Houston BOMA Successfully Advocates for Updates to Detention Requirements

The final updates to Chapter 9 of the Infrastructure Design Manual (IDM), effective May 16, 2025, reflect many of Houston BOMA’s key advocacy points from last August’s City Hall Advocacy Day.

Houston BOMA supported three core reforms:

1. De minimis impact exemption
2. Credit for existing impervious cover
3. Recognition of regional detention and legacy projects

All three of these priorities were adopted, showing strong alignment between the final updates and Houston BOMA’s positions. View the announcement and updated IDM Chapter 9 language here.

Key Wins for Commercial Property Owners and Managers

1. Detention Not Required for Minor Improvements
• No detention is required for improvements on existing impervious cover (e.g., generator pads, charging stations, awnings) as long as they don’t alter the stormwater drainage pattern.
• This eliminates a major barrier to small-scale improvements and repairs that previously triggered costly detention requirements.

2. Credit for Redeveloped Impervious Area
• Commercial redevelopment projects may now deduct 0.4 acre-feet per acre of removed impervious cover from their detention requirement.
• This is a direct response to concerns about prohibitive costs for underground detention on previously developed sites.

3. Recognition of Legacy Projects
• Developments with previously approved and constructed regional detention systems (post-2002) are exempt from providing new detention for new tracts if consistent with the original plan
• This supports phased buildouts without revisiting old detention calculations.

4. De Minimis Impact Allowance for Commercial Sites (≤20 acres)
• Commercial sites up to 20 acres can receive a one-time de minimis credit if their addition is ≤1% of the total tract area.
• Simplifies permitting for very small additions like canopies or new equipment pads.

5. More Predictable Rules and Streamlined Math
   • The confusing "curve detention table" has been replaced with a flat detention rate for non-residential developments, simplifying calculations and design.

Examples of Positive Impact for Houston BOMA Members

1. Industrial Parking Lot Repairs
• Previously: Replacing damaged pavement or subsurface utilities triggered detention requirements, even if no new impervious area was added.
• Now: If the project does not alter the existing impervious cover or drainage pattern, detention is no longer required, eliminating costly underground detention retrofits for basic repairs.

2. Office Building Adds EV Charging
• Under old rules: Required costly detention or underground tanks for adding chargers in a parking lot.
• Under new rules: Likely qualifies for de minimis credit; no detention needed.

3. Medical Campus with Master Drainage Plan
• Sites with regional detention built under approved plans post-2002 are now exempt from revisiting detention for future phases—saves time, cost, and legal review.

4. Redevelopment of Aging Commercial Property into Mixed-Use
• Can now get detention credit for impervious areas removed during redevelopment, significantly reducing detention volume, and potentially avoiding the need for underground storage.

Thank you to everyone who participated in our 2024 City Hall Advocacy Day, and to all of those who worked with us on this important issue.

Updated IDM Chapter 9 Language

IDM Chapter 9: Stormwater Design and Water Quality Requirements Updates FAQs