I'd doubt there's massive investment in housing/building around Cornwall at the moment. Beautiful place, tricky to buy a house though, as the locals are constantly fighting the holiday home crowd pushing them out of their hometowns.
Dorset gives you access to plenty of big cities, London and Cornwall/Devon.
I live in Carbis bay next to St Ives in cornwall (moved here a year ago), there seems to be plenty of building projects going on all around us (So many apt buildings, house extensions etc, I would not know about commercial premises building sorry), 2 of my mates at my local golf club are builders and they are always converting barns or doing house extensions/loft conversions etc so seems to be work around, in fact my parents are about to have a loft conversion on their house.
Someone else mentioned housing prices in cornwall being expensive due to holiday-makers, perhaps for the most they are higher than normal, but not compared to where I lived in the north west, if you live in St ives expect to pay premium prices especially if it has a Harbour/sea view, otherwise no more expensive than a lot of places tbh!
I am from the north west myself and would also vote for that, friendly people, lots around to do and see.