Cost per sqm 1st Floor

[Bobobab]

Hello! Wondered if anyone could tell me roughly the difference in cost per sqm of a double extension versus a single? As a % is fine I can see costs really vary but I am assuming the sqm cost for just a ground floor includes foundations so the top floor should be cheaper?

You can find guide figures on Checkatrade or similar. Yes upstairs usually cheaper because foundations are a big cost.

having just costed an extension I can tell you it will probably be more expensive than you could imagine and a pain in the ass to get any quotes, let enough an array to choose from :-(

How large is the extension going to be?

We are looking at 4x4m. We are definitely doing the ground floor. The top floor would add an extra bedroom so great for the value of the house but got to think about the cost now as we don't "need" a 4th bedroom just wondering if overall its worth doing while we are at it.

You're thinking about this the right way. The second storey adds cost, but not double, because you're already paying for the expensive bits: foundations, groundworks, drainage, scaffolding, and site setup. Those costs don't change much whether you go one storey or two.
As a rough guide, adding the first floor typically costs around 40-60% on top of the ground floor price, not another 100%. For a 4x4m footprint, if your ground floor comes in at, say, £80-100k, the additional storey might add £35-50k depending on specification and your area.
The value question is worth thinking about carefully though. A 3-bedroom house becoming a 4-bedroom house can make a meaningful difference to resale value. Often more proportionally than the extension itself.
Check what 4-bed houses on your street sell for versus 3-beds. If the gap is larger than the cost of adding the storey, then it may be worth it.
The strongest argument: it's significantly cheaper to do it now than to come back and add it later. Retrofitting a first floor onto a single storey extension is a far bigger job.