Since the two residential units make up more than 51% of the square footage,  I still qualify for a FHA loan. 

What is the square footage of the house itself?

Have you thought about converting existing square footage to additional units/ADU/JADU, or, if zoned, correctly, to a triplex?

I also have some drone footage that I would like to have lots layered into. 

When it's time to sell you will need to add that bedroom back in which will cost you UNLESS you can make more money when it's time to sell by adding square footage  on to both units to enlarge and give good closet space.Regards

Shall I alert the property appraiser to this fact so that the square footage will appear as "living area" in the public records?

Thanks to my excel spreadsheet, I'll know by square footage, or type, or bed/bath count the desired price range I'm looking for.

. - Keep it simple SimonAdding sq. footage would be a different story, as you should always get permits for that.

Normally if your home is already in the best condition for the neighborhood the only way to increase value is to add square footage and increase the number of bedrooms and or bathrooms.Also if your house hasn't been updated in years.a new gourmet kitchen and refurbished bathrooms,new flooring,and new paint can help increase values to a future buyer.

Other than checking on the current owner, square footage and lot size or the amount of the property taxes, DO NOT RELY ON THE VALUES