LInas1985: well IwubCheeze if i would knew this before this game ( i mean building lower level houses on the same estate, and add to them repairman) now i will keep that in mind, when i will build, next game. but as i mentioned i chose to build in frree mode, and i was trying to build every single house, from level 1 to level 5. to uncover that one secret building, which is in questionmarks ??? coudl you tell me the purpose of that secret building, what it is?
It's possible the secret building you're referring to is the prison and that only gets unlocked after you've trained a certain number of police. I think you had to have 500 but don't quote me on that. (EDIT: Coming to think of it, it might be 200). Because you aren't playing with any AI opponents, you don't need police so that's probably why you're missing it.
If that's not it, then I can only think of it being your "super houses". Level 5 tenants can make plans for these buildings (a fifth building icon will appear). However, when a building can be made, it must be built before the tenant making the plans will unlock the next building. If not, the effort was wasted and he'll just start over again.
If neither of these sounds correct, could you post a screenie and I'll be able to give you a better answer :) Everything else is unlocked by building one of each house and each level and building factories.