ADU income counted at 75% by the lender but I'm only getting 68% of market in real life

The appraiser comp issue is real and it burned me similarly on my fourth unit, a small detached in a pocket that looked like the adjacent neighborhood on a map but absolutely was not. The appraiser pulled rents from two streets over from a completely different rental pool and my qualifying number was $920 where the unit actually filled at $810. That's $110 a month, $1,320 a year, same bad spreadsheet feeling you're describing. What I've learned since is that you can request the appraiser's comparable rentals before the appraisal is finalized in some cases, or at minimum ask your loan officer to flag it if the comps are crossing a market boundary. Most people don't know to ask and the appraiser isn't going to volunteer that they grabbed rents from a stronger zip.

The 75% figure itself is also not the universal number people treat it as. On two of my six units I've seen lenders use 70% in softer rental markets when they're being conservative, and in one case it was negotiable because I had 12 months of actual landlord history on another property. The $1,040 market rent estimate is what I'd really push back on here though, not the percentage. At 75% of a real $920 number you'd be at $690 which is basically what you're collecting. The problem wasn't the multiplier, it was the starting figure, and that's harder to catch before closing unless you've already rented something nearby and know the ground truth. Your signed lease approach on the other county property is the cleaner method every time.

Signed lease method wins every time for accuracy, appraisal-based is just a qualifying tool.

The $85 gap is smaller than what I walked into on a similar deal in Bibb County about two years back. Appraiser pulled comps from the Riverside area when my unit was firmly in a softer pocket three miles east, and the spread between qualifying rent and actual collections ran $140 a month for the first year, call it $1,680 gone from the projection. What stung more than the money was that I'd flagged the comp addresses to my agent before closing and neither of us pushed it to the underwriter because we were scared of reopening the file. That's the real cost, the not-pushing-back part. On your specific question about whether you could have challenged it pre-close, yes, a borrower can submit a rebuttal with alternate comp data and the appraiser has to respond in writing, most people just don't know that or don't want to risk the delay. The market rent figure being optimistic and you wanting it to be right are probably both true at the same time, that's usually how it works.

The zip code bleed on the comps is the part that almost never gets flagged until after closing. Appraisers are not penalized for optimistic market rent estimates the way they are for optimistic sales comps, so there is less friction pushing them toward accuracy on that number.

The thing I haven't seen mentioned yet: FHA's 75% factor was originally sized to absorb exactly this kind of variance, vacancy included, but lenders treat it as the floor for qualifying rather than a built-in cushion, so borrowers mentally treat $780 as the expected deposit instead of $1,040 minus a rough vacancy buffer. Your $695 is actually inside the cushion FHA anticipated, the problem is the appraiser's $1,040 starting point was wrong, not the 75% math.

Macon County rent ceilings are real and $725 is probably the honest number for that pocket right now.

The thing that tends to get skipped is that the appraiser's market rent is underwritten, not audited. Nobody at the lender is checking it against actual collections 14 months later, so the gap just lives with you quietly. On FHA specifically, the 75% vacancy factor is supposed to absorb exactly this kind of miss, but when the gross estimate is already inflated by 15 to 20% from comp bleed, the 75% haircut doesn't save you. Your $85 is actually a pretty tight outcome given the starting number was wrong at the source.

Pushing back on the appraiser pre-close rarely moves the number more than $20-30 anyway.