Is a small extension actually any cheaper than a large one?!

[Nonemoreblack]

Just wondering whether most of the costs of building a (single storey) extension actually come from the building work ie knocking down walls, putting up walls, putting up the roof, plumbing etc rather than actual square footage? We want to buy a house that really needs an extension, it's only small, about 1.9 x 6m, going along the side return of a semi detached house, but I have a horrible feeling that it's still going to cost virtually the same as a larger one (ie about 45k, London/Surrey prices). I know small side returns can be very expensive on terraced houses, because of the layout, party wall etc, but this is on the unattached side of a semi-detached house. Anyone willing to share costs/sizes of their more bijou extensions?!

1mx6m £30k. (Don't ask why...)

Ah feck. Please tell me that's because you built it out of gold?

no...we're pulling down two useless part extension/lean to bits that comprise 2m & rebuilding it properly as a 3m extension. so redoing all foundations etc. so we are doing it properly, and the extra 1 m means we get 5 separate zoned functions from 1 big space rather than just 2 plus a lot of swearing as we pull in tummies passing in the galley.

Ah, but Pinkfizzy, doesn't that mean your £30K is buying most of a 3m x 6m extension, seeing as you're redoing everything from the foundations up?

Following with interest, as my current ten year plan (which changes fairly frequently ) is to add a 2.5m x 3.5m extension to the back of our house. I can see that a few of the costs (knocking out walls, getting equipment and materials to and from the site) may be nearly independent of the size of the extension, but I'd have thought the vast bulk of it would be a function of size. The cost of materials must be pretty well directly proportional. Labour costs not so simple, because some of the set up will be just as labour intensive for any size extension, even though the actual construction work will vary enormously. It might not cost twice as much to build 16 linear metres of wall as 8, but surely closer than to the same price. Same for foundations, and for the roof.

You're right I should definitely keep telling myself that. Glass half full...
Obviously we wouldn't be doing it if it didn't deliver functionality as well as financially viable for the area. It will be worth it!

Our 6m x 2.9m double storey extension will probably cost around 60k but that includes knocking down a conservatory, unkitchening a kitchen, putting in a new kitchen and lavatory and because it is two storey we could only get planning permission in a way that meant redoing the roof of the whole house.

We're looking at this at the moment. For us the way to cut costs has been to find a solution that doesn't require repositioning the boiler and the waste pipe or require digging out the entire garden to level it (all of which was labour intensive). It's been simplifying what's required rather than the size of the extension that has cut the costs.
Still hideously expensive though as will require new kitchen, floors, foundation etc.

I have no cost to offer but I can give you some insight

In terms of cost, side extension will cost around the same as rear one. The challenging bit of construction is to knock down the structural wall, in your case, 6m, (or 3m for rear ext) and build a beam across to support wall above. The projection bit, 1.9m, is a lot more straight forward. But I am a big fan of side ext (planning to do one myself) because..

1. use dead space on the side
2. doesn't eat into your garden space like a rear ext.

If you build a side ext on the unattached side of a semi house, then you don't have a part wall issue unless you are very close to your neighbor (3m I think). Even it does, its a a lot less intrusive for your neighbor to accept your proposal.