One major factor about septic tanks is the capacity or number of gallons that is related to how many restrooms, house square footage, or whatever and there is a chance your tank if too small.

If you buy a house with and "illegal" addition insurance companies will not cover the addition and in some cases wont cover the entire house and they might not cover an accident caused by the addition like bad wiring causing a fire or bad plumbing in the "illegal" addition that causes damage to exiting structure also if you sell and don't get the addition permitted the square footage wont count towards the sell price or they might count it if the addition looks good and has a good flow but for half of what sq ft is selling in the area. 

In contrast, I would probably need to bump the size of my loft by stealing footage and space from the adjoining master bedroom.

I look at the tax value, deed date, last sold price, location, what's around it, bedrooms, sq footage, back taxes owed, lot size, condition from outside, look at the google street view and if I like the numbers on paper I drive by and check it.

Would that be based on the percentage of square footage of the overall building?

Alex Friesen Some things you can compare:-The property condition-square footage-number of bedrooms-what does the more expensive one have to offer(is it better condition, pool, more updated, etc.)

What's the linear footage of kitchen cabinetry to be painted?  

Property Count - 4,028BPO Value - $692.2MMTop 3 States Represented - FL, GA, AZAverage Square Footage - 1,978Average Home Years - 13

What are houses selling for in that area similar to your property in square footage, beds, baths, year built etc.....If you know for sure this property needs 50K in repairs, I would be thorough in my due diligence and make sure there is enough profit in the deal for you to wholesale and still leave a piece of the pie for your end buyer.Hope this helps!

Find properties with the ability to add square footage or beds/baths.