Many seem to think they are horrible things, but really, people buy and sell homes with unpermitted additions all the time.
Unpermitted additions are worth what they are worth. Sometimes they are great little additions or extra sheds or garage conversions. Sometimes they are nightmare projects with lots of liability associated with them.
If you are a buyer and refuse to buy anything that has unpermitted space, you may miss out on something useful. If you do buy such a place, make sure you know how to evaluate the quality of the space. Is there anything dangerous in the garage conversion? Bad ventilation? No insulation? Unvented dryers in a living space? etc. Make sure your inspector goes through it thoroughly. Decide if it really adds or detracts from the value.
If you are a seller, just make sure you disclose what space is unpermitted. If you have a nightmare unpermitted space, think about “undoing” it. If you have a nice, safe, usable space, leave it alone. It will add value.
