Territory Intelligence · drainage & waterproofing
The basement map, township by township.
The Cook County assessor records a foundation type for 1,114,049 residential parcels. 915,331 of them sit over a full or partial basement. That single figure is the ceiling on every sump, drain tile, egress and foundation job in the county. Where those basements sit is the more useful question, and the answer is lumpy: one township is 98.2% below grade, another is 62.6%.
915,331Homes with a full or partial basement (82.2%)County assessor
198,718Homes on slab or crawl space (17.8%)County assessor
35.6 ptsSpread between the top and bottom townshipAssessor · computed
785,338Below-grade homes that are owner-occupiedAssessor ownership
01 · The county floor
Four values, one market.
The assessor's basement field takes exactly four values, and every residential parcel gets one of them. Full: 671,371 homes, 60.3%. Partial: 243,960, or 21.9%. Slab: 138,947, 12.5%. Crawl space: 59,771, 5.4%. Only 286 parcels come through blank, which is why the denominator here is 1,114,049 rather than the full 1,114,335 the county file carries.
Add the first two and you get the addressable stock: 915,331 homes, 82.2% of the county. The other 198,718 are not a soft market or a longer sales cycle. A house on a slab has nowhere to put a sump pit and no wall to seal from the inside. Mail it interior waterproofing and the money is gone before the envelope is opened.
The read for contractors
Foundation type is the cheapest suppression filter in this trade and almost nobody applies it. One in six Cook County doors is structurally incapable of buying the core interior job, and the county tells you which ones for free.
Foundation type, county-wideAssessor basement_type · n=1,114,049
02 · The township league
Rogers Park is 98.2% below grade. Schaumburg is 62.6%.
That gap is 35.6 percentage points, and it decides the hit rate on every door you knock. In Rogers Park, 7,897 of 8,041 homes on the file sit over a basement. In Schaumburg, 11,466 homes are on slab or crawl, and only 19,158 of 30,624 go below grade. Same county, same postage, wildly different odds.
Rate is only half the picture. The townships with the most below-grade homes in absolute terms are Lake (112,538), Jefferson (98,672) and Hyde Park (60,094), and Lake gets there on a middling 78.9% rate carried by sheer size. Rate tells you what a mail drop returns per thousand pieces. Count tells you whether a territory can keep a crew busy for a season. You want both, and the file has both.
The read for contractors
Pick territories on rate when you are buying attention, on count when you are hiring. Jefferson at 88.2% and 98,672 homes clears both bars; a 62.6% township with a big population clears neither cheaply.
Highest basement share, top 10Assessor · full or partial
03 · Full versus partial
Cook County has a partial-basement belt.
County-wide, full basements outnumber partials nearly three to one. In seven townships the ratio flips. Orland is 51.1% partial against 39.0% full, Wheeling 44.9% against 42.6%, Palos 42.8% against 35.6%, Bremen 42.3% against 25.1%, then Rich, Elk Grove and Schaumburg by narrower margins. Together those seven hold 138,387 below-grade homes, 75,634 of them partial.
Look at when they were built and the belt explains itself. Median build year: Elk Grove 1963, Wheeling 1965, Bremen 1966, Palos 1972, Rich 1972, Schaumburg 1973, Orland 1987. These are the post-war and late-century suburban tracts. Compare the full-basement end of the county: Oak Park's median home went up in 1916 and 88.9% of its stock is full basement, 9,990 homes. Bremen's median is 1966 and its full-basement count is 8,141 out of 32,499.
The distinction is operational, not cosmetic. A partial basement means a shorter perimeter of below-grade wall, a shallower footing on part of the structure, and frequently a step-down transition where two foundation depths meet. Scope, crew hours and material both differ. Pricing a partial belt off full-basement job averages will cost you either the bid or the margin.
The read for contractors
Segment the estimate template by foundation type before you segment by anything else. Orland and Bremen are big, active markets where the average job is a different animal from the one your Oak Park pricing assumes.
Partial-dominant townshipsAssessor · partial share vs full share
| Township | Partial | Full | Median built |
|---|---|---|---|
| Orland | 51.1% | 39.0% | 1987 |
| Wheeling | 44.9% | 42.6% | 1965 |
| Palos | 42.8% | 35.6% | 1972 |
| Bremen | 42.3% | 25.1% | 1966 |
| Rich | 36.3% | 35.0% | 1972 |
| Elk Grove | 35.8% | 33.8% | 1963 |
| Schaumburg | 33.4% | 29.1% | 1973 |
04 · The age curve
Basements did not fade with age. They changed shape.
The obvious guess is that older housing means more basements and newer housing means slabs. Cross the two fields and that guess fails. Basement presence by build era runs 86.5% pre-1940, then down to 76.0% for 1940 through 1959, back to 82.1% for the 1960s and 70s, 83.5% for 1980 through 1999, and 87.0% for homes built since 2000. The curve dips in the middle and recovers. The high point is the newest stock.
What actually moved is the kind of hole. Full-basement share collapses across the same eras: 78.3% of pre-1940 homes, 59.5% of the 1940s and 50s cohort, and just 36.4% of homes built between 1960 and 1979, where partial basements peak at 45.7%. Then it climbs back to 48.1% and 71.7%. The post-war rush also produced the county's crawl-space peak: 10.3% of 1940 to 1959 homes, nearly double the county rate of 5.4%.
Median build year by foundation type says the same thing in one line. Full basement: 1949. Partial: 1965. Slab and crawl both land on 1955.
The read for contractors
Do not use build year as a proxy for basement stock. A subdivision built after 2000 is likelier to have a basement than a post-war ranch, and the 1960s and 70s tracts that look like prime territory on age alone are majority partial when you check.
Full-basement share by build eraAssessor · n=1,114,033
Any basement, same erasFull or partial
05 · Where water meets structure
The flood layers do not pick out basements.
Joining the assessor file to the county's flood layers gives 1,110,897 parcels with both a foundation type and a flood record. Across all of them the basement rate is 82.2%. Now filter to the exposed cohorts and watch what does not happen. Inside a FEMA Special Flood Hazard Area: 11,096 homes, of which 8,065 go below grade, a rate of 72.7%. That is nearly ten points under the county. Homes carrying a First Street flood factor of 5 or higher: 108,292 parcels, 86,143 below grade, 79.5%. Also under.
The cohort that matters most by volume is the one outside the mapped floodplain: 103,148 homes with a flood factor of 5 or higher and no FEMA designation, 82,499 of them with a basement at a rate of 80.0%. Its size is the story, not its basement rate. Only at the extreme does the relationship invert: among the 7,359 parcels at flood factor 8 or higher outside a Special Flood Hazard Area, 85.2% have a basement, above the county baseline.
Read honestly, foundation type and modeled flood exposure are close to independent across Cook County. That is a useful thing to know, because it means you cannot infer one from the other and you have to carry both layers. A high-exposure block sitting inside a township that runs 69.7% below grade is a worse territory than its flood score suggests, and a moderate-exposure township at 96.8% may be the better week of work.
The read for contractors
Score territories on the intersection, never on either layer alone. Basement count times exposure is the number that ranks a block; a flood map by itself will send you to slab country.
Basement rate by flood cohortAssessor × FEMA / First Street · n=1,110,897
06 · Who owns the hole
785,338 basements have the decision-maker living upstairs.
Below-grade work is discretionary capital spending on a space the owner uses, or wants to. Whether the owner sleeps in the house changes everything about that pitch. In Cook County, 85.8% of below-grade homes are owner-occupied: 785,338 of them, against 129,967 held by absentee owners.
The relationship runs the other way too. Owner-occupied homes carry a basement at 83.0%, absentee-owned homes at 77.2%, a gap of nearly six points. And the combination a finished-basement or full-scope job actually needs, a full basement with the owner in residence, describes 568,783 Cook County homes.
For the absentee slice the pitch is different and it is not softer. A landlord buys drainage as loss prevention on an asset, not as a family room, and the economics of that conversation reward a different letter than the one you send a homeowner.
The read for contractors
Split the file before you write copy. 568,783 owner-occupied full basements is your finished-space and upgrade market; the 129,967 absentee below-grade homes are a protect-the-asset market that most contractors never write a second version of the letter for.
Ownership and foundationAssessor owner_occupied × basement_type
Basement rate by occupancyAssessor
The honest limit
What this field does not tell you.
The assessor records that a basement exists and what type it is. That is the whole claim, and it is worth being blunt about the gap between that and a sold job.
The file does not say whether a basement is finished. It does not say whether it has ever taken water, whether there is a sump, whether the drain tile is original, or whether the foundation has moved. No public county field carries any of that, and anyone selling you a list that claims otherwise is modeling it and not telling you.
So treat these 915,331 as the outer boundary of the market and nothing more. Every number on this page is a structural precondition: it says a job is physically possible at that address, not that it is needed or wanted. The layers that narrow the boundary into a route are the ones you already have or can buy, and they are age, exposure, occupancy and permit history, applied in that order. The value of the foundation field is that it removes 198,718 homes from consideration before you spend a cent on any of the others.
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Method & sources
Foundation figures come from the Cook County Assessor's residential property-characteristics file, field basement_type, which takes four values: Full, Partial, Slab and Crawl. Of 1,114,335 residential parcels in the file, 1,114,049 carry a populated value and 286 are blank; every percentage on this page uses 1,114,049 as its denominator unless stated otherwise, and blank values are excluded rather than assumed. "Below grade" throughout means Full or Partial. Build-era percentages use the 1,114,033 parcels with both a populated foundation type and a year built. Ownership splits use the assessor's owner-occupancy flag.
Flood figures come from FEMA Special Flood Hazard Area designations and First Street flood factors as published in the county's open-data universe file, joined to the assessor file on parcel identifier. That join returns 1,110,897 parcels carrying both a populated foundation type and a flood record, which is the denominator for every figure in section 05; roughly 3,100 parcels have a foundation type but no flood record and are excluded there. Township figures cover the county's 38 named townships. Everything on this page is aggregate. No individual home is identified, and nothing here is a claim about any specific property's condition, water history or need for work.
The same layers, scored home by home and neighborhood by neighborhood, are what a Territory X-Ray runs on. This brief is recomputed as the underlying county files update.
Cook County Assessor · foundation type, build year, ownership
FEMA · Special Flood Hazard Areas
First Street · flood factor
Want your townships ranked on basement stock and exposure together?
This is the county-level read. The Territory X-Ray runs these layers on the towns you actually work: below-grade counts and foundation mix by neighborhood, flood exposure crossed against them, ownership, and the first campaign worth your money. Start with the Intelligence hub if you want the rest of the briefs first.
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