I had an idea that could possibly be an assist for this to work. Instead of changing the exterior environment to allow open interiors, just either make an entrance purchasable or free of charge to teleport you into the room. Instead of hallowing it out, just teleport the player to an unreachable area outside the map with the interior room (e.g. random building that's unlocked, player clicks door, teleports player to the inside. It looks like the inside, but it's not located in the building you were once outside of.) Garages on the other hand where you have vehicles parked in, I have no clue what to do with that.
What if someone plays for a few days, buys an apartment and leaves? I'm guessing each apartment would come to about 1k bricks when finished, so how will that work? We have thousands of unique clients that have joined the server, personal apartments arent something which I will include again just due to the brick count, I've even decided to remove admin ones just to save some extra bricks.
When I have some time i'm going to extend the map a little, add some more features to the north west corner of the map, but for now I'm going home for the weekend so I won't be able to get on BL till Sunday afternoon.