How accurate is the £1500 per sq metre frequently quoted for extension work?

[sugarhoops]

Posted a couple of times recently about extensions, cost of etc and had some great responses. Most posters quote an average rate of £1,500 per square metre....

I've worked out that our two storey extension will create 2 new rooms, one with a total space of 10 square metres, one about 15, so approx 25 sq metres in total.

Using the £1500 figure, this will mean that our new extension might only cost £37.5k - that seems awfully cheap to me! We're in Wiltshire, so not a hugely expensive SE area.

What else do I need to add on? We will need new roofing done on the upstairs room, windows, some internal fiddling, steel beam downstairs. I'm abit confused as to whether those sorts of things are included in this £1.5k figure? We don't need any plumbing work or new kitchen though - we're hoping to create just a new upstairs bedroom plus knock-thru from existing kitchen into new downstairs room to create a huge kitchen / diner.

I only ask because I'm trying to work out funding for the extension - I originally thought it could be £75k or similar, so a complete remortgage, if around the £40k mark we can use savings & similar.

Any thoughts or experience much appreciated!

Our extension cost us about double the 1,500 per sqm, so it didn't work for us! A chunk of this cost was down to our choices of fixtures and fittings (ours included a kitchen), plus we got the builders to do some remedial work to gutters before they started. They also built a deck for outside as part of the job. If I strip the costs back to basics, it probably still came in at around 2k per sqm. (I am counting the sqm to the external walls - not just the usable space inside - it makes quite a difference, as the walls were 30cm thick).

Some costs that we incurred that may not apply to others included use of stock bricks to match existing brickwork; large bifold doors; five velux windows; and some considerable steel work to hold up the back of the existing house. We are also in outer London, so had those prices to contend with.

The other factor to bear in mind is that the extension inevitably leads to other work required in the house. We ended up having to replace a bathroom (tiles came loose and fell into the bath, which cracked it...but as it was avocado this is a bit of a blessing in disguise!). We also need to replace various carpets that no longer fit around new doorways, and also replace a couple of internal doors that no longer are in keeping with the rest. All of these jobs remain outstanding and we finished the extension in January!

Many thanks Lemon - yes I can see that we will have various remedial things to do after the extension (that we'll probably live with until we can afford to put right!) plus several jobs to sort before we're even started eg moving the outside gas meter box, potential drain works etc. Although we have some money set aside to do those bits and pieces.

We're a period house too so not sure if we'll have to pay extra for the right bricks, although might render part of it.

Thanks though - I just wasn't clear on what the £1.5k includes (windows / doors / roof etc or just pure brick-work). I'm such a building / house extension novice (although learning quickly thanks to mumsnet!).

We are rendering our new extension due to the cost of matching the bricks - Victorian houses tend to be built with bigger imperial sized bricks. We forgot about moving the gas meter, which was nearly a disaster as builder turned up and we then discovered it was a 6 week lead time to move it...

Our extension seems to be costing more,than £1500 per square meter, but we are also remodelling the back two rooms and I don't know what the cost per square meter would be for this? Also bifold doors and new kitchen plus new flooring throughout. I think if will cost about £85k all in (including furnishing).

Apologise if you've already factored this in but don't forget VAT.
Most builders quote without ,and then to have to put 20% on to that can be a bit of a shock naivevoiceofexperience

Ours cost us £50k (3 years ago). The size of the extended space was 4x5 m, so 20m sq. But the entire space (including the existing space which was massively altered) was about 8x5 m sq. So, using one calculation, it was a lot more expensive, using another calculation cheaper. It does include our kitchen, windows, etc - although these were not high end choices, but we added in a toilet, utility room and built in cupboards. An architect told us recently that the cost of materials has gone up hugely and he expected that it would have cost us 10-20k more if we were doing it now !?!

Marmitelover - presumably rendering is considerably cheaper than sourcing the right bricks? We do have a side extension that was done a few years before we bought our house - never really took much notice of the bricks before but I'm not sure they were matched and you can't honestly notice the difference between those and the main house.

Also, do you have to call the gas company to move the gas box, and do you know how much it costs? I'm kicking myself now as they only fitted it last year onto the back of our house - huge amounts of arguing with them because they originally wanted to fit it right next to our front door, which I point blank refused as I said a large white box on the front of a victorian house would look completely hideous and make me sad every day when I arrived home [mad woman emoticon]. The gas boys looked at me like I was crazy, but eventually agreed to place it round the back (all done free as part of ongoing gas works up our road). I am now going to have to move it again, and pay I guess....arghhh!!

Ok, i'm upping my very conservative estimates to an equally conservative £60k to be safe....

Moving gas meter cost us bout £520 + £50 to builder for building recess. We had to get Wales And west to actually move it, who own the gas pipes, rather than our actual gas provider. We have one that is recessed into the wall, so not as obtrusive, but it is down the side rather than on front. I would allow 8 weeks to be on the safe-side (I had to phone every day and cry in order to get them to do it in 4 weeks).

Right, my builder wasn't VAT registered because he likes to take lots of holidays so doesn't earn above the threshold.

Before anyone starts thinking this is dodgy I paid him directly into his bank account and have invoices and receipts.

But that meant I personally paid for everything else, ie I paid the builders merchant for the deliveries, door & window, guy to dig out footings etc directly to them. These were all arranged by the builder but I merely paid him for his labour and the sub contractors for their labour and goods.

So the £1500psqm was about right for me. But mine was a kitchen extension so I had the new bit built and the knock through into the old kitchen, a large velux in the new roof and then the plasterer.

But I paid the plasterer extra as I didn't want just the new bit doing but the old bit too as I was having new LED downlights fitted in the old and new ceiling so there were holes to be filled in and the wall was a mess from the old tiles being removed (by me)

I paid an electrician for his work which was sockets, lights, garden light feed, a couple more cctv cameras and outside lights and a shower cable for a shower to be fitted at a later date. So clearly that involved paying for work that had nothing to do with the extension. I bought an Ikea kitchen, two internal doors and paid a local joiner to fit it and I bought flooring.

I would say allow £1500psqm for the shell, or to first fix (electric wires but no sockets etc

Hope that helps

My builder also not registered for vat and I also pay the suppliers direct - think this must be common with smaller builders. My cost also included new boiler and 5 veluxes, which I had forgotten about. We had 5 quotes from builders ranging from £45k to £85k excluding kitchen, floor, windows and doors (except veluxes) - can't believe that there was such a massive variation.

Ah ok, thanks contended and marmite, interesting to know when we're considering which builders to use.

I did wonder whether we could do more of it 'ourselves', in terms of appointing our own subbies etc - we have our own business & own a large warehouse, so have good contact with electricians, plumbers (Not that we need much plumbing done I don't think, unless we move washing machine etc), a great chippy, plasterer etc - people we have used for years and we trust, so perhaps we can appoint them to do the interiors bits for us but get builders to do shell work.

TBH we're quite a way off doing anything yet - architectural tech guy only just done survey so not even drawn up plans yet so no idea how big complete extension is likely to be, but i'm abit of a figures geek and want to make sure I understand everything prior to starting.

ps marmite - if you'd accepted the £45k quote, and it ended up costing nearer to the £80k, do you know how that works? I.e. does the builder just say 'oh shite I quoted wrong and now you owe me more money', or 'oh shite I quoted wrong and now I (the builder) have to take that hit'??

No! We accepted a slightly higher quote of £47k but this excluded various things

• kitchen £14k
• floor £3.5k
• bifolds and matching doors & windows £8.5k
• built in cupboards £2.2k
• new alarm £0.5k
• move meter £0.5k

Plus new tv and furnishings etc etc its amazing hos if all mounts up...

With regard to the VAT, we're getting 4 businesses to quote for our work and I suspect our non VAT guy will try and charge more (net of VAT) just because he's not having to add it.

With the VAT registered businesses I'm assuming they will have more manpower so will do the work more quickly.

The difference on VAT is only on labour as the non-VAT registered builders will be charged VAT on matetials etc, so will have to pass this extra bit on.

my £1500 / sqm is based on my start to finish spreadsheet
most of the labourers I paid directly so that no VAT was due on their wages
materials I bought direct
but with bank transfers and signed receipts - and a huge spreadsheet - i know how much I spent
NB I added 85 sqm to my house in total

I'm a building contractor with 30+ years of experience. I'm not here looking for work/tout for business.
Please all remember that the 1.5k (or whatever) per metre square is only a very rough guide as jobs can vary hugely even between what seem to be two similar projects.
My advice is to make the effort to get at least three very detailed quotations with the work broken down into as many section/categories as possible. This helps cover your back as well as the builders and never forget , communication is pivotal to a smooth project.