War story time. A fellow I know signed a contract to purchase a tract to build chicken houses on. Too antsy to wait for the bank to process his loan he immediately sent a dozer to start constructing the building pads... Banker goes to inspect the place (pre-licensing) and finds the dozer working. Cancels the loan. Won't touch it. The dozer op has a clear prior right to lien. The bank is not going to allow themselves be to 2nd lien holder. Another back up buyer ends up with the property, doesn't want the CHs where this guy was building them so those leveled humps are still visible today while 2 chicken houses are built further off the road.
A couple I know started clearing the grown up fence line on a property they made an offer on and got a quarter mile of it cleaned out when they found the property was not going to appraise out. They had to walk away after working a week clearing brush. Ask me how I know...
C2C is considered adequate adjustment to value. Since it is nigh impossible to find a house with ADU partially constructed to sell, it is also impossible to calculate, estimate, or even make a wild***ed guess as to the impact. And if you were to find a single sale, 1 sale is poor support for such an adjustment. So a cost to cure sans any further adjustment is far more defensible than making a PFA adjustment. You cannot prove a negative as they say...and without some minimum burden of proof, and without a comp sold with an partially built ADU, you have no such evidence.
Me, too. Even did weekly inspections after the bank paid a contractor to finish it. The contractor was so pleased with the job, he bought it himself. So what was the discount? The bank came out with their investment and the builder got a nice house he built himself... But I have never seen a house sell with a partially remodeled garage, or unfinished ADU.