how much for an extension?

[MixedupMartha]

Considering buying a new house but would like to extend it.

It has a double garage that we'd like to put a bedroom over (doesn't need an ensuite - has two already). Other properties on the road have done the same so I'm assuming the foundations are up to the job.

We'd also like to add a garden room with some sort of feature glass roof like an orangerie roof.

Any ideas of a rough figure for this? It will affect what we can afford to offer for the house.

have a look here here

Thanks - that's a really helpful link.

I've been told to budget roughly £1000 per square metre for a finished extension (but would obviously be more for eg kitchens and so on).