In Austin, an uncovered wood deck with gaps between the boards over soil or grass counts as 50 percent impervious cover. A covered deck counts as fully impervious. On a lot near its limit, that can decide a deck’s size.
Impervious cover in Austin is the share of a lot covered by surfaces that keep rain from soaking into the ground, like roofs, driveways and patios. The city caps it by zoning and by watershed, and decks count too, just not always in full. If your lot is already close to its limit, how a deck is counted can shape the whole design. Here’s how the rule works, and how our crew plans around it. To check your own lot, book a site visit.
The Rule for Decks
Austin’s Environmental Criteria Manual spells it out. An uncovered wood deck that has drainage spaces between the boards and sits over a pervious surface counts as 50 percent of its area. A covered deck counts as fully impervious. So a 300-square-foot open deck over grass counts as 150 square feet, while the same deck under a solid roof counts all 300.
The city applies the same 50 percent approach to uncovered wooden decks when it calculates the drainage charge on utility bills. That doesn’t change what you can build, but it shows how consistently the city treats open decks.
How Much Cover Is Allowed
Limits depend on zoning and on the watershed. Many single-family lots zoned SF-3 are capped at 45 percent impervious cover, and that total includes the house, garage and driveway. In water supply watersheds and in the Barton Springs Zone, limits are tighter.
Lots along Jester Boulevard, for example, sit in the Bull Creek watershed with lower caps, which our Jester Estates page explains. In the Barton Creek watershed, Save Our Springs limits run from 15 to 25 percent of a site.
Your survey, or the plans from your last permit, usually shows what’s already on the lot. If they’re old, or you’ve added a patio or walkway since, we’ll measure what’s there now, since the calculation has to match the lot as it is today.
A Worked Example
Say a lot is 6,000 square feet and zoned SF-3, so its cap is 2,700 square feet of impervious cover at 45 percent. If the house, garage, driveway and walks already use 2,450, there are 250 left. An open 400-square-foot deck over grass counts as 200, so it fits. Put a solid roof over the same deck and it counts as 400, which doesn’t. That’s the kind of math our permit partners run before a design is final, and it’s why we check the numbers first.
Where It Bites: Small and Built-Out Lots
Many Central Austin lots in neighborhoods like Travis Heights and Hyde Park are small and already carry a house, a driveway and sometimes an addition. On those lots, a few hundred square feet of deck can be the difference between fitting under the cap and not. That’s when the 50 percent rule earns its keep, and why we check where a lot stands before the deck’s size is set.
Design Choices That Change the Count
Keep It Open
An uncovered deck with drainage gaps over soil counts at half its area. Building over an existing patio is a different case, since that patio is already counted, so the deck adds little or nothing new. If you’re also changing what’s under the deck, like replacing grass with gravel or pavers, ask how that surface counts before you commit, since it can change the total.
Think Before You Add a Roof
A solid roof turns the whole deck area into impervious cover. If you want shade on a tight lot, an open deck pergola is a different question from a solid cover, and we’ll confirm how your plans will be read before it’s designed in.
Ask About Composite
The rule’s wording covers wood decks with drainage gaps. If you’re choosing composite deck options, confirm how the reviewer will treat the boards on your plans rather than assuming the same 50 percent. We raise that question early so it doesn’t surprise anyone during review.
How We Plan for It
Our crew starts by finding out where your lot stands: its zoning, its watershed and how much cover the house and hardscape already use. Then we size and shape the deck to fit, and show you the result in 3D renderings through our custom deck design process. Our trusted third-party permit partners prepare the calculations with the permit application.
Most decks need a building permit anyway, and our deck permit guide covers the basics. Impervious cover is one more number the plans have to get right.
Start With Your Lot
If you’re not sure where your lot stands, the site visit is the place to find out. We’ll measure what’s there, check the limits that apply and design a deck that fits under them. Ace Deck Builders has been building decks across Austin since 2008. Call (512) 566-7519 or contact our team.