Phoenix City Code 32A-24: On-Site Stormwater Retention for Commercial Sites

The requirement is short: "On-site retention of stormwater shall be required for all developments." The only way out is narrow, for "isolated developments under one-half acre where there will be no critical drainage problem." The size comes from the manual: retention areas "shall be adequate to contain the volume of water required by the latest edition of the City of Phoenix Stormwater Policies and Standards Manual in effect at the time of the first submittal of plans."

Section 32A-17 ties every drainage facility to that manual. Its key rules:

• Design storm. "All new developments shall make provisions to retain the storm water runoff from a 100-year, 2-hour duration storm falling within its boundaries" (Manual 6.8.1). Volume uses V = C × (P/12) × A.
• No public outfall, more volume. A basin that doesn't outfall to a public right-of-way or drainage easement needs "an additional 25% retention volume."
• 36 hours. Stored runoff "shall be emptied completely from the facility within 36 hours after the runoff event has ended" (Manual 6.8.10).
• Bleed-off cap. "Maximum bleedoff rate into a City of Phoenix storm drain system shall be limited to ONE cfs."
• Parking lots. "The maximum depth of ponded water within any parking lot location shall be six inches."
• Drywells. Designed to ADEQ guidelines, at no more than 0.1 cfs per well unless a certified soils report supports more (Manual 6.8.13).
• Private upkeep and a sign. Basins are privately maintained and stamped "Private Drainage Facility - No City Maintenance" in three inch letters.

• Retention. Holding runoff on site until it soaks in, drains through a drywell, or bleeds off slowly.
• First flush. The first half inch of runoff. For infill and redevelopment lots, Phoenix policy TRT 00768 calls first flush "the absolute minimum retention requirement that can be applied to a lot."
• Bleed-off. A small pipe that slowly drains a basin to a storm drain, capped at one cubic foot per second into the city system.
• Dry-up calculation. The engineer's proof that a basin empties in 36 hours. TRT 00332 says to assume "no surface percolation," so drywells alone must do it.
• Percolation test. A field test of how fast water soaks in. TRT 00332 accepts ADEQ's standard percolation test or the ASTM D3385 double-ring test.

All new development inside Phoenix city limits, public and private, commercial and residential. Section 32A-23 says the manual "shall govern all drainage requirements for development, public and private."

This does not apply to:

• Unincorporated county islands. Those follow Maricopa County's Section 1205.
• Small isolated sites the city waives under half an acre with no critical drainage problem.
• Existing sites with no new development. The upkeep duty still applies to basins you already have, under Chapter 32C.

A two-acre flex building in south Phoenix. The basin can't outfall to a public street, so it gets 25 percent more volume. The dry-up calculation assumes drywells only at 0.1 cfs each, which sets the drywell count on the plans. Percolation tests during construction can bring that count down.

A drive-thru redevelopment on an infill corner. The old site was mostly paved. If the new layout doesn't add runoff, TRT 00768 lets the engineer retain the first flush instead of the full storm.

A basin that still holds water on day three. The answer flips from design to upkeep. The manual says drywells "that cease to drain a facility within the 36 hour period shall be replaced/refurbished by the owner," and the checklist's plan note puts adding drywells on the owner if basins fail to drain.

1. 1Find the manual edition in effect at your first submittal. The city's stormwater documents page links the 2013 third edition.
2. 2Have a civil engineer prepare the grading and drainage plan using the TRT 00047 commercial checklist and the retention formula.
3. 3Run the dry-up calculation per TRT 00332: drywells only, 0.1 cfs each, 36 hours.
4. 4Schedule the city drywell inspection. "The City inspector must inspect and approve the well before backfill."
5. 5Inventory the drywells with ADEQ in myDEQ. The checklist makes the owner or developer responsible.
6. 6On projects of one acre or more, include the site-specific operations and maintenance plan with the grading and drainage submittal, required since March 1, 2024.

• "Small sites are exempt." The waiver is only for isolated sites under half an acre with no critical drainage problem, and only if the city grants it.
• "The newest rules apply." The code applies the manual in effect at your first submittal.
• "Parking lots can pond like the county allows." Phoenix caps ponding at six inches; the county allows 12 inches in public parking.
• "Percolation will handle it." Plan approval assumes drywells only.
• "The city maintains the basin." Basins are private, and the stamp says "No City Maintenance."

The penalty section, 32A-27, sets a civil sanction "of not less than $500.00 nor more than $2,500.00. The $500.00 minimum sanction may not be waived," and each day a violation continues is a separate violation. Failing to keep a finished basin working falls under Chapter 32C, covered in Phoenix basin and drywell upkeep.

Retention design belongs to your civil engineer. When stormwater is backing up into the building through floor drains or the sewer, that's plumbing, and a licensed plumber can camera and clear the line.