A realtor explained to me that you would measure the exterior dimensions of the foundation, not the interior dimensions.
I don't think that unfinished basements are normally included in the square foot calculations. Basements, converted attics, split level homes can make it confusing sometimes on what should be included and what should not.
It depends on what you need for a answer.
If you are looking for the percentage of lot the house covers you measure the exterior of the foundation plus cantilevers.
Real state often does not include the attached the garage. Some times they will state so many sq.ft. with basement, other times they will just add them together and double the sq. footage.
The purpose determines the method. For general usage, any enclosed space under the roof counts. The easy way is to measure outside, then deduct 5% for interior walls etc. which will be close enough. For loan/realtor/mortgage purposes they may want a more exact figure measured inside and may not want attatched garages included. One builder I worked for included porches to make the homes eligable for VA/FHA financing minimums but this is not normally done. For HVAC purposes just use the outside measurements, deducting any unconditioned space.
Most houses are roughly rectangular, and I use the "box method" when measuring them. Take the measurements of the main "box", then deduct the inset 'boxes' (like flush porches) from that or add the projecting 'boxes' (bump-outs and additions) to it. Makes it fast and easy. Deduct 5% for interior walls if needed.