Flood Zones and Erosion Hazard Setbacks in Pima County, Arizona

Pima's own "regulatory floodplain" is broader than a FEMA flood zone

FEMA's mapped 100-year floodplain (covered in our national flood-zones guide) is not the only floodplain that matters in Pima County. The District defines its own, separately-regulated "regulatory floodplain": any area along a wash whose base flood discharge equals or exceeds 100 cubic feet per second, plus any area subject to sheet flooding, regardless of whether that specific reach happens to be mapped on a current FEMA Flood Insurance Rate Map. In practice this means a rural desert parcel can sit outside any FEMA-mapped Special Flood Hazard Area -- clean on a flood-insurance lookup -- and still fall inside Pima County's own regulated floodplain because of a wash that hasn't been formally remapped by FEMA. Confirm both maps separately; treating a clean FEMA lookup as the whole answer is the most common mistake here.

The Erosion Hazard Setback: a second, separate map most buyers never check

Every regulatory wash in Pima County -- meaning any wash carrying 100 cfs or more -- also carries its own mapped Erosion Hazard Setback, a buffer measured from the top of the wash's bank where the District has determined the channel is likely to migrate or erode over time. The standard setback runs from as little as 25 feet on a small regulatory wash up to as much as 500 feet on a major watercourse where no bank protection exists, with the exact figure depending on the specific wash's discharge and channel characteristics rather than a single countywide number. Unlike a flood-hazard boundary, this setback specifically restricts structures and septic systems -- it does not apply to fences, walls, ramadas, or swimming pools, which can generally still be built inside the setback area.

If the standard setback distance would make an otherwise-buildable lot impractical, the ordinance allows an alternative: an Arizona-registered civil engineer can perform a site-specific erosion analysis and submit it to the District as part of a Floodplain Use Permit application, and the District can approve a reduced setback based on that analysis or on documented erosion-protection measures installed on-site. That's a real, available option, but it adds engineering cost and review time to a project -- budget for it before assuming a parcel near a wash is a straightforward build.

The Floodplain Use Permit: what actually triggers it

A Floodplain Use Permit (FPUP), issued by the District's Floodplain Management staff, is required for virtually any improvement inside a regulatory floodplain or an erosion hazard area -- the District's own guidance lists site-built structures, manufactured homes, accessory structures, walls, fences, septic systems, and grading as activities that generally require one. This runs on a separate track from your zoning and building permits, and permit fees are set on a fee schedule adopted by the District's own Board of Directors (available directly from the District's office) rather than through Development Services' standard building-permit fee table. Because the FPUP process folds in floodplain status, erosion setback status, and (where relevant) an engineer's analysis, plan for it as its own line item in your due-diligence timeline, not an afterthought to the building permit.

The District's own advice to buyers: check before you close, not after

Pima County's own published guidance for real estate professionals makes a point of stating that the standard FEMA-floodplain disclosure required in a typical transaction covers only part of the picture -- local floodplains, erosion hazard areas, and riparian habitat all extend over a larger footprint of the county than FEMA's mapped floodplain alone, and none of the broader local layers is guaranteed to show up in a routine flood-zone disclosure. The District's own recommendation is direct: before purchasing a property, confirm its status yourself with Floodplain Management, or have your agent do it, rather than relying on a FEMA-only disclosure to tell the whole story.