Impervious surface
Also called: impervious cover, impervious cover limit, lot coverage, impermeable surface, hardscape limit
Any surface rain can't soak through — roofs, driveways, patios, walkways, pools. Many cities cap the share of a lot that can be impervious (often 30–60%) to control runoff, and count a driveway extension against it. Gravel and permeable pavers are sometimes exempt; check before paving.
Stormwater has to go somewhere. When a lot is mostly roof and pavement, rain runs off it into the street and the storm system instead of soaking into the ground, and enough lots doing that floods the neighborhood downstream. So zoning codes set an impervious-cover limit — a percentage of the lot area — and every addition of pavement counts against it. A driveway extension, a new patio or a shed pad on a lot that’s already at the limit is a denied permit, or an unpermitted surface with a removal order attached when it’s noticed.
What counts varies. Roofs, concrete, asphalt and mortared stone always do. Standard pavers on sand usually count in full, though some cities credit them partially. Gravel is often exempt or counted at 50%. Permeable pavers over an open-graded stone reservoir, and pervious concrete, are usually exempt or credited — which is why they exist. Decks with gaps often don’t count. The city’s zoning office or GIS site will show your lot’s current cover and the limit; the calculation takes five minutes and decides what you can build.
Related rules that ride along with it: a maximum driveway width at the street (commonly 20–24 feet at the curb), side-yard setbacks that keep pavement off the property line, and a separate right-of-way permit for anything touching the sidewalk, curb or apron. HOAs add their own layer on material and layout. Ask about all of them before ordering stone.