How to Find Square Footage of a House the Right Way
Most people asking how to find square footage of a house expect an arithmetic problem and discover a records problem instead. Your house already has a square footage. It has three or four of them, actually, sitting in different databases, produced by different people using different rules, and they rarely agree. The county assessor has one. The last appraisal has another. The listing that sold you the place had a third. The builder's plans, if you have them, have a fourth that is bigger than all of them.
None of those is wrong exactly. They answer different questions. This page shows you where each number lives, why they diverge, which standard actually governs when it matters, and how to walk your own exterior and produce a defensible figure you can hold up against the record.
Updated September 2026
Need the raw area math instead? If you already know your dimensions and just want the totals, run our free square footage calculator. It handles rectangles, L shapes, bump-outs, and the room-by-room adds so you can spend your attention here on which spaces belong in the total.
The four numbers that all claim to be your square footage
Before you measure anything, pull the numbers that already exist. It takes about fifteen minutes and it tells you what you are reconciling against.
| Source | Who produced it | Standard used | Usually includes | What it is good for |
|---|---|---|---|---|
| County assessor or tax record | A mass appraisal office, often from an aerial or a one-time field visit years ago | Local assessment rules, which vary by county | Above-grade finished area, sometimes garages or basements depending on county | Property taxes, a starting reference, and nothing else |
| Appraisal report | A licensed appraiser who measured the house | ANSI Z765 in most conventional lending | Finished, heated, above-grade area only | Mortgages, refinances, estate work, disputes |
| MLS listing | The listing agent, who often copied the tax record | Whatever the local MLS requires, which is sometimes nothing | Anything the agent decided to count | Marketing, and a common source of the numbers people repeat |
| Builder plans or architectural drawings | The designer | Gross area under roof, or the architect's own convention | Garage, covered porches, wall thickness, mechanical chases | Construction, not comparison |
Where to find each one. The assessor record is almost always online: search your county name plus "property search" or "parcel lookup," then enter your address. The record usually shows a living area figure, a year built, a bedroom and bath count, and sometimes a sketch of the footprint with dimensions written on each wall. That sketch is the most useful thing on the page, because it tells you what the assessor thought your house looked like.
The appraisal is in your closing packet if you bought with a mortgage, or your lender can send it. Ask for the full report, not the summary, because the sketch and the area breakdown live in the addenda.
The MLS listing is in your closing documents too, or your agent can pull the historical listing.
The plans may be with the builder, the original owner, or the building department, which often keeps the approved set on file for houses permitted in the last few decades.
Why the three disagree
The gaps are not random. Each source drifts in a predictable direction.
The assessor is measuring for taxes, at scale. A county assessment office is responsible for tens of thousands of parcels and cannot send someone to measure each one every cycle. Many records were created from a single field visit at construction, then carried forward through decades of remodels, additions, and basement finishes that nobody reported. Some counties measure from aerial imagery. Some count a finished basement in the living area, some list it separately, and some ignore it. A garage conversion done without a permit will sit in the record as a garage forever.
The appraiser is measuring to a standard, once, for a lender. This is the most rigorous number in the stack, and usually the most conservative. An appraiser physically measures the exterior, applies the ANSI rules below, and reports finished above-grade area separately from below-grade area. That is why appraisals so often come in under the tax record on houses with finished basements: the basement is real, finished, and used daily, and it still does not go in the gross living area line.
The listing agent is marketing. In many markets the agent simply copies the tax record, which propagates whatever error is in it. In others the agent measures, and the measuring is uneven. Some MLS systems now require ANSI compliant measurements on new listings, which has caused a lot of listings to shrink relative to the tax record without a single wall moving.
The builder plan is gross area. Plans usually give area under roof, or a total that includes the garage, covered entry, and sometimes the wall assemblies themselves. A plan labeled 2,400 square feet frequently describes a house an appraiser will call 1,950.
The practical takeaway: if you are trying to settle a real question, the appraisal method is the one to reproduce, because it is the one a lender, a buyer's appraiser, or a court will reproduce.
The ANSI Z765 rules that decide what counts
ANSI Z765 is the residential square footage measurement standard used by most appraisers and, increasingly, required by MLS systems. It defines gross living area, or GLA. You do not need the document to apply it, because the rules that cause almost every dispute are short.
Finished and heated. A space counts only if it has finished walls, finished ceiling, finished floor, and a permanent heat source that ties into the house system or is a fixed installed unit. A three season sunroom with no heat does not count. A converted porch with a space heater on an extension cord does not count. Add a duct or a permanent mini split and finish the surfaces, and it does.
Measured to the exterior wall face. You measure the outside of the house, not the inside of the rooms. That means wall thickness is inside your total, which is one reason exterior measurement always comes out larger than adding up room areas. Measurements are taken to the nearest inch or tenth of a foot, and areas are reported to the nearest whole square foot.
Above grade only. Any level where any portion of the floor is below the surrounding ground is below grade, and it is reported separately no matter how finished it is. A walkout basement with a full glass wall and a fireplace is still below grade. This single rule causes more homeowner outrage than all the others combined, and it is not negotiable in a lending appraisal. Below-grade finished area gets its own line in the report and its own value adjustment, so it is not being ignored, just not being counted as living area. Our cost to finish a basement page covers what that space does and does not do for you.
Ceiling height. Finished area needs at least seven feet of ceiling height, with an allowance down to six feet four inches under beams and ducts. Under a sloped ceiling, the area with at least seven feet of height counts, and you may also count the portion between five feet and seven feet as long as at least half of the room's finished floor area sits at the seven foot height. Below five feet, nothing counts. This is the rule that governs finished attics, dormered second floors, and bonus rooms with knee walls.
Stairs count on each level they serve. The stair opening is counted with the floor from which the stairs descend, and the stair treads themselves are counted with the floor they land on. In practice, a two story house with one staircase counts that stair area twice, once on each level. That is correct and it surprises everyone.
Open two story spaces do not count twice. A two story foyer or a great room open to above is floor area on the first level only. On the second level it is a hole, and holes are subtracted.
Garages never count, attached or not, finished or not, heated or not. Neither do open porches, decks, patios, carports, or unfinished attics and crawl spaces.
Attached finished areas that are not directly accessible from the main house are reported separately. A finished bonus room you can reach only by climbing stairs inside the garage does not join the GLA, even though it is finished and heated.
| Space | Counts in GLA? | Why |
|---|---|---|
| Finished bedroom, hall, closet, bathroom above grade | Yes | Finished, heated, accessible, above grade |
| Stairwell opening | Yes, on the level the stairs leave from | Counted once per level served |
| Finished walkout basement | No, reported separately | Any floor below surrounding grade is below grade |
| Heated and finished sunroom | Yes | Meets the finish and permanent heat tests |
| Three season porch | No | No permanent heat, often no finished ceiling |
| Attached garage | No | Excluded by rule regardless of finish |
| Converted garage, finished and heated, with interior access | Yes | It is now living area, though the record may still say garage |
| Bonus room over garage, entered from inside the house | Yes | Accessible from the finished area |
| Bonus room over garage, entered only from the garage | No, reported separately | Not directly accessible from the living area |
| Open area above a two story great room | No | Not floor area on the upper level |
| Finished attic with knee walls | Partly | Only the area meeting the ceiling height rule |
| Unfinished basement or attic | No | Not finished |
| Covered front porch, deck, patio | No | Not enclosed living area |
How to find square footage of a house yourself, step by step
You will need a 25 foot tape for detail work, a laser distance measurer for the long runs, graph paper or a tablet, and an hour. Do this on a day the ground is dry and the shrubs are not fighting you.
1. Pull the record and print the assessor sketch. You are measuring to confirm or correct something, so start with the something.
2. Sketch the footprint of each level, roughly to scale. One page per level. Draw the outline first, then mark the garage, porches, and any bump-outs. Write the compass direction at the top so you do not get turned around.
3. Walk the exterior of the main level and measure wall to wall. Measure each straight run from outside corner to outside corner and write it on the sketch as you go. Do not measure from a siding corner trim to the far end of a bay window. Measure the wall plane. Round to the nearest inch.
4. Close the perimeter and check it. The opposite sides of a rectangle should match, and the sum of the segments on one side should equal the single long dimension on the other. If they do not close, you have a transcription error or a wall you did not account for. Find it now, not after you have done the arithmetic.
5. Break the shape into rectangles and total them. Every house footprint, however complicated, decomposes into rectangles and the occasional triangle. Feed the pieces into the square footage calculator rather than doing it on the back of the sketch, because the arithmetic is where careful measuring goes to die.
6. Subtract the excluded areas. Garage footprint out. Open porch out. Any unheated enclosed space out. If a bump-out is a cantilever over open air with living space above, it counts on the upper level and not on the lower one.
7. Do the upper levels. See the next section, because you usually cannot put a tape on a second story wall.
8. Note ceiling heights anywhere they are not standard. Sloped ceilings, dropped soffits, and knee walls need the height rule applied before you can total them.
9. Total and compare. You now have a number to hold against the record.
Linear dimensions and area are different units doing different jobs here, and mixing them up produces the wrong answer on everything from baseboard to siding. If that distinction is fuzzy, linear feet vs square feet explained sorts it out in a couple of minutes.
Measuring the second floor when you cannot reach the outside
You cannot hold a tape against a second story wall from the ground, so appraisers use a documented workaround: measure the interior of the upper level and add the wall thickness back.
Walk the upper level room by room and measure interior dimensions along a common line, adding hall and closet widths as you cross them. Then add the thickness of the two exterior walls in each direction. A standard 2x4 exterior wall with drywall inside and siding over sheathing outside runs roughly six inches; a 2x6 wall runs closer to eight. Measure yours at a window jamb or a door opening rather than guessing, since brick veneer or a rainscreen adds several inches on its own.
Two more checks. First, if the upper level sits directly on the main level footprint with no overhangs and no setbacks, its area equals the main level area minus the open stairwell and any two story voids, and you can use that as a sanity check. Second, look for the parts of the upper floor that overhang or set back from the level below, because those are exactly where a from-the-ground estimate goes wrong.
Reconciling your number against the record
Compare your total to the assessor figure and to the appraisal if you have one.
Within about one to two percent is agreement. Two careful measurers will not produce identical numbers, and the standard itself rounds. Do not chase a twenty square foot difference on a two thousand square foot house.
A gap of five percent or more has a cause, and it is usually one of these:
- A finished basement that the county counts and the appraiser does not, or the reverse.
- A garage conversion or an addition that was never permitted and never made it into the record.
- A permitted addition the county never picked up, which happens more than you would expect.
- A sunroom or porch enclosure counted by one source and not another.
- A bonus room over the garage with garage-only access.
- The record was built from the builder's plan, which was gross area under roof.
- The record is simply from a different house, which happens on new subdivisions where plan types get swapped.
If the record is wrong in your favor, meaning the county says you have more house than you do, you can usually file for a correction with the assessor and lower your assessed value. Bring your sketch, your measurements, photographs, and any permits. Most offices have an informal review before the formal appeal window, and the informal route is faster.
If the record is wrong against you, meaning the county has less house than you actually have because you finished something, understand what correcting it does. It will likely raise your assessment. It may also be necessary for a clean sale or an insurance claim, and unpermitted finished space can hold up a closing. Get the permits closed out first, then update the record.
What each number is actually used for
| Question you are answering | Which number to use |
|---|---|
| What are my property taxes based on | Assessor record |
| What will an appraiser say the house is | ANSI gross living area, above grade only |
| What can the listing legally claim | Your MLS rules, which increasingly means ANSI |
| What should my insurance rebuild coverage be | Total area under roof, including garage, which is a different number entirely |
| How much flooring, paint, or drywall do I need | None of them. Measure the actual rooms |
| Is this house priced fairly against comparable sales | Whatever standard the comparables used, applied consistently |
That fifth row is the one people get wrong most often. Estimating materials from a house's published square footage produces bad orders every time, because published area includes wall thickness, excludes the garage you are about to paint, and says nothing about the rooms you are actually working in. Measure the rooms. For the waste allowance on top of the room area, flooring waste factor explains how much extra to order and why the number changes with the layout.
If you are running the numbers because you are deciding what to spend on the house before selling, home improvement ROI covers which projects actually move an appraised value and which just move your enthusiasm.
The traps that catch people
"Total under air" versus "total under roof." Listings in hot climates often quote both. Under air is conditioned space. Under roof includes the garage and covered lanai. The difference on a Florida ranch can be several hundred square feet.
The heated garage that is still a garage. Finishing a garage and running a duct to it does not make it living area unless the overhead door is gone, the floor is raised and finished, the walls and ceiling are finished, and it is accessible from the house. Half-converted garages count for nothing and confuse everyone.
Knee wall storage. The space behind a knee wall in a finished attic is not living area at any height, because it is not finished and not accessible as living space.
Chases and mechanical closets inside the exterior envelope are inside the measured perimeter and count. You are measuring to the outside wall, so anything within it comes along.
Additions with a step down. If an addition sits several inches lower and its floor is below the surrounding grade at any point, it is below grade for that portion. Check the grade at each wall, not at the front door.
Two measurements of the same house on the same day will differ by a few square feet. That is normal. What is not normal is a sketch that does not close.
Frequently asked questions
Does a finished basement count in a house's square footage?
Not in gross living area under the ANSI standard, no matter how well finished it is, because any floor below the surrounding grade is below grade. It gets reported separately with its own finished area figure. Some county assessors do fold it into their living area number, which is exactly why the tax record and an appraisal so often disagree on houses with finished lower levels.
Does the garage count in square footage?
No. Garages are excluded whether they are attached, detached, finished, heated, or all four. A garage that has been fully converted to living space with the door removed, the floor and surfaces finished, permanent heat, and interior access does count, but the property record will usually still say garage until you get it corrected.
Why is the appraiser's square footage lower than my tax record?
The three usual reasons are a finished basement the county counts and the appraiser reports separately, a below-grade or low-ceiling area that fails the standard, and a tax record that was built from builder plans measuring gross area under roof. Occasionally the record is just stale. The appraiser measured your house this month; the record may date from the year it was framed.
Do you measure the inside or the outside of the house?
The outside, at the exterior wall face. That is what the standard specifies, and it is why exterior measurement always exceeds the sum of your room areas by the thickness of every wall in the house. For upper levels you cannot reach, measure interior dimensions and add the exterior wall thickness back in.
Do stairs count in square footage twice?
Effectively yes, and it is correct. The stair opening counts with the floor the stairs descend from, and the treads count with the floor they land on. A single staircase in a two story house therefore contributes area on both levels. Open two story voids, on the other hand, count only once, on the lower level.
Does a sunroom count toward square footage?
Only if it has finished walls, ceiling, and floor, plus a permanent heat source. A three season room with no heat, or one heated by a plug-in unit, does not count. Adding a duct run or a fixed mini split and finishing the surfaces changes the answer, and the permit for that work is what gets the county record updated.
How accurate does a listing's square footage have to be?
It depends on your state and your MLS, and the trend is toward requiring a measurement to the ANSI standard with the measurement source disclosed. Copying the tax record and calling it a measurement is the practice that generates most square footage lawsuits. If you are listing, measure it or hire someone to, and say in the listing where the number came from.
Can I get my county's square footage record corrected?
Usually yes. Contact the assessor's office, ask for the informal review process, and bring your own measurements, a sketch, photographs, and copies of any permits. If the informal route does not resolve it, there is a formal appeal with a filing window tied to the assessment calendar, so ask about that date at the same time.
Related: Square footage calculator does the area arithmetic for rectangles, L shapes, and room-by-room totals. Linear feet vs square feet explained sorts out which unit each material is sold in. How to measure square footage on Google Maps covers lots, roofs, and driveways you cannot reach with a tape. Cost to finish a basement explains why that space is valuable even though it never joins your gross living area.
