I would be concerned about a plan that is using interior measurements. Builders generally use exterior measurements. In general this will show up as an extra 6-10 inches on exterior walls.

For square footage, you typically calculate the exterior measurements of finished space with some provisions for open areas such as two-storey rooms, depending on the jurisdiction.

Well I assume they are interior measurements, he didn't say either way. They have (for the most part) the room dimensions I asked for. Although he did say about cutting out .5" (I think) for drywall.

Honestly, I just want to know if the square footage he has is correct, because if not, I've got a problem with that.

You should not assume a builder's floor area is to the face of finishes even if the room dimensions are shown that way. Construction floor area calculations are gross and have little to do with rental or sales areas.

Gross floor area is measured to the outside face of framing or the face of the foundation. Room dimensions are sometimes approximate or rounded off but if you have them you should add 4" or 6" at 2x4 or 2x6 exterior walls.

The generally accepted standard for gross floor area calculation is found in "Gross Areas of a Building: Standard Methods of Measurement (ANSI/BOMA Z65.3 – 2009)"

JDS , That answers one of my questions. So is there a way *I* could figure those calculations (gross area) based on room dimensions?

JDS- Ok, thanks for your help on this. So adding 4" only on the exterior walls would hopefully get me closer to what he has.. I can try that.. Its a 486 sq. ft. difference we have.

That should work if the exterior walls have 2x4 studs and the interior finish is 1/2" gypsum wall board. What are the other walls? Is this space in an existing house?

If you are still unsure, post the plan.

Did you include the garage?

I'm not sure if this will be readable.. It doesn't even have windows on it yet. Very first draft of it all.. Things are definitely changing. His measurements were so small I had to go in and make them larger

JDS, 2x4's are normal here, but we haven't discussed materials. I'm going to have to cut sq ft to make it smaller, that's why I would like to understand how he came to his numbers.... May just have to ask him to show me..

It also looks like the interior measurements are not accounting for the walls (which is right, of course) but those 4" walls also take up space / add sq footage. So while you could add up all the rooms along the backside of the house, for example, you'd have to add about 10 4" walls to that dimension. That adds up quick when multiplying.

Yes, while interior dimensions are critical for furniture, fixture and equipment layout, it's exterior dimensions which are critical for construction costs (how many lineal feet of materials for foundations, wall and roof sheathing/finishes, etc.) and other calculations such as for regulatory and property tax agencies. Obviously, interior totals and exterior totals are two different things. Depending on exterior wall design, exterior walls may be approx 4" (2X4 studs); approx 6" (2X6 studs), approx. 8" (2 X 6 studs with veneer, or concrete block), or some other dimension, based on type of construction and finishes being used.

It's a simple matter for whomever does the drawings to provide drawings with the proper exterior dimensions. :-)

Thanks so much for yall's help! Really did clear things up for me. Meeting is now scheduled for Thursday..

If the exterior walls have a masonry cladding that is supported on the foundation wall, your contractor may be measuring to the face of the foundation. That would make the exterior wall 8 to 9 inches thick.

Since the drawing was done on a computer, it is unlikely the contractor has made an unintentional mistake in the area calculation.

In the final analysis it is not possible to accurately determine the cost of a building based on its gross floor area. Such an approach is son unreliable there is no way to tell who is getting the better deal.

The reason no one can calculate the area for you is that only JPG files can be posted here and they will not print to scale and there are no exterior dimensions on the drawings.

If you have the drawings and a scale, you can note the exterior dimensions and then make rectangles out of the plan, calculate the rectangles and then add them up.

This house is roughly 3,920 s.f. (living area)

Scaling it the best I could, the exterior walls are 9".

Naf-Naf, you're about 100sqft over his measurements, so yes pretty close. Thank you!

EDIT that. You're only 47 sq ft more than him

Ask seven architects to calculate the square footage of a building and you will get eight different answers.

And since no architect majored in mathematics, you takes yur chances...! :-)

"... I would like to understand how he came to his numbers.... May just have to ask him to show me."

Its fun to be flippant about such issues but there should be no mystery about how the builder calculates the gross floor area of the house.

Assuming this is a conventional CAD drawing, he or his drafter uses the polygon tool to draw a polygon around the perimeter of the house. Then the program is asked for the area within the polygon and it reports a number accurate to as many decimal places as desired.

The only reason to ask him how he got that number is if you think he didn't get it from the computer or he altered it after getting it from the computer. You should be tactful about how you ask that question.

The only way to accurately check his work is to watch him do it or send one of us a DXF or DWG file of the foundation plan if the builder will give it to you. From the information 7 out of 7 architects would get the same answer give or take a small discrepancy depending on the translation software and the level of accuracy for which the program is set up. I have done this many times and have yet to discover a meaningful discrepancy.

Then there's this way:

https://www.massport.com/media/1106/c_instruction_ex10.pdf

It may be beneficial for you to start a discussion regarding the layout of your house.

IMO review of a house plan with three 25 ft long corridors would be a waste of time. This plan type is one of the most common offered on the internet. Try looking at some of them to see how long corridors were avoided and the gross area reduced. At some point a second floor should be considered if only for the cost savings.

Agree with JDS. Let circulation define the floor plan, not the other way around. The circulation here looks like the white squares in a crossword puzzle.

We measure the area of a plan to the exterior of the wood frame structure. Some Realtors in our area will measure it to the outside of the masonry which, in the case of a brick veneer home, will make the home appear large--and more of a value-for-money proposition to those (misguided) individuals assessing value on a $/SF basis.

On our concept plans, we indicate the inside dimensions of rooms so that our clients can get a better sense of the finished room dimensions and to aid in furniture placement.

For folks who are in the $/SF camp, we suggest a way to scale a project is to include all areas--conditioned living areas, unconditioned spaces, patio, deck, garage, porches, etc. since these areas all contribute to the overall construction cost.