Can anyone help with building costs?

[Houseplanning]

We are having some serious conversations re: life changes. Maybe looking to move GP in with us etc. Extend ours to support this.

We live in Greater London and trying to work our theoretical costs which seem to vary widely!

Could anyone advise a Rough costing for a double storey side extension (maybe 35m sq) per floor. A loft extension (maybe 40m sq) and a rear extension (maybe 50m sq)

I realise this will vary hugely on finish etc but looking for a ballpark figure.

Ps not a stealth boast at all. Would be selling GP house to help fund. Just trying to work our what is best for our family.

They say work on about £1500 per square metre for single storey, lofts can be more or less depending on your roof structure. I spent £65k on 40 sq metres but not London. Obviously fittings vary dramatically in price and mine includes overruns to fix my building (underpinning)

Thanks. Is hurting my head to think about. I guess the best thing is to speak to builders etc too

As it's a large amount of work there may be some economies of scale. I would contact firms with experience of big renovations though they may need plans first before they can quote