Impervious surface

Impervious surface is any surface that prevents water from soaking into the ground: roofs, driveways, patios, sidewalks, and conventional paving. Jurisdictions cap the impervious share of a lot to manage stormwater, because runoff that cannot infiltrate has to go somewhere, and that somewhere is usually a neighbor or a storm system.

Why the limits are tightening

Stormwater regulation has become steadily stricter, and impervious limits are a primary tool. Many jurisdictions now enforce caps that older neighborhoods do not meet, which matters when adding to an existing property.

What typically counts

• Building roofs, including overhangs
• Driveways and parking areas
• Patios, decks over impervious material, and walkways
• Pool decks, and in many places the pool surface itself

Pervious alternatives

Where a project is over the limit, the usual routes are pervious pavers, permeable concrete, gravel, or a designed retention feature. Jurisdictions differ on how much credit these get, with some counting them as fully pervious, some at a partial rate, and some not at all. That decision is worth confirming before designing around it.

Where it shows up

Impervious calculations are typically shown on the site plan and are checked at zoning or engineering review. On a project involving a new driveway or a pool, it is one of the first numbers worth running.