Some people inflate for marketing, many times the county record is wrong. I sold an old house with incorrect county numbers. Looking at Zillow the current value is way off, it appears it is using county square footage to determine value.
It is common in some place to add on without a permit, there is no record of current square footage.
Good reason listings state BTV.

A while back I asked the same question to a friend who had been a local realtor at one time. He told me that they go by the county records here. Sometimes the county has used the overall roof size -- the overhangs completely around a home including covered porches instead of actual living space. We have large porches both front and back and they still are off by another three hundred odd square feet! Our county is getting every tax dollar that they can squeeze out, grrr...

Cmarlin -- What is BTV?

Maybe they are including basement square footage where the county isn't? I've heard in some areas of the country, basement sq ft doesn't count for taxes...but that is definitely habitable space.

In my area, basements aren't common, but when they are there, they get included in the sq ft.

I've also heard, in one southern state, they don't list house sq ftages at all on real estate advertisements/search engines. It is definitely state by state.

BTV - Buyer to Verify is my guess

Remember that most square footage includes the wall thickness dimension. A 50x50 pure rectangle is 2500 square feet, even though you lose approximately 40 square feet to the wall thickness. Start adding more interior walls, and the actual usable square footage starts dropping.

The houses I know most about are in my neighborhood and are mostly less than 15 years old with no basements. I know they haven't had any add-ons. The county is spot-on with ours - at least according to the blueprints. Not a single house in my neighborhood has been listed with the square footage on the county tax records lately. Not one. They have all been advertised at around 300 or more square feet larger, some more than that. At least one was listed that much over the first time it was sold and then that much again the second time. Also it seems to be quite common in at least one adjacent county too. Would a garage be included in the square feet in a real estate listing? A screened porch? If they're including those it makes more sense with the difference, although I don't know why a house would grow each time it's sold. If we list our house with the taxable heated square feet, it looks like we're going to get killed at appraisal time with the comps. I just wondered if the real estate agent measured or just went by whatever the homeowner says or what? Because they sure aren't going by the tax records. I'm so inexperienced with selling/buying. I've sold one house and it was a FSBO and was such an easy transaction. And I bought it as a FSBO. And we built this one, so I really haven't dealt much with real estate agents. We don't intend to go the FSBO route with this one.

I just realized my previous post is not really saying what I mean as far as the appraisal. I assume the appraiser will measure our house for himself, but where will he get the square footage figures for the comps?

That is a good question... Legally, I hope he'd pull the legal comps. And, I'd question what the REagents in your area are doing.

Here, in NC, listing agents MUST verify sq. ftg. by measuring the home themselves or hiring someone to do it for them. We can not use tax records, blue prints, or what the seller tells us. We are allowed to be within +/- 5% of actual.
I used to teach the class on how to measure a home properly. Some homes are very straight forward, and some are real bears to measure.
Appraisers measure the home themselves. They use the MLS data (or tax records) to get values for the comps.

Well here the listing shows the sf and the source. Usually it is tax records. On one house I showed we had an appraisal that showed a higher square footage and we used the appraisal and the listing showed the appraisal as the source.

I have seen a few that were questionable, mostly because they were counting garage apartment on guest house space as part of the SF. We had a guest house on our property but I never included that as part of the SF in the listing.

Thank you ncrealestateguy. This IS in NC, so that's exactly what I wanted to know. But now it's really got me wondering how the measurements are so different. One house in particular is 2400 sf on the tax records. The first time it was sold it was listed at 2700 sf and the second time it was listed at 3200 sf. If it's really 3200, they sure are getting a deal on their taxes. That's quite a difference. Can measurements really be that far off? I wonder what our house will be measured at? We've always thought it was 2200 sf, but who knows? I sure hope it's not less. It's not anywhere close to being a simple square or I would go out and measure it myself.

Maybe your blueprints reflect liveable sq footage? Have you measured your house (physically) to see if it matches your blueprints?

HollyCliff,
If you are near Charlotte, I would be happy to measure your home for you. I don't care if you are a seller or not. I would be glad to help.

I appreciate the offer, but we are three hours from Charlotte. It will be at least another 1 1/2 years before we're ready to put the house on the market. We still have a child in high school and don't want to move until she graduates. We haven't even decided where yet. We just know this property (acreage - barn - pasture, etc.) has about gotten to be too much maintenance for us "old" people. I've been researching everything and noticed the difference in the square footage and wondered how it worked. We've also been looking at houses in another part of the state and the advertised square feet there match up pretty close to the tax records, but not so around here.
And no - we haven't measured our house. Probably should to at least see if it matches up with the specified dimensions, The interior room dimensions look right. But I wouldn't know how to calculate square feet with the way this place is laid out. It's all on one level and sticks out all over the place. I guess I'll just have to be patient and see what it comes out to. But now I know that the agent is supposed to measure for themselves.

Just out of curiosity...would the architect's blueprints be the most accurate square footage #? I seem to recall that our blueprints contained at least two different sq footage numbers...one that was from "brick" and another from "stud"...the brick one was larger and I assume it was the measurement from the outside walls. But even the "stud" one was a little larger than what we thought the actual heated sq ft was. Is actual sq footage the interior size of the rooms? That makes the most sense.
Anyway, after we built, the tax assessor came to check it out...and low and behold he measured from the outside. And when we finally got the assessment, it says the house is about 300 sq ft bigger than we thought. We are in the process of appealing it as it makes a very considerable difference in our taxes. But now I wonder which *is* the accurate #...I guess if they always use the outside #, then we don't have a case.
Sorry for this ramble!

In my area, they use the outside. Always.

Homes are measured from the outside, not from the inside. The space has to be heated and be finished with conventional materials. Any space that is unfinished can not be counted, and the ceiling height must be at least 5 feet tall (bonus rooms with jack walls, for example)

I have foot-thick walls interior and exterior ... there's a serious difference in sqf if you measure rooms and add them up versus measuring exterior.

Typically assessors use the outside because it's easier to get to.

A 300 sq foot difference is probably a garage.

Architectural blueprints can not be used for determining the sq. ftg. of a home because there are many times when during construction that the homeowner modifies the house. These additions and/or deletions of sq, ftg. will not be noted on the original house plans.

If a home is measured from the outside, how do two-story rooms get accounted for in the space?

Example: the house is 30ft x 35ft (1050 sq ft) x 2 stories = 2100. But if 1/4 of the house is a two-story great room, there's no "floor" on that second floor for that 250 or so sq ft, right?

People around here knowingly lie about the square footage on their listings.

Mary,
Two story rooms only get counted once. Stairs get counted twice... one for the top of the stairs and then again for the real estate under the stairs. No space under 5 foot ceilings can be counted (a lot of bonus rooms have jack walls), bay/bow windows need to be accounted for, and so on.