If the work done involves increasing the square footage of the property (a forced appreciation strategy which I expect to use frequently), lenders who only perform drive-by appraisals will use the older, lower square footage figure, and therefore will return a lower appraisal. 

Find the price per square foot and multiply by the square footage of your house you want. 

I would get the monthly rate, sq. footage, what is included in the rent, how far from the school, year built, and amenities.

I would have them verify the sq footage, if that is correct then you can try to contest and get another appraisal as everyone else here was saying. 

@Marta RodriguezWanted to share with you a great way to tap into the equity built from your ADU.The problem today: most appraisers used by traditional lenders would give ADU square footage a second-class treatment than the main house (as some of your mentioned above) since there aren't that many comps and they're trying to be conservative.However, you can consider taking out a HELOC/doing cash-out refi through Figure.com (HELOC up to $250k, Cash-out refi up to $1M).

Square footage? 

consider remeasuring the space to see if living area is more than recorded (perhaps property grew over the years), and look for any additional areas like attic/basement that can be finished and included in rentable square footage. 

For example, if you are looking at a villa that has a 95% occupancy rate compared to a 16% occupancy rate for a "similar" villa both with square footage and location - first it would depend on which villa complex it was in.

Square footage estimate he gave was half the size of the actual building (1500 square ft vs 2900 square ft). 

After calculating the square footage we take up 17% for personal use and 83% for business use.Now the question is, can we do a bonus depreciation from the renovations on 83% of the property?