If it has separate access, it is not considered in the main living area. In the cost approach you figure actual construction cost and depreciate it like any other super adequacy. That is, unless garage apartments are typical for the area and they get full value for them. In this area, it is treated just like a pool or workshop and has a lower contributory value than actual cost (like a pool or workshop or garage conversion). It may be different in your area so investigate and do what is typical.
If it's not finished, heated and cooled (similar to the rest of the improvements) and, if it does not have access from the inteiror of the home, then, it's not GLA. I'd try to find similar comparables. (In-law, office, etc.) detached from the rest of the home. (if access is from outside)
Funny - just today had same/similar. Mine had exterior entrance through breezeway, appears unheated (too much clutter to actually determine & father in law didn't know about heat). So, in my case the additional sq/ft was actually listed in (public records/metroscan). However, I chose not to count as GLA, doesn't really make the house with or without it. If its detached, and in this case with no internal bath you have to exit the area go through front door, through living are to use bath or other areas - get real - so its not GLA, but does have value & contribute towards marketability.
GLA is interior entrance, with heat, functional floorplan, permits. Each situation has to be considered case by case with no hard / set rules - FEEL THE APPRAISAL FORCE... However, it still has a value - just find a comp with cottage, bonus area, sunroom, whatever and justify some value (if warranted) in that manner.
If it is not rented and/or not zoned for multi-family, I call anything that is set-up of independent living on site as "Guest Quarters". If they are rented, I don't do the appraisal, as we don't appraise income property. We have a lot of these up here, especially on the lakes. Often the old garage is made into a 1 or 2 bedroom with kitchen/living and bath. Since they are not rented, I do not include it in the GLA and just assign a contributory value to the unit. There is not enough market data to show what a unit is worth so showing an estimated contributory value satisfies the owner and calling it Guest Quarters does not upset the UW. It is sort of a soft peddling my way around this issue but I honestly can't measure how the market reacts to these things.
In the past week, I've had two like that. The first was a 74 year old traditional home with a detached single car garage. Above the garage was a 300s.f. guest apartment with a bath and living/bedroom. The second was a log home several years old. This home had a rear screened porch. Attached to the rear side of the porch was a 360s.f. guest cottage with a kitchenette, living room and bath.
Not sure about the rest of the country, but here in NC they simply couldn't be considered part of the main home's GLA. Even in situations where a bonus room is located above a garage and the only entry to this room is through the garage and up a flight of stairs. If you have to pass through an unheated portion of a property to access the additional heated living area, it must be counted seperately.
as noted the area is brand new and does offer heat; ac; kitchen; BR & Bath. It does offer a separate entry so we did not include it in GLA and adjusted accordingly. We used a similar sale of recent date and three others; it is "Legal" per town PZC and therefore, no special acceptions were noted.
It appears that we (in general) across the country view this type of product almost similarily.
never said I hadn't done others or that there were few; but-due to most of the New Zoning, there will be a lot less. Zoning in most towns is now under scrutiny based on the great influx from Western Ct.; Noo Yawk; and Jersey. The past few years has seen a tremendous growth in new construction middle towards the eastern half - land is now becoming a very rich item.