Research by DoINeedAPermit Research Team · Updated May 2026
The Short Answer
Any deck attached to your house in McHenry requires a permit — no exceptions. McHenry's frost line runs 36-42 inches deep depending on location, and the city enforces strict ledger-flashing and footing inspection before you pour concrete.
McHenry sits at the boundary of two frost zones: 42 inches in the northern part of the city (closer to Chicago), and 36 inches in the southern portion near the Kane County line. This matters because the Building Department won't sign off on footing depth unless your plan shows footings below frost line for YOUR specific address. Unlike some neighboring municipalities that allow one-size-fits-all 42-inch depth across Cook County, McHenry's permit portal cross-references USDA soil maps and occasionally requires a geotechnical letter if you're proposing anything shallower than 36 inches — even if you're at the south edge of town. The city also enforces IRC R507.9 ledger flashing without variance: the ledger must be mechanized (bolted with 1/2-inch lag or screws, 16 inches on center) and flashed with step flashing and housewrap overlap. McHenry inspectors red-tag decks on first inspection if the ledger is just nailed or missing flashing detail. Plan review typically takes 2-3 weeks; the online portal is functional but doesn't auto-issue decisions — you'll get a phone call or email with comments.

What happens if you skip the permit (and you needed one)

McHenry attached-deck permits — the key details

McHenry requires a permit for any deck attached to a dwelling. This is non-negotiable under IRC R105.2; the city has zero exemptions for attached decks, even if they're under 200 square feet or under 30 inches high. A freestanding, ground-level platform (no attachment to the house) under 200 square feet and under 30 inches from grade can sometimes avoid a permit if it has no electrical or plumbing, but the moment you bolt a ledger to the house rim joist, you cross into permit territory. The Building Department reviews plans for ledger attachment, footing depth, stair geometry, guardrail compliance, and electrical/mechanical work. If your deck includes a hot tub, rigid plumbing, or 120-volt receptacles, those trigger additional inspections under the Illinois Plumbing Code and NEC Article 680 or 690. Most homeowners underestimate the ledger-flashing requirement; McHenry inspectors will reject plans that show the ledger nailed only or missing the step flashing and house-wrap overlap that IRC R507.9 mandates. Budget 2–3 weeks for plan review and 1–2 weeks for inspection scheduling after you've passed review.

Frost depth in McHenry is not uniform. The northern part of the city (closer to Crystal Lake and Chicago) sits in USDA hardiness zone 5A and requires 42-inch-deep footings. The southern part (toward Huntley and Kane County) is zone 4A and technically allows 36-inch footings per the International Building Code Table R301.2(1). However, the McHenry Building Department often errs toward 42 inches city-wide to avoid callbacks. When you submit your plan, include your address and reference the USDA soil map for your property. If you're on the south side of McHenry and want to propose 36-inch footings, provide a written justification citing zone 4A — some inspectors will accept it, others will require 42 inches anyway. Pier-and-beam decks (concrete footings with post-to-footing connections) are standard; frost-protected shallow foundations (FPSF) are not yet accepted by McHenry, so don't propose them unless you have an engineer's design. The soil in McHenry is primarily glacial till (dense, clay-heavy, good drainage), so 8-inch-diameter holes with 24-inch gravel base and concrete footer to frost depth is typical. Some southern-edge properties have loess or coal-bearing clay; if your soil investigation turns up anything unusual, the city will ask for a Phase 1 geotech letter before stamping the plan.

Ledger flashing and rim-joist attachment are the #1 inspection failure in McHenry decks. IRC R507.9 requires the ledger to be bolted (not nailed) to the rim band of the house with 1/2-inch bolts or 1/2-inch-diameter screws spaced 16 inches on center. The ledger must be covered with step flashing (metal, 5-6 inches tall, bent to fit under siding and over the rim board). Above the flashing, the house wrapping must overlap the flashing by at least 6 inches. Below the flashing, install a moisture barrier (felt, ice-and-water shield, or housewrap) to channel water away from the rim. Many homeowners skip this detail or assume nailing is okay; McHenry inspectors will fail the first framing inspection if the flashing is missing or non-compliant. If you hire a contractor, insist that the quote and scope explicitly list step-flashing installation. If you're building the deck yourself, this is the place to hire a professional or have an experienced friend oversee it. The cost to retrofit flashing after the fact is $200–$500 and creates a 2–3 week delay.

Stairs, landings, and guardrails trigger specific code scrutiny in McHenry. Per IRC R311.7, stair treads must be 10 inches (minimum) and risers must be 7-3/4 inches (maximum) on any set of 3 or more steps. Landings must be 36 inches wide (perpendicular to direction of travel) and as deep as the stair width. Guardrails must be 36 inches high (measured from the deck surface to the top of the rail) on all open sides of decks 30 inches or more above grade. Some inspectors interpret this as 42 inches, so include the 36-inch dimension on your plan. Balusters (the vertical spindles) must be spaced so a 4-inch sphere cannot pass through (gap no wider than 4 inches). This is a common failure: homeowners install horizontal cables or wide-gap spindles thinking they look modern, but the code is rigid. If you're planning any non-standard stair geometry (spiral stairs, curved landings, split-level decks), flag this early with the Building Department — it may require an engineer's design, which adds 2–3 weeks and $400–$800 to your timeline and cost.

Electrical work on the deck (receptacles, lighting, heated patio systems) requires a separate electrical permit and NEC compliance. If you're running a 120-volt circuit to a deck-mounted light fixture or outlet, you need an electrical permit, a licensed electrician, and GFCI protection (ground-fault circuit interrupter) on all deck-level receptacles per NEC 210.8. Hot tubs and rigid plumbing (deck-mounted spa, gas line to a built-in grill) also trigger plumbing and possibly gas permits. Many homeowners try to wire a deck light as 'part of the building permit' — that won't fly in McHenry. The Building Department will require a separate electrical permit and will not sign off on the deck final inspection until the electrical work is inspected by a licensed electrician and signed off. Budget an additional $200–$400 for an electrical permit and $800–$2,000 for the electrician's labor. Similarly, if you're mounting a gas grill with a dedicated line, expect a plumbing/gas permit ($150–$300) and a separate inspection. Do not attempt to run gas or rigid plumbing yourself; McHenry requires a licensed contractor for these systems.

Three McHenry deck (attached to house) scenarios

Scenario A
14×12 attached deck, 18 inches above grade, no stairs, gravel perimeter, Algonquin Road area (north McHenry, 42-inch frost zone)
You have a modest ranch on the north side of McHenry (near Algonquin Road) and want to add a 14×12 deck (168 square feet) running off the kitchen sliding door. The deck will be 18 inches above grade, with stepped entry (two steps down to grade). This is clearly an attached deck, so it requires a permit. Your frost zone is 42 inches because you're in the northern part of the city. Plan submission must include: a site plan showing the deck location relative to lot lines and setbacks; a framing plan with ledger detail showing bolts 16 inches on center, step flashing, and housewrap overlap; footing details showing 8-inch holes dug to 42 inches below finished grade, gravel base, 4000-psi concrete footer, and J-bolts for post connection; stair geometry showing 7-inch risers and 10-inch treads on the two-step entry; and guardrail details if the deck is over 30 inches (at 18 inches, guardrails are optional, but a 4-inch-sphere test on the perimeter is still required — you can do this with solid skirting or 4-inch-gap balusters). Permit fee is approximately $200–$300 (roughly 1.5% of estimated valuation at $12,000–$15,000 for labor and materials). Plan review takes 2–3 weeks; expect one round of comments (usually ledger flashing detail or footing depth confirmation). Once approved, you'll schedule three inspections: footing pre-pour, framing/ledger attachment, and final. You can pour footings 1–2 weeks after approval. Total timeline from application to final inspection is 5–7 weeks. Hire a contractor experienced with McHenry permits, or if you're building yourself, have the ledger flashing detail reviewed by a friend with construction experience before you nail it.
Permit required | 42-inch frost depth (northern zone) | Ledger bolting + step flashing non-negotiable | Two-step entry no guardrail required | Gravel perimeter preferred | No electrical required | Estimated deck cost $12,000–$15,000 | Permit fee $200–$300
Scenario B
20×16 deck with hot tub, built-in lighting, south McHenry near Huntley border (36-inch frost zone)
You're on the southern edge of McHenry (near Route 14, closer to Huntley) and want a 20×16 deck (320 square feet) with a sunken hot tub, deck-mounted lights, and a built-in composite bench. Frost depth here is officially 36 inches (zone 4A), but McHenry often defaults to 42 inches city-wide — you must confirm with the Building Department in writing before finalizing your plan. The deck itself is 28 inches above grade. Because the deck includes a hot tub (rigid plumbing, electrical circuitry, 240-volt supply), you need a building permit, an electrical permit, and a plumbing permit. The hot tub also requires a bonded ground rod and GFCI protection under NEC Article 680. The deck-mounted lighting (low-voltage LED or 120-volt) triggers an electrical permit; if you're running 120-volt, GFCI is mandatory. Plan submission includes: building permit with full ledger and footing details (ask the Building Department if 36-inch footings are acceptable for your address; if so, note it on the plan; if not, go with 42 inches); electrical permit with hot-tub wiring diagram, GFCI specification, and bonding details; plumbing permit for the hot-tub supply, drain, and pump. The composite bench is non-structural (bolted to the deck frame) and does not require a separate permit, but flag it on the framing plan. Permit fees: building permit $300–$450, electrical permit $150–$250, plumbing permit $150–$250. Total three-permit cost is $600–$950. Plan review is typically 3–4 weeks (multiple departments). Inspections: footing pre-pour (building), framing and ledger (building), rough electrical (before hot-tub installation), rough plumbing (before hot-tub installation), final building, final electrical, final plumbing. Expect 6–8 inspection events spread over 6–10 weeks. Hot-tub installation must be done by the manufacturer or a licensed contractor; DIY installation will fail plumbing and electrical inspection. Budget $25,000–$40,000 for the deck, hot tub, and electrical/plumbing work. This is a complex project; hire a contractor with McHenry multi-permit experience, especially for the hot-tub coordination.
Permit required (building, electrical, plumbing) | Confirm frost depth with Building Department (36 or 42 inches) | Hot tub = rigid plumbing + electrical = multiple permits | NEC Article 680 bonding and GFCI mandatory | Licensed electrician and plumber required | Three separate inspections sequences | Total permits $600–$950 | Deck + hot tub cost $25,000–$40,000 | Timeline 6–10 weeks
Scenario C
12×10 freestanding platform (no house attachment), 14 inches above grade, owner-builder, rural south McHenry
You own a wooded lot on the south side of McHenry (unincorporated or transitional area near Kane County) and want to build a 12×10 freestanding platform (120 square feet) for a gazebo or hot-tub pad. The platform is NOT attached to the house; there is no ledger bolt. It sits 14 inches above grade on four concrete piers. Under IRC R105.2, freestanding decks under 200 square feet and under 30 inches above grade do not require a permit if they have no electrical, plumbing, or structural ties to the dwelling. This platform qualifies: 120 square feet, 14 inches high, no attachment, no utilities. No permit required. However, check whether your property is in the City of McHenry or in unincorporated McHenry County (different jurisdictions). If you're in the City of McHenry, the above exemption applies. If you're in unincorporated McHenry County, McHenry County enforces the same IRC standard, so the exemption still applies, but call the McHenry County Building and Zoning Division to confirm. You can build this yourself without a permit. However, if you later attach any structure to the house (e.g., a roof over the platform that ties into the house rafters, or a railing that connects to the house), you cross into permit territory. Keep the platform truly freestanding. Also, check setback requirements: even though a permit is not required, zoning setback rules still apply. Most rural McHenry properties allow freestanding structures at 5–10 feet from lot lines, but verify your deed and any HOA restrictions. Cost for a basic 12×10 pier-and-beam platform is $2,000–$4,000 (materials and owner labor). No permit fees.
No permit required (freestanding, <200 sq ft, <30 inches, no attachment) | Verify City of McHenry vs. County jurisdiction | Check zoning setbacks (5–10 feet typical) | Review HOA restrictions if applicable | Owner-builder allowed for this scope | No permit fees | Material cost $2,000–$4,000 | Can start immediately

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Frost depth and footing failure in McHenry

McHenry's frost line is among the deepest in northern Illinois, and it's not uniform across the city. The northern part (zones 5A, closer to Chicago) requires 42 inches; the southern part (zone 4A, toward Kane County) technically allows 36 inches per IBC Table R301.2(1). Footings that are poured above the frost line will heave in winter when soil moisture freezes and expands, pushing the deck post upward. This can crack the ledger attachment, misalign stairs, and create a liability hazard. The Building Department enforces this strictly because failed decks are a known hazard in the Midwest. If your footing plan shows less than 42 inches, you must justify it with a geotechnical letter or a zoning variance; most homeowners simply go with 42 inches city-wide and call it done.

McHenry's soil is glacial till — dense, clay-heavy — which is actually good for footings because it's competent and drains relatively well when you add gravel base. A typical footing is 8 inches in diameter, dug to frost depth (36–42 inches), filled with 6–8 inches of pea gravel for drainage, then 20–28 inches of 4000-psi concrete topped with an embedded J-bolt for the post. This costs $120–$180 per hole (labor and materials). For a 14×12 deck with a 2-post support system (two corners), you need 2–4 footings; for a larger deck, 4–6. Total footing cost is $400–$1,200. Do not try to skimp on depth; the city will red-tag it, and you'll have to dig them out and re-pour.

Ledger flashing: the most common McHenry deck failure

McHenry inspectors cite failed ledger flashing on nearly 30% of deck inspections, making it the single most common code violation. The issue is not complexity — it's corner-cutting. Homeowners nail the ledger directly to the rim board without flashing, or they install flashing but don't overlap the housewrap. Water infiltrates the rim board, causing rot, mold, and structural damage that can cost $5,000–$15,000 to remediate. The fix is non-negotiable per IRC R507.9: ledger bolts (not nails), 1/2-inch diameter, 16 inches on center, tightened to pull the ledger snug against the rim. Above the ledger, L-shaped step flashing (5–6 inches) is installed every 16 inches, bent to fit under the house siding and lapped over the rim. The housewrap or felt then laps over the flashing by at least 6 inches. Below the flashing, a moisture barrier (ice-and-water shield or felt) channels water away from the rim.

If you're hiring a contractor, specify ledger flashing in the scope and ask to see photos of a previous installation. If you're building yourself, this is the ONE detail that justifies hiring a pro or having a knowledgeable friend oversee it. The labor cost is $300–$600 for a typical ledger; the material cost is $100–$200. Skipping this costs you far more in the long run. McHenry inspectors will fail the framing inspection if the flashing is missing or non-compliant, forcing a 1–2 week re-inspection delay.

City of McHenry Building and Zoning Department
333 South Green Street, McHenry, IL 60050
Phone: (815) 363-4200 | https://www.mchenry.il.us (search Building Permits or Building Department)
Monday–Friday 8:00 AM–5:00 PM (verify locally for seasonal or holiday changes)

Common questions

Do I need a permit for a freestanding deck in McHenry?

No permit is required for a freestanding platform under 200 square feet, under 30 inches high, with no electrical or plumbing and no attachment to the house. However, if the platform is attached to the house (even with a small roof tie or railing bolted to the rim), it becomes an attached deck and requires a permit. Verify your address is in the City of McHenry, not unincorporated McHenry County, and check zoning setback rules (typically 5–10 feet from lot lines).

What is the frost depth in McHenry for deck footings?

Northern McHenry (near Algonquin Road, zones 5A) requires 42 inches; southern McHenry (toward Huntley, zone 4A) officially allows 36 inches per the IBC, but the city often defaults to 42 inches city-wide. When you submit your plan, include your address and reference the USDA soil map or ask the Building Department if 36 inches is acceptable for your specific location. Most contractors assume 42 inches to avoid callbacks.

How long does it take to get a deck permit approved in McHenry?

Plan review typically takes 2–3 weeks for a simple deck (no electrical or plumbing), assuming one round of minor comments. If the plan includes electrical, plumbing, or structural issues, add 1–2 weeks. Once approved, you can start construction; footing inspection happens before you pour concrete, and final inspection happens after all framing and guardrails are complete. Total timeline from application to final approval is usually 5–8 weeks.

Do I need a licensed contractor to build a deck in McHenry?

Owner-builders are allowed in McHenry for owner-occupied residential properties, but ledger flashing, footing depth, and code compliance inspection are non-negotiable. If you're inexperienced with construction, hire a contractor for at least the ledger flashing and footing work. If the deck includes electrical or plumbing, you must use a licensed electrician and plumber; McHenry does not allow owner-builder work on these systems.

How much does a deck permit cost in McHenry?

Deck permit fees in McHenry are typically $200–$500, calculated as roughly 1.5–2% of the estimated project valuation. A 14×12 deck valued at $12,000–$15,000 costs about $200–$300 to permit. If the deck includes electrical ($150–$250) or plumbing ($150–$250), add those permit fees separately. A complex hot-tub deck can total $600–$950 across all permits.

What is the ledger-flashing requirement in McHenry?

Per IRC R507.9, the ledger must be bolted (not nailed) to the rim board with 1/2-inch bolts or screws spaced 16 inches on center. Step flashing (metal, 5–6 inches) must be installed under the siding and over the rim, and the housewrap must overlap the flashing by at least 6 inches. A moisture barrier (ice-and-water shield or felt) below the flashing channels water away. McHenry inspectors fail approximately 30% of decks on first inspection due to missing or non-compliant flashing; this is the single most common violation.

Do I need a guardrail on my deck in McHenry?

Guardrails are required on all open sides of a deck 30 inches or more above grade. The rail must be 36 inches high (measured from the deck surface to the top of the rail), and balusters must be spaced so a 4-inch sphere cannot pass through (no wider than 4-inch gaps). Decks under 30 inches above grade do not require guardrails, but a 4-inch-sphere balusters test still applies to prevent hazards.

Can I add a hot tub to my deck in McHenry?

Yes, but it requires a building permit, an electrical permit, and a plumbing permit. The hot tub must be installed by a licensed plumber; rigid plumbing, 240-volt supply, ground bonding, and GFCI protection per NEC Article 680 are mandatory. Expect multiple inspections (building, electrical, plumbing, hot-tub setup) and 6–10 weeks total timeline. Budget $25,000–$40,000 for the deck and hot tub combined.

What happens if I build a deck without a permit in McHenry?

If caught, you face a $500–$1,500 stop-work fine and orders to dismantle or remediate the deck. Insurance will deny claims related to unpermitted work. When you sell the property, the unpermitted deck must be disclosed, and the buyer will likely demand a retroactive permit ($300–$800) or removal before closing. Retrofitting ledger flashing and footing verification can cost $2,000–$5,000 and take weeks.

Can I use pressure-treated wood for the deck frame and posts in McHenry?

Yes, pressure-treated lumber (UC-4 grade minimum) is standard for deck frames, posts, and ledgers in McHenry. PT pine, fir, and hemlock are acceptable. Heartwood cedar or redwood can be used for visible surfaces (railings, trim) without treatment. All bolts, fasteners, and hardware must be stainless steel or galvanized (not plain steel) to prevent rust in Illinois's wet climate.

Disclaimer: This guide is based on research conducted in May 2026 using publicly available sources. Always verify current deck (attached to house) permit requirements with the City of McHenry Building Department before starting your project.

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