Redeveloping a St. Louis Site Under MSD's 2027 Rules: Existing Pavement, Cumulative Disturbance and Additions

What MSD counts as development or redevelopment

The draft adds a plain definition to section 4.060.01: projects that change the line and grade of the site, change drainage patterns, change the amount or direction of runoff, or change the use of the land are development or redevelopment. A change of use with little grading can still bring a project into stormwater review.

The draft also keeps MSD's test for new development versus redevelopment: sites with 20 percent or less existing impervious area are generally treated as new development, and later expansions on those sites are treated the same way.

When existing pavement gets no credit

Both the 2018 rules and the 2027 draft say that for areas draining to downstream stormwater problems, differential runoff is calculated from an undeveloped existing condition. The draft changes three things around that rule:

• The stormwater quantity management table that the 2018 rules applied to the Harlem and Baden watersheds, the Grand and Bates sewershed and other areas MSD directed now applies to any project with stormwater quantity management requirements.
• The 2018 phrase "unless directed otherwise by the District" is struck from the undeveloped-condition sentence in section 4.060.01.
• MSD now says what it will look at to decide: available watershed studies, computer hydraulic models, complaint records and the site engineer's pipe hydraulic calculations.

In its October 15, 2026 responses, MSD put it simply: with downstream flooding or surcharging, there may be no credit for existing pavement or roof area; without downstream problems, the existing site may be taken as it is. Ask your engineer to check downstream conditions before the site plan is set, because the answer drives basin size.

Disturbance adds up

For the one-acre water quality threshold, the draft adds: "Any disturbance since October 1, 2006 is cumulative." Several small projects on the same property over the years can add up to a project that needs water quality BMPs. Keep records of past disturbance on the site; your engineer will need them.

The draft also says the existence of downstream stormwater problems may require detention even where the runoff increase is under 2 cfs, and that projects with a differential of 5 cfs or less use the "Zero Increase" approach in the release-rate table.

Adding onto an old basin

If an existing detention basin or BMP will take runoff from a building addition, parking expansion or new phase, the rules require the facility to be retrofitted to current standards for the area it serves. Where physical limits make that impossible, MSD evaluates alternatives case by case, including managing stormwater to current rules for just the new disturbance. For an owner, that means the old basin is not a free outlet. Budget for regrading, a new outlet structure or added volume.

New exemptions for small, scattered work

The draft adds examples of exemptions MSD may allow:

• Several small disturbances on a large, mostly impervious site that cause zero, negative or minimal runoff increase, with no erosion problems downstream.
• Small disturbances where channel protection can be deferred and provided later as part of a larger project on the site.

These are case-by-case allowances, not automatic exemptions. Ask for them early.

Two reports, and liners where seepage is a risk

Any project with a stormwater BMP, credit or basin now needs two separate sealed reports: a design and calculations report and an operation and maintenance plan. The draft also says any stormwater feature must consider downhill properties and the potential for seepage or water table fluctuation, and impermeable liners may be required. On a tight infill site next to existing basements, plan for that conversation.

What this means for your schedule and budget

• Have your engineer check downstream flooding and surcharging before you commit to a site plan.
• Pull records of past disturbance on the property since October 2006.
• If you plan to tie into an existing basin, price the retrofit.
• If you think an exemption applies, raise it with MSD at the start, not at final review.
• Bring the site contractor in during design so basin earthwork and phasing are priced from the real footprint. See our preconstruction services and bioretention basin work.

Kuesel builds MSD-permitted storm and sanitary work in St. Louis City and County. See our MSD work or call 636-978-3478, Monday through Friday, 7:00 AM to 4:00 PM.