City of Houston IDM 2026 Update: Is Your Project Compliant?

The City of Houston IDM just changed how stormwater is handled.

The updated Chapter 9 rules took effect on June 1, 2026.

If your plans still use the old numbers, your project may no longer be compliant.

At JRH Engineering, we live by one promise: "Engineering Excellence. Delivered on Time. Built on Value."

Here is a quick scan of what changed and what to check.

What Is the City of Houston IDM?

The City of Houston IDM (Infrastructure Design Manual) sets the design standards for public infrastructure.

Chapter 9 covers stormwater and drainage design.

This is the section most likely to affect your site yield and project budget.

For background, see our earlier post: 2026 Houston IDM Update: What Developers Must Know.

What Changed on June 1, 2026?

The updated Chapter 9 stormwater rules include:

• A new 0.8 acre-feet per acre flat detention rate for projects under 20 acres.

  
  

• Updated redevelopment credits for sites with existing impervious cover.

  
  

• Revised low impact development (LID) and stormwater quality expectations.

  
  

These changes can shift how much land you must set aside for detention.

Review the official manual through the City of Houston Permitting Center.

You can also read our detailed breakdown: City of Houston 2026 Stormwater Detention Update.

Is Your Project Compliant?

Run through this quick checklist:

1. Confirm whether your detention design uses the new 0.8 AF/acre rate.

   
   

2. Recheck your site acreage against the under-20-acre threshold.

   
   

3. Review your eligibility for redevelopment credits.

   
   

4. Confirm LID and water quality features meet the current standard.

   
   

5. Verify your rainfall data aligns with NOAA Atlas 14.

   
   

If any answer is unclear, a licensed Civil Engineer can help you confirm.

Why This Matters for Developers

Stormwater rules drive real project economics.

• More detention can mean less buildable area.

  
  

• Redevelopment credits may help recover lost site yield.

  
  

• Early planning can reduce costly redesigns.

  
  

Smart Land Development starts with the right drainage strategy.

Our Civil Engineer and Structural Engineer teams help align plans with the current City of Houston IDM.

For green infrastructure background, the EPA offers helpful guidance on LID.

How JRH Engineering Can Help

JRH Engineering supports clients across Texas, North Carolina, and Florida.

We provide Civil Engineer services, Structural Engineer design, and full Land Development support.

In the Houston metro, find us on our Houston Google Business Profile and our Manvel Google Business Profile.

Want More Information on this Topic?

We are here to help you stay compliant under the updated City of Houston IDM.

Reach out in whichever way is easiest for you:

• Call us: (800) 227-9635

  
  

• Chat with us: Use the chatbot in the lower right-hand corner of your screen.

  
  

• Send a message: Fill out the form on our Contact Us page.

  
  

We look forward to hearing from you.

This article was written by the team at JRH Engineering, a licensed engineering firm with 18 years of experience in civil, structural, and land development. JRH is a licensed professional engineering firm in the states of Texas, Florida, and North Carolina.