Detached garage with accessory unit on top. 

It probably doesn't matter...conventional guidelines won't lend on a MFR with accessory units.

I think the term you are looking for is accessory apartment.

Or, if it is a legal sfr with an accessory unit, it will be appraised as 1 property with an accessory unit.

The other option is a single family with an accessory dwelling unit.

(A) One ADU or JADU per lot with a proposed or existing single-family dwelling, if all of thefollowing apply:(i) The ADU or JADU is within the proposed space of a single-family dwelling orexisting space of a single-family dwelling or accessory structure and may includean expansion of not more than 150 square feet beyond the same physicaldimensions as the existing accessory structure.

Yesterday, the Raleigh City Council approved an ordinance that allows any detached house or duplex to add a second, smaller, "accessory" dwelling unit with its own kitchen and entrance.

I was wondering what the expected going costs would be for design and engineering plans for an accessory dwelling unit and garage. 

I am in Los Angeles county California and in the process of converting my garage into an accessory dwelling unit.

We also went through the city and paid the impact fees and had it registered as a legal accessory apartment, which was not difficult to do.