Rough cost for extension

[usn90]

We re wanting an extension built to the rear of our house, roughly 4m x 3m, trying to get a rough cost to get the shell built.

For the roof I think as a short-medium term measure it s going to have to be a flat roof, can t see a way it can have any pitch to go with the rest of the house currently, in the medium term we are looking to extend upstairs in which case there would then be scope to put a pitched roof over this.



Windows, electrics and plumbing I can do

Northwest/manchester

Dug. £2k
Foundations ?
Brickie + materials £2.5k
Roof?

Edited by usn90 on Tuesday 13th January 16:35

[mdw]

Something like that you can work the materials from the plans then add for labour. We did a 5x5 flat roof with beam and block floor 2 years ago for 29k all in builder doing all the work. I worked it out at 28k before his quote came in so not too far off. Easy on basic building. Only variable is in the ground.

[BoRED S2upid]

£24k ballpark and hope for the best when you get quotes in.

[kambites]

If you're talking JUST the shell - as in: no windows or doors, bare concrete floor (or no floor at all if you want suspended chipboard), no knock-through into the existing house, bare block walls, no insulation, etc... I reckon that's doable for around £10-15k as long as there's no big windows or doors (even if they're put putting the fittings in, big steels are expensive), access is straight-forward, and there's no services in the way of the footings.

Maybe I'm hopelessly out of date though!

Edited by kambites on Tuesday 13th January 19:22

[usn90]

BoRED S2upid said:

£24k ballpark and hope for the best when you get quotes in.

Just for the shell? If so my man maths are well out, I d understand if that was complete, I m hoping to have the floor walls and a flat roof up for 10-15k, and nearer 10 the better!

May have to grab the spade

Edited by usn90 on Tuesday 13th January 19:23

[gangzoom]

usn90 said:

Just for the shell? If so my man maths are well out, I d understand if that was complete, I m hoping to have the floor walls and a flat roof up for 10-15k.

May have to grab the spade

Your not going to much built for £10k these days by the trades. £2k/sq meter for first fix (shell) is pretty reasonable guesstimate. If you want to save ££££ than doing work yourself will clearly bring the cost down.

[kambites]

gangzoom said:

Your not going to much built for £10k these days by the trades. £2k/sq meter for first fix (shell) is pretty reasonable guesstimate. If you want to save ££££ than doing work yourself will clearly bring the cost down.

Doesn't first-fix usually include things like windows and doors?

ETA: In fact from reading on google, it looks like it usually includes everything to be done before plastering, which is vastly more than the OP is after.

Edited by kambites on Tuesday 13th January 19:28

[usn90]

gangzoom said:

usn90 said:

Just for the shell? If so my man maths are well out, I d understand if that was complete, I m hoping to have the floor walls and a flat roof up for 10-15k.

May have to grab the spade

Your not going to much built for £10k these days by the trades. £2k/sq meter for first fix (shell) is pretty reasonable guesstimate. If you want to save ££££ than doing work yourself will clearly bring the cost down.

Yeah looking to do quite a bit myself/ family, I’ve dug foundations myself once before, most miserable time of my life and sworn never to go through it again, may have to relive it

[v8notbrave]

Have u got plans? I just did in Scotland and rules for % glazing, EPC and insulation are all becoming tougher. Presumably this is everywhere, a quick Google suggests England toughened u values in 2023. Bear this in mind it adds to the paperwork cost and final build cost

[fooman]

I could only give you a cost of a rough extension

[kambites]

gangzoom said:

Your not going to much built for £10k these days by the trades. £2k/sq meter for first fix (shell) is pretty reasonable guesstimate. If you want to save ££££ than doing work yourself will clearly bring the cost down.

I paid about £50k for a 100 square meter extension (over two stories) in the south of England, built to the point the OP is suggesting plus windows, fifteen years ago; so £500 per square meter. I know prices have increased significantly since then but surely not by over 300%?

As far as I can see, most of the cost of building work seems to be silly, easily DIYable things like electrics, plumbing, plastering, flooring, etc.

Edited by kambites on Tuesday 13th January 19:40

[BoRED S2upid]

usn90 said:

BoRED S2upid said:

£24k ballpark and hope for the best when you get quotes in.

Just for the shell? If so my man maths are well out, I d understand if that was complete, I m hoping to have the floor walls and a flat roof up for 10-15k, and nearer 10 the better!

May have to grab the spade

Edited by usn90 on Tuesday 13th January 19:23

I’m thinking they are required to come back in after and plaster it all?

Everything is crazy expensive. Footings that’s just a trench right? No. Building control demand double the depth just in case you go up double the height so double the work, disposal of earth, double the concrete than you actually need.

[joshcowin]

kambites said:

I paid about £50k for a 100 square meter extension (over two stories) in the south of England, built to the point the OP is suggesting plus windows, fifteen years ago; so £500 per square meter. I know prices have increased significantly since then but surely not by over 300%?

Probably not 300% but not far off.

[usn90]

kambites said:

If you're talking JUST the shell - as in: no windows or doors, bare concrete floor (or no floor at all if you want suspended chipboard), no knock-through into the existing house, bare block walls, no insulation, etc... I reckon that's doable for around £10-15k as long as there's no big windows or doors (even if they're put putting the fittings in, big steels are expensive), access is straight-forward, and there's no services in the way of the footings.

Maybe I'm hopelessly out of date though!

Edited by kambites on Tuesday 13th January 19:22

Yep, just the floor, walls and roof.

No big windows or doors

10k was my hopeful figure

[kambites]

joshcowin said:

kambites said:

I paid about £50k for a 100 square meter extension (over two stories) in the south of England, built to the point the OP is suggesting plus windows, fifteen years ago; so £500 per square meter. I know prices have increased significantly since then but surely not by over 300%?

Probably not 300% but not far off.

The various sites I can see which attempt to estimate it are coming up with figures more like 80% over that time frame... which is still a hell of a lot (well above general inflation) but that should still put such a project at no more than about £1000 per square meter?

[kambites]

usn90 said:

Yep, just the floor, walls and roof.

No big windows or doors

10k was my hopeful figure

I reckon you should be able to get close to that if you're willing to project manage it yourself rather than paying a building company to do everything.

Will it need PP or is it doable under permitted development? If the latter, you shouldn't need an architect (although you will need a structural engineer).

[Little Lofty]

usn90 said:

kambites said:

If you're talking JUST the shell - as in: no windows or doors, bare concrete floor (or no floor at all if you want suspended chipboard), no knock-through into the existing house, bare block walls, no insulation, etc... I reckon that's doable for around £10-15k as long as there's no big windows or doors (even if they're put putting the fittings in, big steels are expensive), access is straight-forward, and there's no services in the way of the footings.

Maybe I'm hopelessly out of date though!

Edited by kambites on Tuesday 13th January 19:22

Yep, just the floor, walls and roof.

No big windows or doors

10k was my hopeful figure

It’s just cost me £10k to have some garden walls built, that was around 3000 bricks & 500 blocks, I think you are a bit out, probably double what you are hoping for.

I fitted a warm roof to a 3.5 x4.0m existing extension last year, it did have a roof lantern but was around £6k.

Edited by Little Lofty on Tuesday 13th January 20:00

[kambites]

Little Lofty said:

It s just cost me £10k to have some garden walls built, that was around 3000 bricks & 500 blocks, I think you are a bit out, probably double what you are hoping for.

Am I missing something? That sounds like about 50 square meters of double layer wall? The OP needs under a third of that. Obviously his foundations will cost more and he needs a roof, but small flat roofs aren't particularly expensive.

So he's probably looking at about £3-4k for the walls; say an extra £1000 for a couple of short steels; £2-3k for the footings is probably doable? Say £1000 for joist, hangers and boards for the roof and floor, plus whatever a 12 square meter EPDM layer costs... it might be over £10k, but it's certainly not 20!

I could easily be wrong, but I can't get my head around where £20k would go on such a small project! Certainly if it was me and it was going to be that sort of price I'd just build the whole thing myself!

Edited by kambites on Tuesday 13th January 20:13

[usn90]

kambites said:

I reckon you should be able to get close to that if you're willing to project manage it yourself rather than paying a building company to do everything.

Will it need PP or is it doable under permitted development? If the latter, you shouldn't need an architect (although you will need a structural engineer).

As far as I can tell it’s permitted development

- 4m as were detached
- rear of house
- not taking up the % of garden

Won’t even be visible, we’re a corner plot with hedges all around

[joshcowin]

kambites said:

The various sites I can see which attempt to estimate it are coming up with figures more like 80% over that time frame... which is still a hell of a lot (well above general inflation) but that should still put such a project at no more than about £1000 per square meter?

Ok sure.