean throughout. Well worth the money as it is excellent.

The builder did the kitchen for cheap really, £5000 for the units, range cooker, extractor hood, splash back and kitchen sink from Howdens. I wish we had a better sink though, it is a bit wobbly. And the extractor hood seems too big (and too low and too noisy). Everything else is OK.

The internal work was not brilliant. The builders reused some door frames and skirting and in areas it looks a bit botched. We probably should have had a better finish for the price.

I feel that we did OK, but having since watched the Channel 4 show “How to double your home for half the price” it is depressing to see people do twice as much as we did for £15,000 less! But seeing that some building quotes came in at over £60,000, we did OK.

I have not added on the extra £400 I had to pay a roofer to repair the crap roof work my builder did.

My only regret in the whole build is not having a better builder, but such is life!

10 tips to make a better extension:

  1. Get a plasterer to plaster
  2. Get a tiler to tile
  3. Get a plumber to plumb
  4. Get an electrician to do electrics
  5. Get a decorator to paint
  6. Get a floorer to install the floor
  7. Watch YouTube videos on how various things are done
  8. Speak to your builders every day – ask what they are doing and why
  9. Ask for receipts for all items purchased
  10. Consider DIY – you will probably be as good as a bad builder at tiling, painting etc.!!

More like this in the Planning and Design section

  38 comments for “How Much Will My Extension Cost?

  1. Ian Prior
    October 19, 2014 at 11:02 am

    Do the prices in “double your house for half the money” include VAT / Architects fees?

  2. Masuma
    October 26, 2014 at 9:40 am

    Hi Jon,

    Your article was very insightful and detailed. We are currently looking at doing an extension with at the rear end of our house with pretty much the exact same dimensions as yours.

    We are calculations costs and it’s very confusing as we’re getting very different opinions on how much I spend on labour and on materials.

    Do you have any recommendations for companies that have done the work to your house?

    We had one quote for labour – £30,000 got 12 weeks labour for everything including fitting the kitchen and doing new doorways etc. Is this fair?

    Kind regards,


  3. Jon
    October 26, 2014 at 11:18 am

    Hi Ian, I am not sure. I would think so, but that is only a guess.

  4. Jon
    October 26, 2014 at 11:21 am

    Hi Masuma, £30000 for 12 weeks of building does sound fair I think. Ours cost more. Builder plus a labourer might be £250 a day, so 12 weeks would come to £15,000 for the labour and then the other £15,0000 for all materials. You could ask them for a breakdown of labour and materials costs. Get several quotes though.

  5. Mas
    October 27, 2014 at 3:04 pm

    Hi Jon,

    Thank you for the response.

    We are currently getting a couple of quotes so hopefully will be able to decide where out money is going and if it’s worth it.

    The £30,000 quote was actually just for labour over 12 weeks so I think with materials included we are looking at an overall cost of around £45k ish (quoted by builder)…. I’m not sure if I was clear on my above email (excuse the grammer as was typing on my mobile phone).

    From your above article I wasn’t able to tell how long your overall extension took to complete from start to finish… ?

    Kind regards

  6. Jon
    October 27, 2014 at 3:19 pm

    Ah, that does seem a lot. The actual structure of the new extension was just short of 6 weeks. Then a few more to do the new kitchen and other internal changes. I had two builders each with a labourer working on mine (four people in total) – I could probably have saved money here. At the end one builder admitted that he and the other builder were taking £800 a week each. I guess the labourers were getting about £100 a day.

    So based on that, the labour would have been £12,000 and the remainder materials (the build was about £27,000). If they took 12 weeks it would have been £24,000. But if your build is similar to mine it should not take them 12 weeks – unless there is only one builder and his labourer. But in that case, it would still be around £12,000.

    I think that if you get one good builder who has two labourers and brings you would save too.

    One quote for my extension came in at about £69,000. There can be a huge difference and much of the cash goes in labour. I gave feedback to the expensive quote telling him that other builders put it closer to £30000 and he just said “what are they going to build it out of, straw?”. He wore a very nice suit and drove a very nice range rover (about £70,000 worth I think), whereas my builders drove knackered transit vans. I think I could see where the money was going to be going and it certainly was not on high quality straw.

  7. October 27, 2014 at 4:07 pm

    Hi Jon,

    Thank you for your response, based on other opinions I was questioning the labour cost in terms of 1. time required to do the work, 2. How many workers we realistically needed.

    Thanks for clarifying that… We want to get the most realistic quote. Our planned extension is literally half a metre smaller so hardly any different from yours.

    Thank you for your responses again.


  8. Karen
    March 15, 2015 at 11:05 am

    Hi thanks so much for the detailed breakdown. We are planning a small extension to the side of our flat which measures roughly 3 x1.5 mtrs. If I take rough extension costs at £2k a metre this comes to £9k. The 1 quote I’ve had so far is for £45k which is so far off my estimated budget that I’m starting to worry that I’ve miscalculated the rough cost.

    Are my calculations right or am I missing something?


  9. Jon
    March 15, 2015 at 11:33 am

    £45k for a single story 1.5m by 3m extension is very high. Is it just a brick room? I.e. no kitchen? Get a few more quotes.

  10. June 29, 2015 at 11:27 am

    always better to have one builder and one labourer ,instead of 2 ,than labour price will be reduced ,it takes longer ,but not a double time ,the work will be more eficient ,3 -1.5 should not cost more 15k max ,unless is on island,i am a builder myself and building at the present moment 30 sq meters ,including steel fabrication for posh balcony and byfolding door 45k all together ,and i got to suspend the the existing house end to get the steel correckt

  11. yuvraj singh
    July 16, 2015 at 12:55 pm

    hi, i want to know design and development of extension lab in extension department, so what instrument are require in this lab, please give the detail information.

  12. Jon
    July 16, 2015 at 2:45 pm

    I think you must be after a different type of extension.

  13. a better builder
    July 20, 2015 at 10:11 pm

    whereas my builders drove knackered transit vans…
    no surprise then about your regrets….
    My only regret in the whole build is not having a better builder, but such is life!….

    always u get what you pay for..

  14. Jon
    July 20, 2015 at 11:11 pm

    Yep, although sometimes you get it first, but still have to pay even if you don’t like what you have got!

  15. Georgina
    August 27, 2015 at 8:42 pm

    I have planning for an extension on my corner plot bungalow. In total the extra space will be around 50sqm. The finished space will require steel supports where the corner of the existing building is knocked to extend outwards and to the side.
    Initially I just want the shell built then I can add bathrooms etc in turn. Any idea how much I should expect to pay given the extra structural work?

  16. Jon
    August 27, 2015 at 8:59 pm

    Hi Georgina, best get a few quotes. The size and design of the steels can make a big difference to cost. It really could vary from anywhere from £20k to £50k, depending on the materials used, the finish etc. 50sqm is pretty big, so could cost even more.

    Definitely plan the bathrooms first – they will require drainage and this will very likely need new channels dug. You don’t want the cost of doing this after the extension has been built – for a neater finish you want the drainage to go under the floor, otherwise you will have waste pipes showing on the outside. Also, consider the type of windows, extractor fan etc.

  17. PHILIP
    December 4, 2015 at 7:19 pm

    How much do you pay for labourers cost to install 70mmkingspan insulation under roof between timber joists?please help

  18. Jon
    January 3, 2016 at 8:31 pm

    Hi Philip, if you are supplying the material you just need to agree a price based on how long it should take – my builder did it all as one job, so not sure how much this would have been, but it was done in a couple of days.

  19. gerard maclean
    July 9, 2016 at 2:01 pm

    Hi Looking for a estimate for a single story extension 10 sq meters by 5sq meters do you have any idea what kind of price range i would be looking at your help would be much appreciated

  20. Jon
    September 6, 2016 at 2:45 pm

    Hi Gerard, I’d guess about £35, but depends. Draw up some plans and get some builders to quote.

  21. Kay
    September 22, 2016 at 7:19 am

    Hi Jon, My builder quoted 8-10weeks to build a 6×4 extension. It ran over 14 weeks now saying he has been let down by manufacturers or his worker was injured or ill etc or he has to deal with some emergency on other house.. Now we are on 15 weeks and the knock through is still on hold. When do i say enough is enough? He said he’s back on ours 100% now. However this week, they came in monday to do bits and then not a soul turn up tues and wednesday. Ive asked him for the third time completion date and he said in a weeks’s time. How long does it take from knock through to plastering, electric fixtures, skirting etc? Feel like i’ve been lied to too many times. Thanks.

  22. Derek Toyne
    October 15, 2016 at 9:36 pm

    I am having a 2 metre by 7 metre brick built extension on brick columns. It requires wall to be knocked down so as to extend an existing bedroom and the creation of another bedroom.I’ve been quoted £45,000 for six to eight weeks work is that a fair figure.

  23. Derek Toyne
    October 17, 2016 at 8:37 am

    I left a comment on Sunday but for some reason it’s disappeared anyway I am having a two storey extension 2metre by 7metre on brick columns with a pitched roof and was wondering is £45,000 a fair price,
    sincerely Derek

  24. Jon
    October 17, 2016 at 11:25 am

    Hi Derek, that sounds reasonable. If in doubt get a couple more quotes.

  25. Jon
    October 17, 2016 at 11:26 am

    A tough one. I guess there was no contact? Keep chasing. Do they owe money?

  26. October 27, 2016 at 5:54 am

    It’s impressive how you managed to accomplish all these things with that budget. Any rough estimate on the cost of the design, planning permission, etc. that you spent for the entire extension project? Cheers!

  27. Ally S
    February 1, 2017 at 10:35 am

    Thanks for this breakdown, really useful. Just wondering – are all these costs including or excluding VAT?

  28. Jon
    February 12, 2017 at 11:43 am

    These are all including VAT. Bear in mind that this was first written a few years ago now, so prices have likely risen.

  29. February 15, 2017 at 3:27 pm

    Thanks for the great article, this wil help a lot of people get a better understanding of the costs involved in pricing up their homes.

  30. Maria
    March 8, 2017 at 4:15 pm

    Really interesting article. We are getting 3 x 5 metre extention done at the back and have been quoted £33,600 inc VAT. We are putting in a new kitchen (units already come assembled) size 5.6 x 1 metre (long u shape with cupboards just on one side) and out builder has quoted £5,900 plus VAT and another £2000+ VAT to lay the ceramic (just labour – no tiles). Do I think this sounds right?

  31. Jon
    March 8, 2017 at 5:45 pm

    Hi Maria, that could be about right. The build price seems a little high, even at £1500 per square metre, it is high. Kitchen sounds expensive too tbh, depends on units. We paid about £4000, and that included the Leisure range cooker and extractor fan (one from Howdens, so the cost of units + install was around £3000. Does that price include plumbing and electrics though? You need an electrician, as latest building regs require new cookers to be on their own circuit. So the extra cost could be electrics, water, waste and gas. Are you having laminate or stone worktop?

    £2000 for tiling, again, seems expensive. How many days are they expecting that to take? If they take 5 days, that is still £400 a day for tiling. But saying that, I paid my first builders to do the tiling, and the result was shoddy – I regret that now – the second builders who did my loft were miles better though.

  32. Maria
    March 9, 2017 at 6:57 am

    Hi, the £33,600 inc electrics, moving water supply to new location for sink., moving boiler. The £6000+ VAT was to install kitchen. We did actually go to HOwdens for kitchen, for the white floss (thicker door), where quoted £13,000 incVAt ( it inc appliances which we wldnt get from them, £2500 less), so I’m going back tmrow to c what they are playing at -2 friends of mine got their kitchen at HOwdens and just paid £5000-£6000 pounds and now u have also come up with a much lower figure !
    £2000+VAT for flooring was for ceramic and he wants £1500 + VAT for laminate (+ we have to purchase material). Thanku

  33. Anna
    February 19, 2018 at 4:28 pm

    Hello Jon,

    we are about to undertake a major refurbishment of a Victorian terrace which includes knocking down part of the old kitchen at the back of the house and building into the side return to create one large open plan kitchen/dining/living room. I think using professionals trained to do each specific job (i.e. plasterer to do the plastering, decorator to do the decorating etc.) is sensible, but I am unclear as to when to programme these various trades once the initial extension is built. Can you advice where I may be able to find out out when the trades are needed during an extension project?

    many thanks.


  34. Jon
    March 20, 2018 at 2:55 pm

    Hi Anna, that is a good idea. Best to get some quotes from individuals, then from somebody to do it all. And compare.

  35. February 5, 2019 at 4:02 pm

    Hello all,

    Please advise me if the below sounds reasonable for a 4 x 4.785 m extension in Milton Keynes area.

    Bricks/Blocks with insulation (Material & Labour) – £4,500
    Foundation (Material & Labour) – £3,000
    Wall insulation material – £250
    Roof insulation material – £250
    Roof material (Tiles) + Labour – £2,500
    Velux x 2 (Material & fitting) – £1,500
    Electrics (Material & Labour) – £1,500
    Plumbing (Material & labour) – £1,500
    Plastering (Material & Labour) – £1,000
    Painting (Material & Labour) – £750
    Flooring (Material & Labour) – £1,000
    Skip – £800
    Scaffolding/Digger/Bandstand etc. – £1,000
    Bi-fold door – £1,250
    Drainage movement (Material & Labour) – £1,000
    Timber for Pitched roof (Material) – £500
    Gutter/Facia/Soffits – £500
    RSJ (Material & Labour) – £800
    New Kitchen fitting (Labour only) – £2500
    New partition wall for utility, toilet, sink, standing shower (Material & Labour) – £3500
    Total – £29100

    Does this sound reasonable please?

    Thanks in advance

  36. Jon
    February 5, 2019 at 6:32 pm

    It sounds ok overall. The labour for kitchen fitting and the partition wall both seem high, but the total sounds alright.

  37. M Abdul Ghani
    March 6, 2019 at 2:42 pm

    Hi Jon excellent website and some really valuable information. Wanted to ask is it true you will save money if you get specific tradesmen for each stage/job rather than one builder doing everything?

  38. Jon
    March 6, 2019 at 7:58 pm

    It can be, really it depends. e.g. if you get a builder that pretty much does everything himself, then it might work out cheap if they are just charging materials + their time. But, usually they will employ people, and sub-contract jobs, and for this all costs extra, and you end up paying for a project manager. So it can be cheaper to get contractors yourself. Best to get quotes. I almost did my 3rd extension like this, but found a good builder and left them to it. They did bring in contractors for electrics, plumbing, gas, plastering, rendering outside, and all this cost a fair bit, and more than I’d expect to pay if I went DIY.

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