Build cost per m2 question - internal or full footprint?

[Deleted member 281264]

I see a lot of people estimating build costs on a per m2 basis

Say a 4m a 8m extension would the universal calculation that people throw around be based on the internal 3.6m x 8m space created or the former?

 

[Loofah]

It's an estimate, very rough, for cost per added m2. A 4*8m extension is 32m2 so 32*your chosen value. I use 2.5k per m2 which gives you £80k .

 

[Deleted member 281264]

That was always my impression too, but I've seen some guy on youtube advising people on extension pricing and doing it with internal m2 so it got me thinking.

 

[freddiemercurystwin]

Internal, external, they'll only ever be a few m2 in it anyway so a bit of a moot point, that said I've never known it to be internal.

 

[Loofah]

It's all guesstimate until you get the quotes in!

 

[Mottie]

Surely it must be to the external edge to include the walls?

 

[jeds]

It depends what you mean by estimating, or rather who is doing the estimating, but If you mean building price books e.g. Laxton's, Spon's, BCIS, Griffiths, Wessex, etc. I've used them all over the years and they are all based on GIA - Gross Internal Area - over all floors.

As mentioned above, the prices are average and are also based on a base cost which should be adjusted for regional variation. A lot of prices quoted aren't always adjusted.

 

[Wayners]

Recent YouTube video with prices
I happened to watch it last week