Beyond legality, look at how the two units are actually separated: fire separation between them, independent electrical panels or a properly documented shared one, and separate mechanical systems where possible, since a second unit that shares a single furnace or panel with the main house limits what a lender will finance against it.
Also check what the builder's warranty covers versus what a resale seller would be responsible for, since a two-family bought new carries different protections than an older converted one, and a home inspection, typically $400 to $600, should specifically cover both units rather than just the primary living space.
The paperwork tells you what a lender and a future buyer will actually count; the finishes do not. Get your hand-reviewed valuation from Joel Dyck.