Rough cost for building shell of extension

nickrhymes

Hello.

We are considering buying a house that has planning permission for an extension at the back and so I am trying to work out rough costings for building the extension before we put an offer in on the house.

The house is a chalet bungalow and the extension would be roughly 7x9 meters (so 63m2), with half of it being a pitched roof that will contain a bedroom and the other half being a flat roof - so basically it's a single story extension with a room in the roof space.

I'm quite happy to do most of the internal finishing off work (fitting kitchens, bathrooms, flooring, dry walling, decorating etc) and so am wondering if anyone can give me a rough idea of costs of building the shell of the extension.
e.g. the cost of doing all the ground work, foundations, block work, roof, fitting windows/doors and insulating roof etc.

In case area matters we are just outside Oxford.

Any help would be much appreciated.

Thanks.

Nick...


Can anyone gf

slummymummyof3

Is it brickwork? Blockwork and render? Without this sort of information it is impossible to give any indication. You are probably looking at approx £1,500k to get out of the ground. The pitched roof, is that trusses or does it need to be hand cut? Room in the roof - standard window or Velux?
If you were to buy in the materials yourself eg. got yourself a trade account with somewhere like the build centre - thermalite blocks you are looking aroung £8.80 per m2, cavity wall insulation boards you can get much cheaper from seconds and co (online), there is a lot of money you can save by buying in your own materials (get builder to tell you how many blocks etc from the plans). If you are prepared to project manage and sub jobs out seperately, you can save a small fortune. PM me if you want more info.

nickrhymes

Thank you very much for the reply slummymummyof3.

I knew I wouldn't have managed to give all the necessary info so will try and fill in the gaps with a bit more detail )

Groundwork/foundations:
Think the only complication that I know of is that one manhole needs moving about 1 meter. From what I can see it looks like this is moving it along the direction of the in/out pipes so hopefully not complex.
It will also need a waste pipe run to the end of the new extension and into the moved manhole for kitchen sink waste, and also drainage put in for rain water (not sure what is generally done with this).
Hot/cold water pipes also need to be run to the end of the extension where the new kitchen will be, however I don't know if these are generally run underneath the floor slap or put in at a later stage after main building work completed.

Walls:
This is blockwork and render.
We may decide to have one internal wall built with reclaimed bricks as a "feature" wall, however won't take this in to account for basic costing.

Ground floor:
General concrete floor with all the layers/DPC required for building regs.

Pitched roof:
I'm guess that prefabricated trusses could be used for the majority of the roof (trusses would of course have to take in to account that the majority of roof space is to be a room), however it would definitely need some hand-cut work at the end where it would need to be joined in with the existing sloping roof of the house.
It would also need to be insulated to comply with current building regs.

Windows/doors:
Downstairs extension will have a single door at the side, patio doors on one side at the end, and a large window at the other side at the end.
Room in roof will have one standard window at the gable end, and one dormer window about 1.5 meters wide on one of the sloping sides.

Stairs:
Stairs will require a 180 degree turn in them.

Changes to current structure:
One 3m span of existing wall needs to be removed and so an RSJ put in to replace this. This will need to support part of the existing gable end of the house and so will be supporting blockwork.

I really like the idea of project managing myself and breaking the jobs down to save cost and so if anyone could give very rough guideline broken down costings for each of the parts of the job that would be really useful.

Also love the idea of sourcing the building materials myself. Have had many people suggest that to me so it is definitely something for me to investigate.

Thanks again in advance for any costing information anyone can provide.

If any more info is need on any area of the build to give ideas of costing then please just ask and I'll do my best to answer.

Regards,

Nick...

slummymummyof3

Have you got copies of the plans? Builders usually price up from seeing the plans, so you would get more accurate costing showing them to some builders.

You may also need to take into account the Party Wall Act and its associated costs to you (the building owner) for the appointment of a surveyor and for the adjoining owner. This is dependent on whether you are building to 'line of junction' (the boundary - and your foundations will be projecting under the neighbours land), or you will be digging your foundations within 3 or 6 metres of your neighbours property and your foundations are likely to be deeper than theirs - this is where the difficulty lies. PWA can cost you in the region of £1k - £2k depending on whether your adjoining owner is prepared to use the same surveyor as you (to cut costs) or whether they insist on having their own. Sorry to throw another potential spanner in the finances. Can give you loads more info if you need.

nickrhymes

Thanks again for the reply.

Yes I do have copies of the plans and so am thinking I might well need to try and get some builders to give me an idea of costings.

However as this is a house we are "thinking" of putting an offer in on I was hoping to try and get a ballpark idea of costing from here so that we know whether it's even financially viable for us to be able to buy it and then do the work. From the information I've given, do you think we are looking at £20,000, £40,000, £60,000 or more to get the building shell put up?

Should have mentioned before - the house is detatched and so no problems with party walls.

Thanks.

Nick...

dander

I'm in the very early stages of planning an extension, so no expert but the figure I've seen around for a ballpark figure is to expect £1-1.5k per m2 for an extension. If the footprint of your extension is 63m2, then you'd have to add the size of the upstairs bit in as well.

I haven't had quotes yet myself, so can't vouch for the accuracy of these figures!

nickrhymes

Thanks very much dander.

That £1-1.5k - is that the figure you've seen around for building the shell only or for finishing off the entire thing? I've seen £1k per m2 mentioned quite a lot in these forums but that generally appeared to be the total "finished" cost where as I'm trying to find a ballpark for the shell only as I'm planning on doing most of the internal work myself.

Thanks again.

George_Bray

Nicky - to get a rough idea of costs you could try this website:

http://www.growyourhome.com/

I wouldn't spend more than 10% of the value of my house on improvements because you might not get it back. No way would I spend more than £20,000 on a two-storey extension or £10,000 on a ground floor extension and that would be for quite a large one. Concrete, bricks, wood and cost-effective windows are simply not that expensive. Many sources suggest at least £1000 per square metre for a ground floor extension but what's that for? I bit of concrete base and a tiny bit of extra wall and roofing for each square metre added. I wouldn't pay that! Mind you, I would expect to source the materials myself, to avoid builders adding their own margin, and I would expect to do a lot of the basic work myself, like digging out a trench for the foundations. I'd only use the builders for anything I probably couldn't do myself to a high standard, e.g. the brick walls.

dander

I have to say, I'm not clear on what you get for those prices either! I think economies of scale mean the bigger the extension the lower the m2 cost would be, so since yours sounds pretty hefty then you'd be looking at the lower end.

I've certainly seen quotes for porches on here that are well over the the £1k per m2 mark, and presumably a porch has a fairly basic interior, so I get the impression you'd be lucky, or be prepared to do a very large amount of work, to come in much below the £1k mark.

Be happy to be told otherwise!

If the people you'd be buying off have got PP, have they had any quotes to do the work? It's worth asking them!

slummymummyof3

nickrhymes wrote: »

Thanks again for the reply.

Yes I do have copies of the plans and so am thinking I might well need to try and get some builders to give me an idea of costings.

However as this is a house we are "thinking" of putting an offer in on I was hoping to try and get a ballpark idea of costing from here so that we know whether it's even financially viable for us to be able to buy it and then do the work. From the information I've given, do you think we are looking at £20,000, £40,000, £60,000 or more to get the building shell put up?

Should have mentioned before - the house is detatched and so no problems with party walls.

Thanks.

Nick...

The party wall act does not just refer to a 'party wall' consider it as party wall et al act- you can live in a detached property and still need the PWA. As I explained in my previous post , if you are within 6 metres of a neighbouring property and there is any chance your foundations are likely to be deeper than the adjoining neighbour (perhaps they have a conservatory, perhaps the building inspector wants to excavate further due to trees.....a whole host of possibilities), then the PWA will apply. You really need to consider this as a real possibility and include this in your budget, otherwise you are liable for a bit of a shock.

ormus

you can a get a bare shell built here for 5k/10k in the NW. (flat roof). so i would have thought 20k would be plenty for even oxfordshire.