Do You Need a Permit to Build a Deck in McHenry County, IL?

When you need a deck permit in McHenry County

Here's the short version that covers almost everyone. In McHenry County and its towns, you need a building permit if your deck is attached to the house, sits more than 30 inches above grade, or is larger than about 200 square feet. Freestanding, ground-level decks under those thresholds are sometimes exempt — but "sometimes" is doing a lot of work in that sentence, because every town draws the line a little differently and the exemption disappears the moment your deck touches the house or serves a required exit door.

If you're not sure, assume you need one. A permit that turns out to be unnecessary costs you a phone call. A deck built without a required permit costs you at resale, when the inspection turns it up, and it can complicate a homeowner's insurance claim if someone is ever hurt on it. When we build a deck, we pull the permit as part of the job — so the question of "do I need one" is one you never have to answer alone.

The 42-inch frost footing rule (this is the big one)

McHenry County sits in the Chicago-metro frost zone, and that dictates how deep your footings have to go. The frost line — the depth to which the ground freezes and thaws each winter — runs deep here, so code requires deck footings to reach a minimum of 42 inches below grade. That isn't a suggestion or a "best practice." It's the number your footing inspection is measured against.

The reason is frost heave. When water in the soil freezes, it expands and lifts everything above it. A footing that stops short of 42 inches gets shoved upward every winter and settles unevenly every spring, and within a few seasons your once-level deck is racking, pulling at the ledger, and popping fasteners. Footings poured below the frost line sit in ground that doesn't freeze, so they stay put. This single detail is the difference between a deck standing straight in twenty years and one that isn't — and it's the corner the cheapest bids quietly cut where you can't see it.

Crystal Lake adds a local amendment worth knowing: the city sets a minimum 10-inch concrete footing (diameter) in its residential code, on top of the county-wide 42-inch depth. Depth gets you below the frost; diameter gives the footing enough bearing surface to carry the load. Your plans need to show both.

The two inspections, in order

A deck permit in this area comes with two required inspections, and the order matters because one of them has to happen before you can pour concrete.

1. Footing inspection — before the pour. Once the holes are dug, an inspector confirms the footings hit the required depth (that 42 inches) and the holes are sized correctly. You cannot legally pour concrete until this passes. Digging first and pouring before the inspection is the most common way a DIY deck fails — the inspector can make you dig it back up to prove the depth.
2. Final inspection — after the build. When the deck is framed, decked, railed, and stairs are in, the final inspection checks the structure as a whole: ledger attachment and flashing, joist spacing and fasteners, guardrail height (36 inches for residential), baluster spacing, and stair rise/run. Pass this and your deck is legal and on record.

Filing the permit correctly the first time is the fastest way through this schedule, and that's exactly the part we take off your plate when we handle the build.

Deck permits by town — McHenry County

Each municipality runs its own building department, and unincorporated addresses go through the county. Here's where to start in the towns we serve. Permit fees vary by town and by project value — expect typically a few hundred dollars — so confirm the current fee with the department when you apply.

Not sure whether your address is inside village limits or unincorporated? That single question decides whether you file with the town or with McHenry County Planning & Development — and it's one of the first things we check before pulling a permit for a build.

What your permit application needs

Departments want to see that the deck is designed to carry a load and attach safely. Whether you file it yourself or we do, the submittal usually includes:

• A site plan showing the deck's location and its setbacks from property lines.
• A framing plan — footing size and depth, post locations, beam and joist sizes, and joist spacing.
• The ledger attachment detail and flashing method (how the deck connects to the house).
• Guardrail and stair details — heights, baluster spacing, and rise/run.
• The fastener and hardware schedule (lag screws, tension ties, joist hangers).

Towns like McHenry publish a Residential Deck Handout that spells out exactly what the drawings must show, which is genuinely useful if you're going the DIY route.

We handle the permit for you

If you hire us to build or replace your deck, the permit isn't your problem — it's ours. We prepare the drawings, file with your town or with McHenry County, schedule the footing inspection before we pour, and meet the final inspection when the build is done. Because we work this county town by town, we already know how Crystal Lake's footing amendment reads, what Huntley's Development Services expects, and how the county handles unincorporated parcels. You get a deck that's on record and built to code, in writing, without spending your week at village hall.

Building in a specific town? Start with our local pages for Crystal Lake, Algonquin, or Huntley, and once you're weighing the budget, our Illinois deck cost guide breaks down where the permit and footing line items land in a total.