He would probably need to resize some of the buildings that would contain rooms, as the rooms are not distributed with the same dimensions, if he were perfecting them, so he would need to move a lot of stuff around, and so it would take a long time to implement a more wider interior to some buildings

As for the new buildings it would force the game to render even more things that are in the map, making it more laggy, but it would possibly open up the opportunity to set some sides of each building to "nodraw" to make it invisible, so then the game doesn't have to render it