Are there construction codes to consider when building a deck?

Building a deck in the Monroe or Snohomish County area can be subject to building codes, permit requirements, structural safety standards, and property setback rules. The requirements depend on the build scope, such deck's height, size, whether it's attached to your house, whether it has a roof, and where your property is located.

While requirements vary by municipality, the current building code framework is generally based on the 2021 International Residential Code (IRC), as amended by Washington State, effective March 15, 2024.

When a permit is required

A building permit is generally required when the walking surface is more than 30 inches above the ground within 36 inches horizontally of an open deck edge.

Additionally, a roof attached to a residential structure generally requires a building permit, regardless of the deck's height. A deck over a basement or another story also requires a permit.

Main construction code requirements

These are the major areas to consider when designing your deck.

1. Footings and foundations: Footings must support the deck's loads and be sized for the soil, post spacing, and deck design.
2. Guardrails and stairs: Guards are generally required where the walking surface is more than 30 inches above the floor or grade below. Typical residential guards are at least 36 inches high, and stair handrails are generally required for stairs with four or more risers.
3. Ledger board and house attachment: An attached deck needs an approved structural connection to the house, appropriate fasteners, and flashing to prevent water intrusion. Freestanding decks, however, have different structural connection requirements.
4. Joists, beams, posts, and connectors: Lumber dimensions and spans must meet applicable span tables or an approved engineered design.

These are general code principles, not a complete approved design.

Property-line setbacks and site restrictions

Another area to consider are land-use rules that can affect where your deck can go.

• Uncovered decks 4 feet high or less: Under certain conditions, they may project into a required setback, provided they are at least 30 inches from a property line and project no more than 6 feet into a required street setback.
• Decks over 4 feet high: Generally must meet all applicable building setbacks.
• Covered decks: Must meet the applicable building setbacks.
• Slopes, wetlands, streams, shorelines, and flood hazard areas: May trigger additional restrictions or environmental review.

The setback allowances depend on zoning, easements, and the adjacent property boundary or right-of-way.

Remember, the instructions that apply to unincorporated county property may vary from those within city limits, such as Monroe, Snohomish, and Duvall.