Impervious Cover in Austin, in Plain English

Impervious cover is the share of your lot covered by surfaces that don’t let rain soak in: the roof, driveway, walks, patios, and slabs. Austin caps that share by zoning and by watershed, so a new patio or wider driveway has to fit inside whatever room your lot has left.

Most Austin homeowners first hear the phrase when a project gets bigger than expected. You want a wider driveway or a patio that finally fits the table, and someone asks how much impervious cover you have left. It sounds like jargon, but the idea’s simple, and knowing your number before you plan saves a lot of redrawing.

This guide explains the concept and how it shapes concrete projects. It isn’t a filing guide. The rules belong to the city, they change, and the numbers here were checked in October 2026, so treat them as a starting point. Our permit triggers overview covers when a review comes into play.

What counts as impervious cover

The city’s definition is any surface that keeps water from soaking into the ground. In practice that’s the house and garage roof, the driveway, front walks, patios, shed slabs, and parking pads. Concrete counts in full. So does asphalt. A gravel driveway counts too, because gravel that’s driven on gets compacted until water runs off it.

A few things are treated differently. An uncovered wood deck with gaps between the boards, built over open ground, counts at half its area. Gravel used only for landscaping or a footpath, without a compacted base, generally doesn’t count. Permeable pavers are the one that surprises people: on a driveway they usually still count, since the city’s credit for porous paving is limited to pedestrian paths in certain areas.

Why Austin limits it

Rain that can’t soak in has to run somewhere. More hard surface means more water, moving faster, into the same creeks and storm drains, and it carries whatever was on the pavement with it. Limits on impervious cover are how the city manages flooding and protects creeks and the aquifer. It’s also why the monthly City of Austin drainage charge on a utility bill is based on how much impervious cover a property has. Adding hard surface can raise that charge a little, on top of using up your allowance.

How the limit is set

Two sets of rules apply, and the stricter one wins. The first is zoning. In Austin’s most common single-family districts, the cap is 45 percent of the lot, and in some lower-density districts it’s 40 percent. The second is the watershed. In the urban watersheds that cover much of central Austin, the watershed rules defer to zoning. In the watersheds that feed the lakes and in the Barton Springs Zone, they can be far tighter than zoning alone would suggest.

That’s why two houses with the same zoning can have very different room to build. A lot in the hill neighborhoods northwest, draining toward Lake Austin or Bull Creek, may have much less to work with than one in a central neighborhood. Many HOAs add their own limits on top, which our HOA review walkthrough covers.

How to find your number

You need two figures: the size of your lot and the area of everything hard on it. Your survey or plat gives the lot size, and often shows the house, driveway, and walks to scale. Add up the roof footprint, driveway, walks, patios, and any slabs. Divide by the lot area and you’ve got your percentage.

Here’s a simple example. On a 7,000 square foot lot with a 45 percent cap, the allowance is 3,150 square feet. If the house and garage cover 2,200, the driveway 600, and the walk and back patio 250, that’s 3,050 already in use. There’s room for about 100 more square feet, which is a small landing, not a new patio. Better to learn that at the kitchen table than after the forms are set.

What to do when you’re near the limit

Being close to the cap doesn’t end a project. It changes the design. The usual moves are trading, trimming, and reshaping. Trading means removing hard surface you don’t use, like an old pad behind the garage or an extra-wide walk, and spending that area where you’ll enjoy it. Trimming means sizing a patio to the furniture you own instead of the biggest rectangle that fits; our page on right-sized patio planning starts from that question. Reshaping can mean a driveway that’s wide where cars park and narrow where they only pass, which our narrower driveway layouts can handle.

What doesn’t work is pouring first and sorting it out later. An overage found during a later permit, a sale, or a complaint is expensive to fix, because the fix is usually removing concrete. When a project does need review, we coordinate it through a permitting partner, and the question gets asked while the quote’s being written. For the rules themselves, the City of Austin Development Services department is the source that outranks every summary, including this one.

Impervious cover questions we hear most