The house has a family room (a deck that was wrapped, which is usable, but in dire condition) with an unfinished rumpus room in the basement.

The City banned decks on top of garages (although that ban has been lifted recently) I cannot see a scenario where they would let you have an apartment. 

From really just decking them out and doing completely right in the beginning.

I do not know the condition of the roof but the top floor tenant owns the rooftop deck on the roof (however, I may still be liable for roof repairs?)

The rental apt. is pretty unique (3 beds, 3 baths, 2 of which are on suites) with a private rear deck in a nice residential neighborhood, so I don't think a locked door will deter tenants. 

@Wayne Brooks @Julia Deck-RussellThis has turned out to be a major PITA! 

Less for basements that aren't finished, garages, and decks.

Deck should have been pressure washed.There's debris and an old truck topper in the back yard.Not a fan of the bathroom with the door opening to the kitchen, but that's a personal thing. 

Jarrod Williams If it was me (and others would do it differently), I would put together a pitch deck /offering summary for this investment showing the amount of money you seeking and the returns to your partner.

@Delmas Edwards You can get a plumber to run a snake up the drain that has rotating cutting blades that will cut out the roots.