Does a basement count as square footage in Illinois?
No, a basement does not count as square footage in Illinois.
Not as gross living area, in Illinois or any other state. ANSI Z765, the measuring standard Fannie Mae requires appraisers to use, puts a basement in below-grade area because part of it is under the ground. A finished basement is reported on its own line; an unfinished one is not living area at all. Illinois has no rule of its own that changes this.
Why a basement is not gross living area in Illinois
The line is drawn by grade, not by finish, windows, or how the space is used. Under ANSI Z765 any level with part of its floor below ground on any side is below grade in its entirety. That covers every basement, including walkouts and daylight basements, and it covers them the same way in Illinois as in every other state.
An appraiser still measures the basement. Finished area goes on the below-grade line, unfinished area is noted as unfinished, and each is valued separately. What never happens is the basement being added into the gross living area of the house.
Basements in Illinois homes: the numbers
In Illinois, 78% of single-family homes sit over a basement, according to the 2024 Residential Energy Consumption Survey, against 43% nationwide. That share is about the same as in the 2020 survey.
Of those basements, 61% are finished. Every square foot of that finished area is measured by an appraiser and reported as below-grade finished area, never as gross living area.
Here is the number that explains the arguments: 33% of Illinois owners with a basement include it when they report their home's square footage. Nationally it is 40%. Under ANSI Z765, none of them should, which is why an owner's number and an appraiser's number so often disagree.
The average Illinois single-family home has 2,559 square feet of total area, basement included, compared with 2,248 nationally.
| Single-family homes | Illinois (2024) | United States (2024) |
|---|---|---|
| Have a basement | 78% | 43% |
| Basements that are finished | 61% | 59% |
| Owners who count the basement in their square footage | 33% | 40% |
| Average total area, basement included | 2,559 sq ft | 2,248 sq ft |
Source: U.S. Energy Information Administration, Residential Energy Consumption Survey. Values are for single-family homes; a small number of states have samples too small to report every line.
Calculator: what a basement adds to a Illinois house
Enter the basement area and the above-grade area. The first figure is the gross living area an appraiser reports; the last is the inflated number a listing shows when it adds the basement in.
The basement does not count. The appraisal GLA stays at 1,800 square feet. A listing that adds it in would show 2,600, which overstates the house by 44%.
How Midwest basements changed since 1971
Illinois is in the Census Bureau's Midwest region. In 1971, 72% of new single-family houses completed in the Midwest were built with a full or partial basement. In 2025 it was 50%. Nationally the share went from 36% to 16% over the same years, as slab and crawl-space construction spread.
What to put on a Illinois listing
Report the above-grade area as the square footage and the basement on the below-grade or "finished basement" line your MLS provides. Most Illinois listing systems have separate fields for above-grade, below-grade, and total finished area. Filling in the total field is fine; presenting it as the size of the house is what causes appraisal gaps and, in some states, complaints.
If a buyer's appraisal comes in with a smaller number than the listing, this is almost always the reason. The house did not shrink. The basement moved to the line where ANSI Z765 puts it.
Frequently asked questions
Does a basement add to square footage in Illinois?
It adds value and it adds below-grade finished area, but it does not add to gross living area. Illinois appraisers follow ANSI Z765 because Fannie Mae requires it, and that standard keeps every basement on a separate line.
Do Illinois appraisers count a basement?
They measure it, report it as below-grade area, and value it with its own adjustment. They do not count it in GLA. There is no Illinois exception to the standard.
Does Zillow square footage include the basement in Illinois?
Often, yes. Zillow and county records frequently show a total that includes finished basement area, and 33% of Illinois owners with a basement count it in their own figure. An appraisal will show a smaller number because it reports above-grade GLA only.