It counts ang GLA for the main house if it is attached and accessible from within the main improvements. Appraiser looks at what did someone else pay for a similar Det Apt. Appraiser will only give value to what is legally permissible. Is it zoned for multi family. Don't fall into the trap that it is a mother in law quarter therefore it is legal. It might be legal for a family member non renter, but not for joe blow. What does this mean. It means you can't legally rent it to the general public and you say no one will ever know who is in there. Well I as appraiser is not going to give much value to it because it is non rentable by law. If the county or city decides to crack down on these properties there goes your renter. I have seen this happen because the noisy neighbor turns you in for it and raises hell until they do something about it. All that being said I appraise in a resort area where these are legally permissible rental units and for short term. This is important for you ARNB guys. Are there any deed restrictions to short term rentals? Some say min 6 months. Read the paperwork. So back to the vacation homes that allow it. In my area these carriage houses, detached apartments, Detached Accessory Units, are sold and marketed including these units. They are considered by the market as part of the overall GLA because it is useable in that fashion. Typical homeowner does not as they are not going to put the new born baby in the garage apartment. Not going to put the 12 year old kid out there either. So it's a different market. So the way these units are handled on a appraisal is by using a separate line item for it. I label it MIL or Carriage House. And adjust separately for it. Now for the value of this unit. If you are in the resort area where they are utilized and the market sees equal value in the unit as part of the overall GLA I give the same value to it as additional GLA to the main improvements. In other words if I am adjusting $100 a sqft for differences in GLA I give it $100 a sqft in value as a separate line item. Now if it is in a general neighborhood that is not zoned for a accessory building that is rentable to the general public what is it's usefulness to a buyer. Not much. Like a detached workshop, storage, auto shop, crafts, a She Shed, whatever you call it has very limited use to the general public. Like a pool not everyone wants one. Some see it as a liability. Who buys a house with a small house in the back yard you can't legally rent to anyone. You can let Grand ma live in it for free. How many of you want your in laws in the back yard thats right there are only 2 of you out there the rest of us don't want a place for her at all. Don't want even thinking she can come here. Joking. Most of these structure's are originally built because of a ailing relative that can no longer take care of themselves. So after they pass there is no use for the building and it just sits. So unless it is legal and rentable to the public you are not going to get anymore value for the structure than a unfinished detached garage. Also if you enclose a garage you loose that feature that has value. I highly recommend you spend time reading a appraisal. Not just the numbers. Read every damn line. You should be able to understand the reasoning and rational that the appraiser is employing if not call him/her and ask. Why did you put all your weight on that sale and not this one. Why no comps with a ADU. How did you come to the additional value of the ADU. It's not just about the number at the bottom of the page. I did a appraisal on a house where a investor renovated a oversize 4 car garage into a 2 bedroom rental unit. Well after he spent around 100k on this thing he got 5k in value for a accessory building. It was not legally rentable. He could let family members live in it for free. You can't even charge family members in the city of PCB rent to live in it. So does it have income value, No. Buyer buying this property ran when he found out he couldn't legally rent it. Owner wanted to shoot me until I told him the law which he never bother to read and how guide lines will only allow me to give value to what is legally permissible. Spend a hour with a appraiser. You need to understand what the guidelines are. They are not the same as the hard money lender. Understand what is a comp to your house. Ask the realtor to show you one that recently sold for, that they are saying the one you are looking, is going to be worth when you rehab it not just what he/she thinks it will sell for. They have pictures of the inside, look at them.