many thanks - very kind of you all.
So, as I suspected - you cannot measure areas directly - but the Room tools are rather good, I can now see - apart from the fact that you don't seem to be able to include the floor space within a recess up to the room door face unless there is an option I have missed... or maybe that doesn't count as rentable/usable space ?
The Area Boundary tool was the bit I couldn't find, thanks - it isn't on the Drafting toolbar but in the top menu.
Umm, still haven't quite got the hang of this - you can create an Area plan, draw an Area boundary...
but the area within the boundary seems to have no special significance to Revit. I guess there is a next step I haven't located yet.
Filled regions works pretty well though, thanks.
cheers