Cost of one-storey extension

jaminphil

Considering buying a new house to do up, including adding a rear one storey extension which would measure approximately 3.2m deep x 13.5m wide internal size (approx 43sqm)

Looking around the internet, there seems to be a guide price of £1,000 per sqm used when calculating the price of extension.

However, my thinking is that with a rear extension, you basically have one wall already built. Furthermore, I would think that given the size of the proposed extension that you would also be able to take advantage of economies of scale compared to one that is half the size.

Assuming that there are no hidden surprises when doing the foundations, what would be a reasonable cost for building the actual shell (zinc flat roof, excluding windows and doors)?

Not interested in getting a detailed quote on here, but just wondering where the £1,000sqm figure comes from (is this so finished product maybe).

I would think that if an extension half the size of that above costs £1000 per sqm, then to double this should probably only cost maybe £500sqm extra as there would be less materials and I would think less labour cost?

tired_dad

Your logic does not work. By that rationale an extension 3 times bigger would be £250sqm and 4 times bigger would be £125sqm.

My architect says to budget for £1200-1500sqm depending on location for a basic build including all the bits such as plumbing , electrics, 1/2nd fix.

jaminphil

My logic is sound, in that for every additional square metre of an extension, the overall cost should be less per square metre.

ie an extension of 1sqm would be well in excess of £1000, but then with each extra sqm, this would decrease.

What I'm saying is that an £500 for each additional square metre on the extension, however big you go, would apply - not halfing the cost every time you double the size, which would be silly. (Please note I'm just using £500 figure for illustration purposes).

Therefore, by my rationale, the larger the extension gets, the closer the average cost will tend towards £500 per sqm.

Just thinking in terms of my example, the length of walls needing to be constructed don't double with the doubling of size - so why would the cost?

3.2 x 6.75 = 13.5m of walls
3.2 x 13.5 = 20m of walls

I would also think that economies of scale would also apply to most other aspects as well.

tired_dad

There are economies of scale. Of course there are. A 2 story extension costs less per m sq than a single story etc. However total cost of a build is a complex calculation including costs of materials, cost to builder of paying for trades, builders own profit and critically time taken to complete the job. Some builders like small jobs some don't.

The £1000-£1500sqm calculation is just a ball park to give people a rough idea of what a build may cost in the most general of terms. Lay people have varying ideas of what extending costs. This guide gives them a basic idea of what the costs might be.

I think you have a specific requirement with a good idea of what you want. Get some quotes in then post back and you will get good advice on whether its reasonable or not.

Ivana_Tinkle

£1k per square metre is a (very low) ballpark figure, not a maximum, which is where your logic is flawed. Not really sure why you think the area of the walls should be the deciding factor, unless you're building in stone or something, in which case you'll need to budget a hell of a lot more than £1k per square metre!

Clive_Woody

We're mid way through the build of a 20 sq metre extension and ours is going to come out around the £1500 per sq metre, but we had to have 2m foundations and we are having some additional structural work done.

I think you will be doing well to get it done at £1000 per sq metre, anything less and I would be suspicious of the quote and predict lots of extras suddenly being added on as the build progresses.

ajsexton

We had a 18ish m2 extension added a few years ago now, and it was something like 1.1k per m2 and that was a cheap building firm - had a few minor issues since too, think the cost of the roof, that probably increases over size due to building regs requiring a certain level of load bearing

I think a better ball park would be to look around 1.4/1.5 per m2 with potentially getting it down to 1.2k (1.1k if your lucky - barely any bricks in our extension its all windows!) for economies of scale

andrewf75

as others have said you're looking at more than £1,000/m. Its a bare minimum.

WentworthFox

I would say the £1k per sqm is not a bad starting point. Like whats been said above it all depends on what specification you are building too, if anything crops up and what fixtures and fittings you are having inside.

Your extension will be cheap if you build a shell with minimal windows and only add flooring cover a few sockets and switches and a pendent light with one radiator inside.

As soon as you start adding other things the cost goes up like kitchens, bathrooms etc. My advice work out what you want in your extension and get a few quotations from local builders. This will give you a rough costing.