It may be a regional thing, but I've been told unless the screen porch is heated living space it can't be included. Locally we have two square footages reported, heated square footage and total square footage.

In TX it wouldn't. I doubt it is anywhere unless it is enclosed and fully protected from the elements (which screens don't do) and heated or cooled depending on the area.

Only HVAC-ed space above ground is counted here in Chicagoland -- both by the assessor and on a listing.

"...nor should closets be included..."

Yes, closets are included. They are above ground & HVACed.

closets are definitely included, esp. since lots of those sq ft calculations are done on the outside dimensions of the house. Hallways, too.

(when I tried once to figure the sq ft of my apt, I added the major rooms together and came up w/ 850. Later, I realized I'd left out hallways, closets, and maybe even both bathrooms. Suddenly, I gained a whole bunch of living space--refiguring, based on external measurements, up to 1,100. And I didn't even have to move, LOL!)

Okay, so the 2-car garage in my recently-sold MD house had it's own heat pump - should I have included that in my square footage?
This house was the 1st model home, so the showing office was in the garage, which, I should mention, also had double french doors, bay window w/window treatments, chair railing, wallpaper, crown moulding, lots of dead phone lines, and a huge chandelier hanging in the middle. If we threw down carpet, we'd have a giant bonus room. If you looked very carefully, you could just see a slight variance in the grass where a path had gone from the office french doors straight to the front of the model home next door.

Okay, so the 2-car garage in my recently-sold MD house had it's own heat pump - should I have included that in my square footage?

Only if it's actual living space.