Because the association parcel is billed on the non-residential or multi-family formula, not the flat residential tier — and a formula scales with surface area while a tier does not.

Work the Wilmette arithmetic on a plausible association. A surface parking lot of 60,000 square feet is about 16.7 ERUs. At $155 per ERU plus the $65 fixed fee, that is roughly $2,655 a year for the parking lot alone — before private drives, walks and any common-area roofs.

Compare that to the $215 a Tier 1 single-family household pays. A large association can find its common-area stormwater charge dwarfing what any individual member pays on their own home, which is exactly the surprise boards report.

The lever, and it has to be applied for

Credit programmes are the only way to reduce it. Wilmette references a Stormwater Incentive Program and Stormwater Credit Programs; the details are not published on its page. Credits typically recognise on-site detention, green infrastructure, permeable surfaces, and downspout disconnection.

Two things follow. First, an association that has already built detention — and many suburban Illinois associations have, because it was required at subdivision — may be paying full freight for infrastructure it already provides. Second, credits are never automatic. Somebody has to apply, with drawings.

That is a concrete, checkable item for a board: ask the municipality whether a stormwater credit exists and whether this parcel has one. The answer is frequently no on both counts and the fix is a form.

The Cook County permitting layer

For associations undertaking common-area redevelopment — parking lot reconstruction, drainage regrading, a clubhouse addition — the Metropolitan Water Reclamation District's Watershed Management Ordinance, effective May 1, 2014, governs permitting countywide.

The MWRD added a regional stormwater detention and volume control trading program: a project may satisfy WMO stormwater requirements through an agreement with an offsite landowner, provided both sites are in the same watershed. The pilot watersheds are the Lower Des Plaines River (as far north as Wheeling, south to Willow Springs) and the Little Calumet River in south Cook County.

Through January 1, 2025, projects in those planning areas did not need to demonstrate site limitations to participate. Whether the pilot was extended past that date is unverified.

The option matters for a specific and common problem: an association whose site physically cannot accommodate required detention. Satisfying it offsite within the same watershed can be materially cheaper than underground storage.

What this is not

No Illinois EPA rule, permit condition or enforcement action specific to association common areas surfaced in this period. Statewide municipal stormwater obligations flow through the MS4 NPDES general permit to municipalities, which then push cost onto property owners via the fees above. The chain is indirect, and it ends at the association's parking lot.

What to watch next

Whether more Illinois municipalities adopt stormwater utilities. The model spreads because it moves cost off the tax roll onto parcels with large impervious surfaces — and community associations are, by construction, exactly that.