Use what is called the "footprint". That is the perimeter of the building. If has two floors, not high ceilings, that is when you add to the calculation. Of course the tax record is a good place to start, but it is not always current. An experienced real estate professional is very useful for this kind of thing.
(Choosing an answer as a best answer, or at least as a thumbs up, helps those who answer questions.)... more
It depends on the rest of the house. If your house is not updated for the past 50 years, new geo thermal system won't probably do anyting to help, but if your house is fully renovated and in good shape and priced well, then you can demand a premium fom the new system.
Good Luck.... more