Floodplain

The regulated version of a floodplain is the area of special flood hazard: land subject to a 1 percent or greater chance of flooding in any given year. FEMA maps it, and the flood it maps is the base flood. Arizona's version is rarely a river. It is a dry wash that runs hard for an hour, or flat desert that floods as sheet flow, which federal rules call an area of shallow flooding, 1 to 3 feet deep on average.

Building there takes a second approval. Under A.R.S. 48-3613, no development may occur in a delineated floodplain without written authorization from the flood control district board, and development covers grading, filling, and paving, not only the house itself. In the Phoenix metro that board is the Flood Control District of Maricopa County, which writes and enforces the floodplain regulations for the unincorporated county and 14 cities and towns. Its floodplain use permit asks for a site plan, an engineer's report, and your finished floor elevations.

Height is where the money goes. The lowest floor has to sit at or above the design flood elevation, and most communities add freeboard, a safety margin in feet above the flood level. Section R322 of the residential code then calls for flood-damage-resistant materials below that line and puts the electrical, HVAC, and plumbing equipment above it.

For example, take a five-acre Rio Verde Foothills parcel with a wash across one corner. The pad moves out of the flow path, the finished floor gets raised, and the floodplain use permit adds fees and staff review on top of the building permit. The flag follows you into other permits too. A rooftop solar array on a floodplain parcel loses access to Maricopa County's faster SolarAPP+ review path.

A floodplain lot is still buildable. Pull the flood status during due diligence, before you buy, because the finished floor elevation drives the foundation, the driveway, and the price.