While 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!
- £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
- 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
- Fascia and soffit
- Roof tiles and felt
- Carpets / wooden floors
- Internal door
- Plaster board
- Skirting boards
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:
- Get a plasterer to plaster
- Get a tiler to tile
- Get a plumber to plumb
- Get an electrician to do electrics
- Get a decorator to paint
- Get a floorer to install the floor
- Watch YouTube videos on how various things are done
- Speak to your builders every day – ask what they are doing and why
- Ask for receipts for all items purchased
- Consider DIY – you will probably be as good as a bad builder at tiling, painting etc.!!