How Does Engineering Permitting Work in Unincorporated DuPage County, IL?

DuPage County's stormwater ordinance requires new development and redevelopment to manage both quantity and quality. Detention facilities must control release rates so downstream waterways see no increase in peak flows, and post-construction best management practices — bioswales, permeable pavement, native plantings, infiltration features where soils allow — address water quality. The ordinance also regulates work in and near floodplains and wetlands, with buffer requirements and compensatory storage calculations where flood storage is displaced. The engineering behind the submittal is substantial: hydrologic modeling of pre- and post-development runoff, hydraulic sizing of storm sewers and outlet control structures, grading plans that direct water to the BMPs, and floodplain analyses where the maps show risk. Reviewers check the math, not just the drawings, so the stormwater report must be sealed by a licensed professional engineer and internally consistent — a detention volume that does not match the grading plan is the classic first-round correction. Projects should budget schedule for at least one full round of stormwater review comments, because the county's program does not rubber-stamp.

The county's building review covers zoning compliance, architectural plans, structural design, and MEP systems against the adopted codes. Structural engineers design foundations below the region's deep frost line, with spread footings or deep foundations sized from the geotechnical report's bearing values for the site's glacial soils. Basements need drainage and waterproofing details suited to the county's clay tills and seasonal high water tables, and commercial slabs need subgrade preparation that accounts for the same soils. Utility coordination follows the parcel's reality. Unincorporated lots on the county's edges may rely on private wells and septic systems, which the county health department permits separately — and the septic field's location, sized from soil percolation testing, can dictate the site layout. Fire protection review runs through the fire protection district, which may require hydrant coverage, fire lanes, or suppression systems that feed back into the civil and architectural design. The AHJ for each piece — county for building, stormwater program for watershed, health department for wells and septic — decides what is approved, so the design team tracks each review thread to its own closeout.