We have a 4 bed house, which was a 3 bed semi which we extended. Downstairs we have a large family room (living room/dining room/playroom), a decent size kitchen, utility and downstairs loo. Upstairs 4 bedrooms, but none of them are particularly spacious, particularly DDs which is a box room. One bedroom is an office with a guest bed squeezed in.
Right now it's perfect, as DC are young (5 and 2), so the big family room works well and the small bedrooms don't matter too much. Technically we could live here forever, but we do have some spare cash and I'm trying to figure out whether it would make sense to move in a couple of years, and versus increased saving for retirement and/or savings for DC as they get older. To significantly increase the space, we'd be looking at minimum of 150k extra on the mortgage, so we'd be looking at taking on a 20 year mortgage at age 40. We could afford it, but it's more a case of the length of the mortgage and the balance between that and saving for retirement etc.