Normally it's just the length times the width (if it's a true rectangle or square)....but if there are any bump-outs, that definitely complicates figuring it.
Something I would ask him is if that includes any porches, decks, garages, and/or- is that heated sq footage or sq footage under roof.
If you're just talking about a 63x45 rectangle, then the sq footage would be 2835.

Unfortunately there is no std and often it isn't specified and based on the incentive of the measurer. So, for example, real estate agents want the largest measurement, tax payers the smallest.

Variation comes from whether you include exterior wall dimensions, interior dimensions only, conditioned space only or total under roof. There has been an attempt to try to unify how sq ft is defined, but not everyone follows it.

Conditioned space s/f is always measured from the outside walls(including the garage if it's part of the house even thou it might not be conditioned), never inside walls. ditto about the conditioned space vs under roof.

My building inspector calculated the vaulted area in the second story over the living room as square footage to base his fee on.

Ugh

Area measurements should use gross (outside of framing/foundation) dimensions unless stated otherwise. If you want interior living space areas you need to make that clear.

jenny......what kind of 'building inspector' are you talking about?

"My builder is showing me a 63' x 45' plan and telling me it's 3100 sq. ft."

The garage may not have been included.

For permit purposes the livable area is often calculated, for permit fees, different than a garage or parking area.

If the plan is scaled or has a specific area that is dimensioned, you can figure out the scale. Or as a for instance, the garage is stated as a specific sq. ftg.

At the end of the day, if it won't fit on the lot, something has to change.

So, know your permit able lot coverage.

Robin,
My initial meeting with our county building inspector was to pay our fees and walk through all aspects of building the house. My calculations of square footage was different than his. Our house is a 1.5 story. So I took the width x length for the first floor then did the same for the second; however I had removed the vaulted section from the calculations for the second story. I wasn't considering it living space. I calculated wrong ⺠all done now, no biggie, but I was confused at the time.
Jen

For us, the square footage was based on "area under roof". So, our bonus room upstairs wasn't counted but porches and attached carport were. To figure this out, they took the length times width of every space on the first floor and added them for a total.

" I wasn't considering it living space."

And who said bureaucrats aren't entrepreneurial?

I guess in simple terms, if you see a room as 10x10 on a floorplan, how much sq. ft is that?

10'x10' = 100...but that's drywall to drywall
2x4 + drywall + 10' = 10' 4 1/2" x 10' 4 1/2" = 107 sq. ft.

If you add up all the rooms in a house and add 10% is that about right?

"If you add up all the rooms in a house and add 10% is that about right?"

"About" being the operative word, because, least we forget to adjust for common walls and add for exterior and rated walls.

Plumbing walls will also be 2x6.

jenny......that inspector took advantage of you.

http://www.bing.com/search?q=calculating+square+footage+of+a+house&qs=AS&sk=AS1&pq=calculating+square+&sc=8-19&sp=2&cvid=bcbc1f265223411da68c2b298f1ab69c&FORM=QBLH&ghc=1

Any Software floorplans - automatically calculates sq ft.

Normally, like with chief architect it takes 5 minutes to reproduce entire plan (no roof pita)

If you builder has website....

Robin,
I agree. His response was that it was heated and cooled space that could have been another room!! But seriously, what could I do. It was my first meeting with the guy. He approves everything past that point. Arguing would have made him mad.. And thus more difficult to work with for the remainder of the project. It was a swing of about 200 dollars. I had calculated what I thought our fees would be prior to the visit, so I made a choice on the spot not to argue. We saved so much in other places. It's just one if those things I still shake my head and wonder about.
Jen