Personally I would:
1: create Post with type Building (you can then add meta to this eg: address, lat lng, name etc)
2: create Post type Apartments and associate with building via apartments post meta
3: create metabox for owner for post type apartments, here you can GET all user info with role as owner/landlord and add to post meta
4: as above but for tenants, GET all users with role tenant, when apartment is let.