Decks
What Is It?
A single-family deck is an outdoor platform built above the ground; it may or may not be attached to a house.
What Permits Do You Need?
You need a permit to build a deck if it is:
- More than 18 inches above the ground
- A roof deck (built on top of part of the building)
- In an environmentally critical area (ECA), for example landslide-prone or flood-prone areas.
Most decks require only a subject-to-field-inspection construction permit.
- Tip 316, Subject-to-Field-Inspection Permits
You need an addition or alteration construction permit if your deck meets any of these criteria:
- It will be more than 8 feet above the ground.
- It includes beams 14 feet or longer.
- It will be a roof deck.
- It will be located in an environmentally critical area.
- It has deck flooring that is a solid surface (no gaps between boards).
- It will be over 750 square feet in size.
Research the Code
Our land use zoning code limits the size and placement of your deck.
If your deck is more than 36 inches above the ground: your deck counts toward the percentage of your property on which you can build (called lot coverage). For neighborhood residential zones (NR1, NR2, and NR3), the total coverage is limited to 50 percent of the lot. We do not limit the size of decks that are less than 36 inches above the ground, unless you plan to build a deck in or near an ECA.
If your deck is more than 18 inches above the ground: you can’t build your deck within 10-15 feet of the front property line, or within three to five feet of either side property line in neighborhood residential zones. You also cannot put the deck within five, 10, or 15 feet of the rear property line depending on the number of units on your lot. We do not limit the placement of decks less than 18 inches above the ground, unless you plan to build a deck in or near an ECA.
If your deck is in or near an ECA: you will need a pre-application site visit before applying for any construction permit (including subject-to-field inspection permits). We will determine what type of permit your project is eligible for based on the site visit. Read the Land Use Code and Environmentally Critical Areas Code for complete requirements, or visit our SDCI Services page for land use, construction permit, or geotechnical coaching.
Main Building Code Requirements for Decks
Structural: Your deck design must provide sufficient structural support and attachments to meet safety standards. The minimum live load design that we require is 60 pounds per square foot. If you have questions about our structural requirements you can contact us via our SDCI Services page for further coaching.
Ledger connection: If your deck is supported at the house by a ledger, the ledger must be connected to your house so that it will not pull away during an earthquake. See pre-approved standards for connecting ledger to your house (called a prescriptive connection) in the Seattle Residential Code Chapter 5, Section R507.9.1.3. You can also hire an engineer to design the ledger connection and the guardrail.
Guardrail (guards): We require guardrails if your deck is more than 30 inches above adjacent surfaces, such as the ground or a lower roof. Your guardrail must be at least 36 inches tall with rail openings that are no more than 4 inches in diameter.
We have special requirements for glass guardrails. See the Seattle Building Code sections 1015.2.1 and 2407.
You may need to have a structural engineer design your glass guardrail.
Stairs: If your deck will have stairs, they must have a riser of no more than 7-3/4 inches, a tread that is at least 10 inches, and a width of at least 36 inches. We require a handrail for stairs with four or more risers. The handrail must be 34 to 38 inches above the nosing of treads and landings and the hand grip must be 1-1/4 to 2 inches in dimension.
Treated Lumber/Wood: You must use treated wood or wood that naturally resists decay for all wood that will be in contact with concrete that is close to earth.
Should You Hire a Professional?
Deck drawings rarely require a professional’s stamp indicating they were designed by an architect or engineer. Anyone with drafting skills may prepare the drawings, or you may hire a professional to help you.
If you plan to build a deck in or near an ECA, you need to follow specific rules for your project. You may want the help of a professional to prepare documents for your application and construction permit.