Do I need a permit for a deck?

Usually. The model IRC exempts a deck only if it's 200 sq ft or less, 30 in or less above grade at every point, not attached to the house, and not serving the required exit door (IRC R105.2).

Under the International Residential Code, a deck is exempt from a building permit only if it meets all four conditions in Section R105.2 (Work exempt from permit, Building item 10): it is no more than 200 square feet in area, it is not more than 30 inches above grade at any point, it is not attached to the dwelling, and it does not serve the exit door the code requires. Fail any one of the four and the model code expects a permit.

"At any point" matters on a sloped yard: if the deck surface is more than 30 inches above the ground at its low side, the exemption is gone even if the high side is a step up. Thirty inches is also where the IRC starts requiring guards on open sides (Section R312.1.1 in the 2021 IRC). A deck fastened to the house with a ledger never qualifies, whatever its size. A freestanding (floating) deck can qualify, but only if it also meets the size, height and exit-door conditions; being freestanding alone is not enough.

Examples under the model IRC: a 10×12 freestanding deck 12 inches off the ground, away from the required exit door, is exempt. A 12×16 deck (192 sq ft) that sits 36 inches above grade on its low side needs a permit (over 30 inches). A 14×16 ground-level deck (224 sq ft) needs a permit (over 200 sq ft). A small, low landing outside the home's required exit door needs a permit (it serves that door). Whether replacing deck boards counts as repair is your building department's call.

The exemption does not mean the rules stop applying. R105.2 says an exemption from permit does not authorize work that violates the code, so a permit-exempt deck still has to be built to code; it just isn't reviewed or inspected. Zoning setbacks, HOA approval and electrical work such as deck lighting can need their own approvals or permits.

Your city or county may have changed this rule. The IRC is a model code that states and local governments adopt, often with amendments. For example, Ankeny, Iowa deletes the deck exemption (item 10) from R105.2, so decks there need a permit regardless of size, while Kent, Washington's deck handout keeps the four-condition exemption. When a permit is required, departments typically ask for a site plan and construction drawings showing footings, framing, connectors, stairs and guards. Always confirm with your local building department before you build.