As a side note, my CPA told me I'd have to prorate all utilities for the sq footage I occupy vs the sq footage I rent out.

What's the average cost per square footage of materials and labor have you spent in the past on this type of work? 

Also, with a lot of the properties being fairly outdated, total square footage and the amount of disrepair you're dealing with could significantly increase the rehab/update expenses on larger multi-unit vs a small single family.

The fin sq footage above grade is actually 1,270.

As a wholesaler, I'm trying to find out what the builders in the area are wanting to pay per lot square footage.  

Check your city's planning and development website to be sure, but I've never encountered a city that allows you to add livable square footage without a permit.

thinking basement square footage appraises close to above grade square footage.7.

They operate off objective data, like square footage, number of beds/baths, etc.

@Ned Carey Very true, the idea is to add a room in the existing structure and not expand the sq footage.

If so, is it recommended that we base the portions on the square footage of the rented bedrooms or by the number of bedrooms (we rent out 2 of our 3-bedroom house)?