All 3 methods. Depends on who is doing the measuring and what it needs to be used for.

Generally builders are measuring from outside dimensions, because those walls cost money and are included in the total cost/sq.ft. Tax and mortgage appraisers also measure from the outside.

The only people I know that might use individual interior room measurements for sq. footage would flooring people and maybe painters. Plumbers and HVAC contractors might use your x and y measurements for new construction if planning materials and long runs.

If you are trying to compare to real estate listings it’s the overall footprint of the house.

Are you comparing your estimated square footage cost to other house's construction cost or their asking sales price?

@Mark Bischak, Architect : I am comparing it to the sale prices of newly listed home. Their Square footage is listed as “Total Square Feet Living”. My house plan currently has “Heated Square footage”, it appears I will have to obtain external dimensions to make an accurate comparison.

#1.

"Total Square Feet Living Space"

In our area, real estate listings use that wording to include finished below grade space as opposed to square footage, which is only above grade space. Outside dimensions only.

There is an ANSI standard on how to measure conditioned living area of homes, ANSI Z765-2003. It's been adopted by most Realtors and MLS at least in the U.S. Finished living area below grade is not included.

Calculating the conditioned living area and using it to derive a cost/SF isn't all that useful for comparing two homes. Cost/SF is affected by too many variables to be useful. That said, if you want to compare two homes, you might want to compare the total constructed area which is the conditioned living area plus unfinished habitable spaces such as basements and/or walk-in attics, plus the area of garages, porches, decks, patios, etc.

Thanks for your inputs. I find option 2 (internal perimeter dimensions) to be more logical as "finished living space" for MLS listings as it:

1) It truly reflects the finished living space that is both finished and conditioned.

2) It is consistent across all external wall construction styles. If 2x4 studs are used instead of 2x6s then you have a different external wall thickness. Another variation is siding thickness etc.

So using the outside dimensions of external walls introduces these variations whereas inside dimensions of external walls will be consistent across all styles.

@Jeffrey R. Grenz, General Contractor @HALLETT & Co.@worthy

I have only used "net square feet", "gross square feet", and "leasable square feet"; anything other than that ask the person that uses the term.

@Mark Bischak, Architect Is this the mapping of your terms ...

Net Sf = Option 2

Gross Sf = Option 1

Leasable Sf = Option 3

@Charles Ross Homes : Thanks, your explanation makes sense from the home access point of view.

When I used "leasable square feet", I was directed to the centerline of the glass (but actually more complicated than that.

@Mark Bischak ,

I think those are for commercial spaces. Not sure they're commonly used for single family dwellings.

The definitions of "net square feet" and "gross square feet" shown in the graphic can be applied to any type of architecture, the other two definitions came along for the ride.

I have no idea what this would help you with . A new build is designed for your life not just from an idea of how many sq ft it has so this info from someone elses house is IMO completely useless. Finishes, lot district all come into play if you want comparisons which IMO are still pretty useless.

Depending on where you live the total living sq. ft can be calculated differently. When I lived in the northeast RE listings only listed the above ground living area, even if the house had a walk-out basement full of windows. The basement was listed separately.

In the Los Angeles area, where basements are becoming very common for new homes (tear downs), the basements are counted in the total living space that is listed on the MLS. Most of those basements are not walk-outs.

Skip to the bonus round. Building new will cost more than buying used, pretty much anywhere.

@Patricia Colwell Consulting: I think it does help for my comp purposes. I do understand your point about no two houses being the same but it does help in establishing a ball park.

@User: That is true you pay more with new but there is also the benefit of lower ownership costs in terms of repairs and you get to enjoy the new house too :)

Almost all homes are measured as gross sf to the outside perimeter (#1) not including garages, patios, porches.

New, resale, FHA, appraisers, energy calculations, etc.

@Jon John, I am pretty sure I know where you are going with this, but it just doesn't work the way you are intending to use the sf measurements. Select your SOLD comps based on similarity to your proposed new build. The actual end price of those other listed to be built homes is usually no where near the list price. It's much higher than list. It is very common that the builders of the new homes have extras or options not baked into the original list price IME (I'm a Realtor). It isn't uncommon to have $100k+ in extras added to the contract price - lot premiums and extra's that you don't see (electrical etc) as well as extra's you do see (kitchen and bath finishes etc). The hard part is knowing which comps to select because most people have a tendency to select recent (or not recent) sales based on price rather than actual similar properties based on proximity and other features of the subject property.

As to measurements, use the exterior perimeter (#1). Sum of interior dimensions is never correct for the type of comparison you are planning. As mentioned by Charles Ross, check ANSI. Celadon is right: it is more expensive to build new than buy a resale if you are comparing apples to apples even after you have taken into account repairs.

As a side question, I think what everyone is saying is that if there is a cathedral ceiling then that square footage is counted on the 2nd floor even though it is not usable space. Is that correct? So a two-story family room will add more square footage than is actually usable living space. I assumed this was true as we looked at houses for resale.

" . . . IMO are still pretty useless."

Ask five architects what the square footage of a building is and you will get six different answers.

@Tanya Shelly,

Spaces open to the floor below would not be included in the calculation of finished living area if following the ANSI standard. For example, the void space on the 2nd floor for the stairs (the area of the stairs would be counted once in the area of the 1st floor.) Ditto for vaulted two-story spaces.

Thank you! This would explain why some of the real estate listings do not match the tax listing. I think in a hot market some realtors are being ambitious with their listings.

Regardless of how you calculate it, existing home prices and even tract home prices have zero comparison basis to a custom home. Our 3200 SF custom home priced out 30% higher per square foot than the nearby 4000 SF tract home (we priced out both since we had an inkling that our custom might come in higher than we were aiming for at $200/SF - which it did by a significant margin). And that was just our build cost (the tract home cost included the lot). We'd never make up the cost difference. We didn't expect to, but we were a bit blown away by the final numbers. Maybe our area has a bit higher premium on custom, but I think the difference is similar everywhere. Bottom line, don't expect a custom house to cost anything near the price of an existing home. Or if you must, take existing home prices, multiply them by at least 1.2 and maybe you'll get in the ballpark. Either way, the differences in calculating square footage aren't going to matter if your end goal is estimating a custom home.

@Denita, @anj_p : Thanks for the info. Yes i am making sure i compare pricing to custom built homes (not production homes)

@Jeffrey R. Grenz, General Contractor has the right definition for real estate value purposes. If you look on Zillow or Redfin what homes list for, they will also include the square footage (minus garages, storage attics, and patios/porches) as well as the cost/sf. So, when I do my sq. ft calculations, it's based on the interior perimeter of the living spaces.

@Sabrina Alfin Interiors

Thanks and understood. Cost/sf is based on SF computed from external dimensions .

For the typical Zillow/Redfin listings you mentioned, interior dimension are provided for main rooms:- living, master, other rooms, kitchen etc. just for reference?