There is a reasonable chance that you could be required to convert it back to a 2 bedroom, so you would have paid for the extra square footage I would expect to pay for the original square footage and have the price discounted from that for the extra hassle and risk of dealing with the code enforcement officer 

Aside from maxing out your rentable square footage, I would look at the condition of the kitchen and bathrooms since those often add the most value. 

Question: is there anywhere to get educated on square footage? 

The Realtor should have in his flyer the P&L based on actual not projections, the square footage, pictures of the property from across the street and inside, and a rent roll that breaks down of number of units and type of units, and how many are rented out. 

Getting the ADU permitted will increase the total square footage listed in the county records and increase the value of the home.

It should list the square footage there. 

These two particular individuals did not have the sq footage- for those, what they were being rented for, what the lease terms were for those tenants -NOTHING!!

We normally do it half by square footage & half by number of residents in the home. 

We’re under contract at the moment on a doublewide and the booklet for foundation requires requires piers spaced at certain distance apart,a certain square footage of venting, piers on footers below frontline, mortared , etc etc. they require an engineers inspection to approve it.

The sq. footage is 2,438 and 2,625 respectively for a 4bd/3ba and 3bd/3ba.