How much value does it add to the living space? That depends on the buyer. Seller wants $100,000 more for it over a comparable property down the street probably won't get it unless the buyer has that classic expensive car and it's worth it to them. All value comes down to how much the buyer wants to pay.
I know that answer isn't really quantitative, but the only thing that you can do is ask yourself "how much is it worth to me" and then throw out numbers toll you get there.