Topic: Areas?

I just was wondering if anyone could answer this question for me. When creating an area is it possible to create one large area, and then make it possible for the players to change particular tiles, say staring out the whole area is a grassland but if a palyer wanted to create a house or something along those lines who it be able to change that one tile. or since it was first made a grassland would it always stay a grass land. I'm kind of horrible at explaining things sometimes it's prolly some newb question, but I just wanted to try to find out before I get to ahead of myself.