Really would appreciate advice from those that knows...

I am thinking of changing the attached double garage to living space. I
would extend the back and produce another living room, bathroom and study.
This would be accessed from the hall and could also be closed off to form a
seperate annex.

The house is a typical 70's style with the flat roof garage attached to the
side and there is room to park about four cars in the drive. I haven't
looked in to the cost of this yet but am wondering whether the loss of
garage is a problem, whether an annex adds much value, etc.

Any views really would be appreciated.