The 30 Inch Deck Rule Is Real. It Is Also Not the Whole Test

Ask how high a deck can be before it needs a permit and the internet answers 30 inches. The figure is real: the International Residential Code exempts decks not more than 30 inches above grade, and plenty of jurisdictions adopt it as written.

Here is what the answer looks like across the 137 jurisdictions where we have read and dated the deck rule. Eighty-two of them publish no measurement at all, and in 73 of those the answer is simply yes, a deck needs a permit. Only 55 publish a numeric trigger, and the trigger is not always 30, and not always a height.

Where the number is a height

Thirty-two jurisdictions draw the line at a height above grade:

A 24-inch-high platform deck that is comfortably exempt under the famous rule needs a permit in Seattle and Miami-Dade County, where the line sits at 18 inches, in Prince William County, Virginia, which triggers at 16 and a half inches, and in Fort Worth, Texas, which sets the lowest bar in our data at six inches above grade. Marion County, Florida has since taken the title: its fee schedule prices a deck permit for decks “over 1 step in height,” a threshold measured in stairs rather than inches.

Denver deserves its own sentence, because it runs two bars at once: an uncovered deck 12 inches or more off the ground needs a zoning permit, and at 30 inches or more it also needs a building permit. A homeowner who checks only the building code number clears the second bar and walks into the first.

Where the number is an area

Nineteen jurisdictions put an area number on the line. Fifteen use 200 square feet: Contra Costa, Riverside, Sacramento and Santa Clara counties in California, plus San Diego, San Jose, Boston, Detroit, Grand Rapids, Austin, Corpus Christi, Houston, Plano and Tacoma. Philadelphia uses 216, San Bernardino County 120, and Leon County, Florida draws its line at 100 square feet. Manatee County, Florida draws two lines at once: a detached deck needs a permit over 120 square feet or over 30 inches, whichever comes first, and an attached deck needs one regardless. Leon County runs the same two-bar test: over 100 square feet or more than 30 inches above grade, either one triggers the permit. Philadelphia’s number is also a different kind of threshold: a deck up to 216 square feet still needs a permit, it just qualifies for the no-plans EZ permit; a larger deck files full plans.

A 12 by 20 floating deck, 240 square feet and 20 inches off the ground, is permit-exempt under the pure height rule and permit-required in all nineteen of those places.

The exemption is a checklist, not a number

Most of the published exemptions require passing every part of a compound test, and the height is only one part. Among the 55 jurisdictions with a numeric trigger, 20 pair the limit with a 200 square foot area cap, 29 void the exemption if the deck is attached to the house, and 24 void it if the deck serves an exit door or an accessible route.

So the same 10 by 12 deck can flunk three different ways: by sitting an inch too high, by being screwed to the band joist instead of freestanding, or by being the thing you step onto out the back door. The last one catches people most often, because almost every deck is built at a door.

In most places, the answer is just yes

The largest group in our data is the quiet one: 82 of 137 jurisdictions publish no measurement, and 73 of them require a permit for a deck, full stop. Virginia is the clean example: of the 12 Virginia jurisdictions we cover, 11 answer yes with no exemption stated, and only Prince William County publishes its 16.5 inch trigger.

If you remember one thing, remember that 30 inches is where the question starts, not where it ends. Look up your own jurisdiction’s rule, with the fee and the office link, on its deck permit page in our verified index.

Method

Every figure above is derived from the permit rules we hand-verified against each jurisdiction’s own published code or guidance. Each locality’s deck permit page carries the verbatim rule, the source link, and the date we last checked it.