@Austin Shute if you intend it to be storage for the whole building, you'll need to figure out how to connect it to the common areas / hallway so all tenants can access it.But usually what folks do in this situation is combine the top floor with the closest unit and make it a bigger unit (a "townhouse" unit), rather than using a top floor for storage for the whole building.You might have to remove a door or two to connect the two units so the building inspector doesn't think you're going to keep renting the top floor as an illegal 5th unit, but as long as that's done I think you might be able to keep the bathroom up there.

I live in Houston where this setup seems relatively common but am not sure if that comes with tenant issues around a shared yard, etc.Thanks!!

Very common for the lockout policy to be outlined in the lease.

Commonly, not always. 

It is a small urban town so crime in urban areas is common but I'm not afraid to walk around. 

Not impossible, but far from common. 

The property is in a foreign country (EU member) that does not seem to offer the kind of cost segregation studies that are common in the US.

They are not wanting these on Loopnet, but I think I am going to just try to get them to allow me to post there at least as that seems to be the common thread.

Some useful info here:https://www.opploans.com/oppu/articles/8-money-pitfalls-to-avoid-when-co-signing-a-lease/Pitfall No. 7: Not putting all roommates on the leaseFalen Cox, attorney at Cox, Rodman, & Middleton, LLCReaders should be aware that if they allow another person to live with them, even without the intent to acquire a roommate or create a landlord-tenant relationship, they may have to go through the formal eviction process if that person refuses to leave.”

It’s very common.