Implications of a property on top of another property...

[Payton Gonzales]

Hello!  I'm interested in making an offer on an apartment on top of another building in San Francisco, CA.  The building is a two unit building, with the property I'm interested in being apartment, residing on top of a commercial building (which houses a daycare).  

My question is what would the implications of buying a property on top of another that I don't be?  Is this a terrible idea? 

A few of my concerns being:

1) You don't have control of how the building on the ground level is being taken care of...

2) I'm interested in making the roof of the apartment a deck... if it's structurally sound, would you be allowed to do this if you own the top unit?

3) Apartments are rent controlled in San Francisco.  Because this building is mixed use, the apartment is not rent controlled.  Would this change the status if I were to own only one unit (i.e. it would become rent controlled)?

There are a lot of questions here, that I'm sure would be followed up with questions to attempt answering this, just starting here as I'm not sure where else to go :)

Thanks in advance for any response!