Yes, the living area (i.e. heated/cooled space) is what should be used for total square footage when they calculate this. Which would exclude your garages, porches, patios and even lanais. Some people try to use their "florida rooms" as add'l square footage because they have an a/c window unit in them...I was told this is a no-no. It needs to be under roof. fully ducted air & heated space to count 100% in SF and I'm sure other appraisers would also argue that.
And remember price per square foot is only one way to calculate value. I would much rather use true sold comparables to the subject property and look good/hard at the upgrades, condition and location of the property in order to give you a better feel for market value rather than simply using a general cost per sf. Just think if you had a home that had all new hardwood flooring, updated kitchen with higher end appliances and fully remodeled baths on a lake with a new roof vs. same SF home in same neighborhood with laminate flooring, older kitchen/baths, original roof and backed up to the busy highway. One should have a lot more value than the other agree? Great example why just using cost per SF would NOT be a fair comparison of these two homes.