This article was co-authored by Maria Ramirez, PE and Jessica Dennis, PE

Effective May 15, 2025

After five years under the current drainage and detention requirements, Houston Public Works has released significant updates to Sections 9.1 and 9.2 of the Infrastructure Design Manual (IDM). These long-awaited changes mark a pivotal shift in how detention is evaluated and designed within the City, with the goal of promoting more development and redevelopment within the Houston city limits.

👉 Read the full supplemental changes here.
📘 Review the FAQs for a quick overview here.

1. Simplified Detention Rate for Tracts Under 20 Acres

One of the most impactful changes is the replacement of the previous sliding scale (0.75–0.98 acre-feet per acre) with a flat detention rate of 0.8 acre-feet per acre for all tracts under 20 acres. This provides a consistent standard and makes detention volume calculations more straightforward for developers and engineers.

2. Detention Credit for Redevelopment Projects

Qualifying redevelopment projects can now earn detention credit for removed impervious area at a rate of 0.4 acre-feet per acre. The updated detention volume calculation is:

Detention Volume = (Ap × rd) – (Ae × 0.4)
Where:

Ap = Proposed impervious area

Ae = Existing impervious area removed

rd = Detention rate (from Table 9.5)

To claim this credit, applicants must submit a survey signed and sealed by a Texas-licensed land surveyor (dated within the past five years) along with photos verifying site conditions.

3. Legacy Project Exemption

Under the updated criteria, developments will be recognized as legacy projects if:

• • • There is a master drainage plan approved after the year 2002, and
    • The detention system and storm sewer infrastructure have been fully constructed for the entire development

This recognition allows eligible projects—especially large, phased developments—to proceed under their originally approved drainage plans, avoiding new detention or stormwater requirements. By honoring prior approvals that align with permitted infrastructure, the City provides certainty for developers and helps prevent costly redesigns or retrofits.

4. De Minimis Impact Credit

The City has introduced a new de minimis impact credit, a change that provides flexibility for small-scale site improvements with minimal effect on overall drainage.

This credit is designed to streamline the review process and reduce unnecessary detention requirements for proposed improvements such as:

• EV charging stations
• Decks or small structures
• Swimming pools
• Minor parking lot additions

Who Qualifies?

• For existing single-family tracts ≤ 15,000 sf, a one-time credit of up to 5% of the lot size.
• For existing single-family tracts > 15,000 sf, a one-time credit of up to 5% with a maximum of 750 sf.
• For existing commercial tracts ≤ 20 acres, a one-time credit of up to 1% of the total tract area.
• For existing commercial tracts larger > 20 acres, a drainage study must be submitted to determine whether the proposed improvements qualify for the de minimis credit.

What Do We Know Now?

With these changes in effect, here’s how property owners and project teams can move forward:

• ✅  Projects under review or approved but not yet built
  → Can be resubmitted to take advantage of the new standards.
• 🚫  Projects already under construction
  → Must continue under the previously approved drainage criteria.
• 🏗️  Legacy projects
  → May proceed under approved master drainage plans (post-2002) without additional detention if criteria are met.

These changes present a unique opportunity for developers and property owners to reduce detention obligations and simplify design. For help evaluating how these new requirements affect your current or upcoming project, reach out to our drainage experts: Maria Ramirez, Jessica Dennis, or Rob Erni.