Drainage

Site drainage is the shape of your ground and the route water takes over it. It is a separate thing from the drain lines inside the house. This is the outdoor version: the finished grade, the swales, the driveway crown, and where a monsoon storm sends water once it comes off your roof.

The base rule lives in the foundation chapter of the residential code. Lots have to be graded so surface water runs away from the foundation walls, and the grade must fall at least 6 inches within the first 10 feet. Where a property line, a wall, or a slope blocks that, you build swales or drains instead, and hard surfaces within 10 feet slope at least 2 percent away.

Arizona adds its own problem. Desert storms arrive fast and sheet across flat ground rather than running in a channel, and the caliche and tight clay under many Valley lots absorb little of it. So the plan has to carry runoff around the home and off the property without pushing it onto the neighbor. That last part is the correction that costs the most to fix later.

Drainage is a permit, not only a detail. In unincorporated Maricopa County, the drainage review uses a grading matrix to estimate how much engineering your plan needs. Paradise Valley is tighter: add 999 square feet of hard surface, or price the work above $500,000, and a grading and drainage plan attaches to the house permit, signed by an Arizona registered civil engineer along with the survey and the stormwater plan.

For example, on a Rio Verde Foothills parcel with a wash across one corner, the engineer sets the pad clear of the flow path and shapes a swale to route storm water around the home. One thing owners miss: that 6-inch fall has to survive the landscaper. Check the finished grade again after the yard goes in.