I know this is a big one, but I don't think it makes sense to have two different kinds of buildings. How about making houses and towers the same thing and adding permissions?
That way, players could choose to have a street with no building, or with as many buildings as they had space for. And instead of two different mechanisms, the game would have a single more flexible one.
What's different between houses and towers:
- houses are only accessible to key-holders and by knocking
- houses can be increased in width
- houses have a back yard
- towers can be nine stories tall
The replacement buildings would:
- either have a back yard or not, perhaps max one per home street
- be up to nine stories tall
- have an optional elevator, otherwise one would have to build doors
- be width adjustable, perhaps with a maximum width across all buildings
- have a permission setting: owner, key-holders, friends, everyone