Research by DoINeedAPermit Research Team · Updated May 2026
The Short Answer
Any attached deck in Des Moines requires a Building Department permit. The shallow 12-inch frost depth in the Puget Sound basin and ledger-flashing details specific to Washington's 4C climate zone drive strict plan review.
Des Moines is situated in the Puget Sound lowlands (IECC Climate Zone 4C), which means a 12-inch frost-depth requirement — the shallowest in King County and well below the 30–36 inches standard east of the Cascades. This shallow frost line affects deck footing design but also means you don't need the deep footings that Snoqualmie or North Bend require. However, the City of Des Moines building inspectors are notoriously detail-oriented on ledger flashing (IRC R507.9) because the region's heavy winter rain and seasonal saturation cause catastrophic ledger rot if flashing is substandard. The city also enforces the full IRC R507 deck specification without local amendments, but they've adopted the 2021 International Building Code — two cycles ahead of some neighboring jurisdictions — which tightens guardrail and stair geometry rules. Unlike some Puget Sound cities that allow over-the-counter plan review for decks under 300 sq ft, Des Moines routes all attached decks through standard plan review (not fast-track), which adds 1–2 weeks to the timeline. Expect a 3–4 week turnaround from submission to approval.

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

Des Moines attached deck permits — the key details

Des Moines adopted the 2021 International Building Code (IBC) and IRC, which means your attached deck must comply with IRC R507 (Decks) in full. The critical requirement is the ledger board connection: IRC R507.9 mandates flashing that wraps the top and sides of the ledger, with a 1-inch air gap between the ledger and the rim joist of your house. This flashing must extend 4 inches up the rim joist and 2 inches down the face of the rim joist, and it must be sealed with caulk or tape. The City of Des Moines building inspectors will red-line your plan if the flashing detail is missing or if you show flashing nailed directly to the rim joist (a common mistake). The reason this matters so much locally is the Puget Sound climate: winter rain (avg. 37 inches/year) and spring saturation create ideal conditions for ledger-board rot if water gets behind the flashing. A rotted ledger is a collapse waiting to happen — the deck can separate from the house, dropping anyone on it. The city requires flashing to be corrosion-resistant (galvanized or stainless steel, per R507.9.2), and they inspect the ledger connection before you frame the deck, so you can't hide it behind rim joist sheathing.

Footing depth in Des Moines is 12 inches below grade, per the Puget Sound frost-depth map. This is shallow — sympathetic to glacial-till and alluvial soils common in the valley — but it still requires footings to extend below the 12-inch line. The City of Des Moines requires a footing-depth certification on your plan (either a geotechnical report for questionable soils or a note from you stating 'footings below 12-inch frost line per IECC Table R301.2(1)'). Post footings must bear on undisturbed soil or 4-6 inches of gravel in a sonotubes pier (never sitting on the surface). Beam-to-post connections require galvanized post bases (Simpson LUS210 or equivalent per R507.9.2) that are bolted — not nailed — to the post. The city's plan checklist explicitly calls out IRC R507.9.2 for 'positive connections' between beams and posts, meaning you can't just set a beam on a post and rely on gravity. If your footing design shows footings at or above 12 inches, the plan will be rejected and sent back for revision — this is the #1 resubmission reason for Des Moines decks.

Guardrail and stair requirements follow IBC 1015 (Guards and Handrails) and IRC R311 (Stairs and Ramps). Guardrails must be 36 inches high (measured from the deck surface to the top of the rail) on any deck 30 inches or higher above grade. Guardrail balusters (vertical spindles) cannot be spaced more than 4 inches apart — the standard 'sphere test' means a 4-inch ball cannot pass through any gap. Handrails on stairs are required if the stair has more than 3 risers, and the handrail must be 34–38 inches high (measured from the nosing of the stair tread). The stair stringers must have a minimum run (tread depth) of 10 inches and a rise between 7–11 inches. Des Moines inspectors measure these dimensions during framing inspection, and they carry calipers and a sphere gauge. If your deck is over 200 sq ft or elevated more than 10 feet above grade, the city may also require a professional engineer's seal on the structural calculations — this typically adds $500–$1,200 to your design costs but is worth doing upfront rather than failing plan review.

The permit application process in Des Moines is online through the city's Permit Portal, but you'll need to upload PDF plans (not hand-sketches). Plans must include: site plan with property lines and setbacks, deck framing plan with joist spacing and fastening, ledger detail (critical), footing detail, guardrail elevation, and stair section if applicable. The city's plan-review staff will typically take 10–14 business days for the first review and 3–5 business days for resubmissions. Plan review is not fast-track in Des Moines, even for small decks under 300 sq ft — all attached decks receive structural review. Once plans are approved, you schedule the footing inspection (city inspector verifies footing depth and soils), framing inspection (ledger flashing, beam connections, guardrail blocking), and final inspection. Total timeline from permit application to sign-off: 3–4 weeks if your plans are complete and correct on the first submission, 5–7 weeks if you have resubmissions.

Owner-builder permits are allowed in Washington State for owner-occupied primary residences, and Des Moines honors this. You do not need to hire a licensed contractor to build your deck if it's on your primary residence. However, you must pull the permit in your name, obtain an owner-builder affidavit from the county assessor, and be present at inspections. Some owner-builders hire a surveyor to certify footing depth and setbacks — this costs $300–$600 but is worth it to avoid a failed inspection. The permit fee for a 200–300 sq ft deck in Des Moines is typically $200–$350, based on the valuation of the structure (roughly 0.5–1% of the project cost). If your deck includes electrical (outlet or lighting), plan review adds 1–2 weeks and the fee increases by $50–$100 for the electrical sub-plan.

Three Des Moines deck (attached to house) scenarios

Scenario A
12x16 attached deck, 36 inches above grade, no stairs, no electrical — Tall Firs neighborhood
You're building a 192 sq ft attached deck on the back of a 1970s rambler in Tall Firs, elevated 36 inches above grade on posts (this is common on sloped lots in Des Moines). The deck triggers a permit because it's attached to the house and over 30 inches high. Your plan must show: ledger flashing detail (top and side flashing with 1-inch air gap, sealed), post footings at 12+ inches below grade (glacial-till soils, so 12-inch frost depth applies), beam-to-post connections with galvanized post bases bolted to 4x6 posts, guardrail 36 inches high on the open sides with 4-inch-max baluster spacing, and deck framing with 2x10 joists at 16-inch centers. The city will require a footing-depth certification note ('Footings extend 12 inches below finish grade per IECC 4C frost depth'); you can do this yourself without a geotechnical report. Since the deck is over 30 inches high, the city inspector will also verify that the guardrail is blocking-backed (2x12 blocking behind the balusters to prevent entrapment). Plan review takes 10–14 business days; footing inspection happens before you pour concrete; framing inspection verifies the ledger flashing and beam connections. Permit fee is $200–$250 (valuation typically $8,000–$12,000 for a basic 12x16 deck). Total timeline: 4–5 weeks from application to final sign-off. No electrical, so no electrical sub-plan required.
Permit required (attached + over 30 inches) | Footing depth 12 inches (Puget Sound zone) | Ledger flashing detail critical | Galvanized post bases required | 36-inch guardrail with 4-inch baluster max | Permit fee $200–$250 | Plan review 10–14 days | Total project cost $8,000–$15,000
Scenario B
16x20 attached deck with 12 stairs, 48 inches above grade, 1 outlet — corner lot near Hwy 516
You're adding a 320 sq ft deck to a 1980s colonial on a corner lot near Highway 516, elevated 48 inches above grade. This triggers full structural review because the deck is over 200 sq ft and over 30 inches high. Your plan must include detailed stair geometry: each stair has a 10-inch run and 8-inch rise (12 treads to get 4 feet of rise); the stair landing at the bottom must be 36 inches wide and the landing at the top (at the door) must be at least 36 inches deep; handrails on both sides of the stairs (34–38 inches high, graspable diameter 1.25–2 inches). Because the deck is at 48 inches, lateral wind loads become a factor — the city may require an engineer's stamp on the structural calcs to verify that the beam-to-post connections (post bases + through-bolts) can handle the overturning moment. You're also adding an outlet, which requires a separate electrical sub-plan showing the outlet location, GFCI protection (required per NEC within 6 feet of water or ground), and a conduit run from the house panel. The deck plan must show: ledger flashing (same as Scenario A), post footings at 12+ inches below grade, beam sizing (likely 2x12 or 2x14 depending on span), joist sizing (2x10 at 16 inches), and stair stringers with reinforcement at the connection to the deck (bolted, not nailed). The city will assign a structural reviewer and an electrical sub-plan reviewer; plan review takes 14–21 business days because of the stair and electrical details. Footing inspection, framing inspection, electrical rough-in inspection, and final inspection add up to 5–6 inspections over 2–3 weeks of construction. Permit fee is $300–$450 (valuation roughly $15,000–$22,000 with the stairs and electrical); electrical sub-plan adds $50–$100. Total timeline: 5–7 weeks from application to final sign-off.
Permit required (attached + over 200 sq ft + over 30 inches) | Stair design per IRC R311.7 (10-inch run, 7–11-inch rise) | Handrails on stairs (34–38 inches high) | Electrical sub-plan required | GFCI outlet required | Engineer stamp may be required (height + span) | Permit fee $300–$450 | Electrical fee $50–$100 | Plan review 14–21 days | Total project cost $18,000–$28,000
Scenario C
Freestanding ground-level deck, 180 sq ft, 18 inches above grade, no attachment — Maple Lane
You're building a 15x12 freestanding deck on a corner lot on Maple Lane, set 18 inches above grade on a gravel pad (not attached to the house, no ledger). This deck does NOT require a permit because it meets the exemption criteria in IRC R105.2(2): it's less than 200 sq ft and less than 30 inches high. You can build it without filing any paperwork or scheduling inspections. However, there's a critical catch: the exemption applies only to 'one-story decks not connected to a dwelling.' If you later decide to attach a ledger to the house or elevate the deck above 30 inches, you must get a retroactive permit. The city can see unpermitted decks on aerial photos or from neighbor complaints, and they do chase these down. An unpermitted structure discovered during a property sale or refinance will cost you $3,000–$5,000 in remediation costs or a price reduction. That said, for a true freestanding deck you can build on your own. Your design should still follow basic framing practices: footings on a 4–6-inch gravel bed (not directly on soil, which can settle or frost-heave), 4x4 posts on 12-inch centers, 2x8 joists at 16-inch centers, 2x6 deck boards at ½-inch spacing. You don't need a city inspector, but a neighbor might call if they see work. The safest approach: get the permit anyway (cost is $150–$200) and get sign-off, which takes 1 week and costs less than the headache of a retroactive enforcement action. If you're certain the deck will stay freestanding and under 200 sq ft, go ahead and skip it — but keep this rule in mind if you ever expand.
No permit required if truly freestanding AND under 200 sq ft AND under 30 inches | Risk: unpermitted structure may be flagged on sale or refinance | Recommended: pull permit anyway for $150–$200 | Footing pad 4–6 inches gravel (no concrete required) | 4x4 posts, 2x8 joists standard framing | Total DIY cost $3,000–$6,000 | Can be owner-built with no inspector

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Why ledger flashing is the #1 rejection in Des Moines deck permits

Des Moines sits in IECC Climate Zone 4C (marine/oceanic west of the Cascades), which means heavy winter rain and year-round moisture saturation in the glacial-till soils. The Puget Sound basin receives an average of 37 inches of rain per year, much of it falling between November and March. Winter ground saturation and spring runoff create the perfect conditions for ledger-board rot: water gets trapped between the rim joist and the ledger, the wood stays damp, and fungal decay sets in over 3–5 years. A rotted ledger is catastrophic — the entire deck can separate from the house in a single season, dropping a family barbecue and an umbrella stand into the yard 4 feet below.

IRC R507.9 requires flashing that wraps the top of the ledger (extending 4 inches up the rim joist and 2 inches down the face of the rim joist), with a 1-inch air gap between the ledger and the rim joist. This gap allows moisture to drain and air to circulate. The flashing must be galvanized steel or stainless steel (never aluminum — it corrodes in contact with pressure-treated lumber). Des Moines inspectors check the flashing detail at framing inspection, and they measure the gap with a feeler gauge. If your plans don't show the flashing detail, the city will reject the plan on the first review cycle and send it back with a comment: 'IRC R507.9 requires flashing detail; provide section view showing ledger, rim joist, flashing, and air gap.'

The most common mistake Des Moines homeowners make is installing the flashing after the deck frame is already nailed to the rim joist. Once the ledger board is bolted in place, you can't slide flashing under it — you have to tear the ledger off, install the flashing, and reinstall the ledger. This costs $500–$1,000 in extra labor. Get the flashing detail right on your plan before the first nail goes in.

Footing depth, frost heave, and glacial-till soil in Des Moines

Des Moines is built on glacial outwash and till deposited by the Puget Sound lobe during the last ice age (18,000 years ago). The soils are a mix of clay, silt, sand, and gravel — variable and unpredictable. The 12-inch frost-depth requirement in Des Moines (much shallower than the 30–36 inches east of the Cascades) reflects the marine climate and the fact that the Puget Sound moderates winter temperatures. However, the glacial till itself is prone to frost heave because fine silts and clays in the soil matrix absorb water and expand when frozen. A deck footing placed at only 6 inches depth can heave 1–2 inches in a hard winter, settling back down in spring — this repeated movement cracks the concrete and loosens the bolts in the post base.

The City of Des Moines requires footings to extend 12 inches below finish grade, but the inspection process also requires the inspector to verify that you're bearing on undisturbed soil, not fill. On sloped or filled lots (common in Tall Firs and other neighborhoods), you may need to dig deeper to hit native soil. A geotechnical report costs $600–$900 but is worth it if your lot has questionable fill or if you're building on soft soils near a stream. Most owner-builders in Des Moines use concrete piers in sonotubes set 12–14 inches deep on a 4-inch gravel base — this is the standard, cheap, and acceptable approach.

One more frost-heave risk: if your deck footing holes fill with water (common in spring in Des Moines, where the water table rises), you must pump them dry and place 4–6 inches of gravel at the bottom before pouring concrete. A water-filled footing will freeze, heave, and crack. The city inspector will not sign off on a wet footing, and they'll make you bail it out and dry it before pouring.

City of Des Moines Building Department
22015 Lakeview Drive, Des Moines, WA 98198
Phone: (206) 870-6526 | https://desmoinespermits.org (or search 'Des Moines WA permit portal' to confirm current URL)
Monday–Friday, 8:00 AM–5:00 PM (closed weekends and city holidays)

Common questions

Do I need a permit for a small freestanding deck under 200 sq ft?

No, freestanding decks under 200 sq ft and under 30 inches above grade are exempt from permit requirements under IRC R105.2. However, the exemption ends the moment you attach a ledger to the house or raise the deck above 30 inches. If an inspector spots an unpermitted deck during a property sale or refinance, the city can order removal or require a retroactive permit ($200–$400 plus potential fines). For peace of mind, pull a permit anyway — it costs less than the risk.

What is the frost depth requirement for Des Moines?

Des Moines is in IECC Climate Zone 4C (Puget Sound), which requires deck footings to extend 12 inches below finish grade. This is shallow compared to east-of-the-Cascades locations (30–36 inches), but you still must verify that footings bear on undisturbed soil, not fill. If your lot has questionable fill or soft soils, a geotechnical report ($600–$900) is worth the cost to avoid a failed inspection.

Why is the ledger flashing so important in Des Moines?

The Puget Sound climate receives 37 inches of rain per year, creating ideal conditions for ledger-board rot. IRC R507.9 requires flashing with a 1-inch air gap between the ledger and rim joist, wrapping 4 inches up the rim joist and 2 inches down. Galvanized or stainless-steel flashing is mandatory. A rotted ledger can cause the entire deck to separate from the house — this is the #1 reason the city red-lines deck plans in Des Moines.

Can I build my own deck without hiring a contractor?

Yes, owner-builder permits are allowed in Washington for owner-occupied primary residences. You must pull the permit in your name, obtain an owner-builder affidavit from the county assessor, and be present at all inspections. You don't need a licensed contractor, but you're responsible for code compliance. Many owner-builders hire a surveyor to certify footing depth ($300–$600) to avoid rework.

What is the permit fee for a deck in Des Moines?

Permit fees are based on project valuation, typically $200–$450 for decks under 300 sq ft. A 12x16 deck (192 sq ft) is usually $200–$250; a 16x20 deck with stairs (320 sq ft) is $300–$450. If the deck includes electrical (outlet or lighting), add $50–$100 for electrical review. The fee is paid when you apply for the permit online.

How long does plan review take for a deck permit in Des Moines?

Des Moines does not offer fast-track plan review for decks; all attached decks receive standard structural review. First review takes 10–14 business days; resubmissions take 3–5 business days. If your deck includes stairs or electrical, add 1–2 weeks to the timeline. Total time from application to approval is typically 3–4 weeks if plans are complete and correct on the first submission.

What inspections do I need for an attached deck?

Typical inspections are: footing inspection (before pouring concrete, verifies depth and soil), framing inspection (checks ledger flashing, beam connections, guardrail blocking, and stair geometry), and final inspection. If the deck includes electrical, there's also an electrical rough-in inspection. You schedule each inspection through the online permit portal after the previous stage is complete. Inspections typically take 1–2 weeks apart, so plan 3–4 weeks of construction time from footing to final sign-off.

Do I need an engineer's seal on my deck plans?

For decks under 200 sq ft and under 10 feet high, engineer review is usually not required. For larger decks (over 200 sq ft or over 10 feet high), or if you're building on problematic soils, the city may require an engineer's seal on structural calculations. An engineer's report costs $500–$1,200 but prevents plan rejection and can save time in the long run. Ask the city during initial consultation whether your deck requires engineering.

Can I add electrical (outlet or lighting) to my deck?

Yes, but electrical requires a separate sub-plan showing the outlet location, circuit breaker, and GFCI protection (required within 6 feet of the ground per NEC). The electrical plan must be submitted with your deck plans and reviewed by the city's electrical inspector. Electrical review adds 1–2 weeks to the timeline and $50–$100 to the permit fee. Running wiring under the deck (in conduit) requires a trench and additional inspection; running it above the deck to an outlet box is simpler but visible.

What happens if I build a deck without a permit and the city finds out?

The city can issue a stop-work order within 48 hours of discovery, with fines of $500–$1,000 per day. You'll be forced to obtain a permit and pass retroactive inspection, or have the deck removed at your expense ($3,000–$8,000 for demolition). Unpermitted structures must be disclosed on any future home sale, causing a $5,000–$15,000 price reduction or buyer demand for removal. Your homeowners insurance will deny any claim for injuries on an unpermitted deck, leaving you personally liable for medical bills or lawsuits.

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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 Des Moines Building Department before starting your project.

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