Extension - how are costs calculated?

ringthealarm2009 · 5 Jul 2011

Hi there,

I want to have a small extension put on, and was wondering how builders calculate the costs. Do they go on the *internal floor space* that you're adding?
And does the space occupied by the existing wall (that will be knocked down when extending) come into consideration when the floor space costs are being calculated?

Thanks in advance.

jeds · 5 Jul 2011

There are published costs per square metre but these should be adjusted for regional variation and also for any special features. Personally I don't think it's appropriate for small extensions other than to get a ball park figure. (I hate that expression) In my neck of the woods you would be looking at about £1250/m² of floor area.

Most builders work to elemental costs, which are normally based on the last couple of jobs with a bit of spadework on particular items. An example would be facing bricks which can range between £250 and £750, or more, per 1000.

ringthealarm2009 · 5 Jul 2011

ok many thanks. That's good to know builders go on internal floor space (if the external space were included it would be so expensive!)
We are actually using stone for the walls, however we have most of it ourselves, so should save on costs.

noseall · 5 Jul 2011

Either Jeds post has mislead or you have misunderstood but the true costs are estimated based upon the buildings' external footprint.

stuart45 · 5 Jul 2011

You will make some savings on materials as you have some stone, but the labour for stonework is much more expensive.

ringthealarm2009 · 6 Jul 2011

Ok so now I'm confused - builders charge on the external space of the extension? My architect told me they usually go on internal floor space.

freddiemercurystwin · 6 Jul 2011

I've always used external face of wall areas.

jeds · 6 Jul 2011

Industry standard in the UK is gross internal floor area.

jeds · 6 Jul 2011

Although I would just add that gross external area is the preferred method for Insurance rebuilding costs. But construction costs are definitely always quoted GIA. Don't know why it would be different.

ringthealarm2009 · 6 Jul 2011

ok thanks guys. So, it looks like they charge on internal floor space.
Our small extension will be *stone*, not brick built, measuring about 2 x 3.5 sqm. We have the windows, and most of the stone, and the tiles - any rough idea how much it may cost?

^woody^ · 6 Jul 2011

ringthealarm2009 said:
Ok so now I'm confused - builders charge on the external space of the extension? My architect told me they usually go on internal floor space.

No that is not the case.

Any builder that costs an extension on a m2 basis, is either clueless or putting in a inflated price. And your Architect seems equally clueless for thinking that

The extension is priced up based on what work is involved, how much stuff is required and how long the job will take

Even such basic items such as a 20p brick or a 50p brick can make a difference ... and that will not be affected by floor area

ringthealarm2009 · 7 Jul 2011

Ok thanks for your reply. Will get quotes from more builders, anyway - had one ridiculous one for £15,000 excluding VAT - seems ludicrous for such a small job, esp as we've got the windows, most of the stone, and the tiles for the roof.

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Extension - how are costs calculated?

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