Valuation of upstairs garage apartment vs. primary home

[Stacy Litwin]

If a primary home on ground level is valued at $200 a sq foot as an example, what is a safe estimate to value an above the garage apartment as that is on the same lot? Would the value be 50% that of the primary, IE $100 a sq. foot value for the apartment and $200 for the primary residence? I'm just curious if there is some sort of "appraised standard" for how an upstairs apartment should be valued. The situation is 2 homes on 1 lot, 1 of the homes being above a garage but given a second address because it's in a multi-family zone, so it's sort of a duplex without shared walls but technically a shared roof to cover a set of stairs to the upstairs apartment.