Permit guide · Deck
Almost certainly yes. Attached decks require a permit in virtually every jurisdiction in Illinois, Wisconsin, and Indiana. Even freestanding decks need permits if they're over 30 inches above grade or exceed 200 square feet. Decks involve structural engineering, frost-depth footings, and specific fastener requirements, and the inspections are strict.
Enter your address to find your building department, then answer a few questions to see if you likely need a permit.
What's an AHJ?
The specific city, village, or county office that issues permits. Their boundaries don't always match your mailing address.
GPS-verified
We cross-check your coordinates against municipal boundary polygons, not just ZIP codes.
Wrong AHJ = weeks lost
Filing with the wrong building department means your application sits unreviewed.
The short answer
If your deck is attached to your house, you need a permit. If it's freestanding but more than 30" off the ground or over 200 sq ft, you almost certainly need a permit. The building department will check footing depth, ledger board attachment, joist sizing, railing height, and stair dimensions. Deck collapses are a leading cause of residential structural failures, which is why inspections exist.
What we check
Every deck post must sit on a footing that extends below the frost line. In Illinois, that's 42 inches deep. In Wisconsin, 48 inches. In Indiana, 36-42 inches depending on the county. Footings are typically 16-24" diameter concrete piers poured on undisturbed soil. This is inspected before you can pour concrete.
The ledger board connects your deck to your house, and improper attachment is the #1 cause of deck collapses. Code requires lag bolts or through-bolts (not screws or nails) at specific spacing patterns. Flashing between the ledger and house siding is mandatory. You cannot attach a ledger to brick veneer alone, cantilevered floors, or engineered I-joists without manufacturer approval.
Joist size, beam size, and post size all depend on span distances, spacing, lumber species, and grade. The IRC provides span tables (R507), but your AHJ may have adopted a different edition with different values. Post-to-beam connections must use approved hardware, not toe-nailing.
Required when the deck surface is more than 30 inches above grade. Minimum 36 inches tall (some jurisdictions require 42"). Balusters spaced no more than 4 inches apart (the "sphere test"). The top rail must resist 200 lbs of concentrated force.
Maximum 7-3/4" rise per step, minimum 10" tread depth, minimum 36" width. Handrails required at 34-38" height with a graspable profile. Landings required at top and bottom.
Decks usually must meet the same setbacks as your house. Some AHJs allow reduced setbacks for uncovered decks. Deck area may count toward lot coverage/impervious surface limits.
Process
Most AHJs require three inspections for decks: a footing inspection (holes dug to depth, before concrete), a framing inspection (structure complete, before decking installed), and a final inspection (everything complete including railings and stairs).
Per state
IL
Follows IRC with local amendments. Chicago has its own building code (not IRC-based). Most suburban AHJs use 2018 or 2021 IRC. 42" frost depth statewide.
WI
UDC (SPS 320-325) applies statewide for 1-2 family dwellings. 48" frost depth statewide, the deepest in the Permitech service area. State-controlled inspector licensing.
IN
No statewide residential building code. Each AHJ adopts its own (most use IRC 2012 or 2018). Frost depth varies 36-42" by county. Check your specific jurisdiction.
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