Maryland Stormwater: New Development vs Redevelopment Thresholds

ESD-to-MEP: the core requirement

Maryland's Stormwater Management Act of 2007 established ESD-to-MEP as the state's primary stormwater approach. The policy intent: mimic pre-development hydrology by prioritizing small-scale, distributed, non-structural practices (bioretention, microbioretention, swales, permeable pavement, disconnected rooftops, conservation of natural features) before resorting to larger structural best management practices (ponds, constructed wetlands, underground storage).

The performance target is typically expressed in terms of rainfall volume captured and treated — historically the 1-inch event, with updated manual versions extending to 2.7 inches for ChesBay and trading considerations in some contexts. The "MEP" qualifier means "to the maximum extent practicable" — the designer and reviewer are negotiating what's truly practicable given site-specific constraints, not just claiming a generic practicability excuse.

ESD-to-MEP is a site-layout rule before it's a BMP-selection rule. See our MD ESD-to-MEP Site Layout essay for the layout-first framing.

New development threshold

For new development:

• Trigger. Land disturbance of 5,000 square feet or more, or any disturbance that creates new impervious surface above certain thresholds.
• Performance requirement. ESD-to-MEP for the applicable rainfall-volume target (typically starting at 1 inch, with designer working toward higher targets as practicable).
• Design reference. Pre-development reference typically is "woods in good condition" hydrology — the natural-condition benchmark.
• Result. The site, post-development, must replicate that pre-development hydrologic behavior to the maximum extent practicable.

New development ESD-to-MEP design begins at schematic layout: building placement, grading, impervious location, preserved-area delineation. Bolted-on BMPs at the end of design don't satisfy the standard.

Redevelopment threshold — structurally different

Redevelopment projects engage a different performance target:

• Trigger. Projects that increase impervious area or disturb existing developed land.
• Performance requirement. A 20% reduction in existing impervious area, or treatment of an equivalent water quality volume, or demonstrated water quality improvement to an equivalent standard.
• Focus target. Sites with more than 50% impervious are the primary redevelopment focus — the state is trying to harvest water quality improvement on previously-degraded sites.
• Design outcome. Net improvement on the water quality trajectory of a previously-compromised site.

The practical effect: redevelopment doesn't demand "as if the land were woods." It demands meaningful water quality improvement relative to the existing degraded condition. That's both easier to reason about and harder to achieve on tight urban infill sites.

Why the distinction matters for fill, infill, and adaptive reuse

Projects on previously-developed land that fit the redevelopment definition include:

• Site rebuilds after demolition. Building demolished, site rebuilt with new structure; the redevelopment framework applies.
• Adaptive reuse with substantial exterior work. Existing building shell retained, but grading, parking, and site drainage modified.
• Brownfield redevelopment. Contamination issues layer over the redevelopment stormwater framework.
• Parking lot reconfigurations. Even without new building, impervious changes trigger the analysis.
• Fill operations on developed land. Grading and fill on previously disturbed/developed sites engage the framework; ESD-to-MEP is required for disturbance at or above the trigger.
• Infill on previously-developed lots. The lot's prior state is the baseline, not "woods in good condition."

The common mistake is applying the new-development framework to these projects. The designer calibrates to "woods in good condition" pre-development, discovers the site has no room for the required volume, and either over-designs or gets pushed back in review. Applying the redevelopment framework instead — 20% impervious reduction or equivalent water quality volume — often yields a feasible design on tight urban sites.

How MDE and delegated local jurisdictions handle the distinction

MDE (Maryland Department of the Environment) sets the state standards. Implementation is delegated to counties and municipalities approved under COMAR 26.17.02. Each delegated jurisdiction:

• Adopts a local ordinance consistent with state minima.
• Reviews plans and approves or requests revisions.
• May grandfather certain pre-2010 plans.
• May allow flexibility for infill or Priority Funding Areas when ESD efforts are demonstrated.
• Must document perpetual maintenance for stormwater BMPs.
• Is subject to MDE oversight of ordinance compliance and program administration.

Practical variation shows up in:

• How aggressively a jurisdiction polices the distinction between new development and redevelopment designations.
• How readily it accepts alternative water-quality-volume approaches on tight sites.
• Whether it permits off-site mitigation or fee-in-lieu for portions of the requirement.
• Specific BMP acceptance patterns — green roofs, permeable pavement, underground infiltration.
• Detail requirements in design submittals.

Carroll County's Chapter 151 ordinance is an example of a well-articulated local adoption: ESD-to-MEP mandated, with specific treatment thresholds for ≥50% impervious sites. Montgomery, Baltimore, Anne Arundel, and Howard counties each run robust local programs with different specific ordinances. Don't assume the same design will land identically in two different counties; verify local program-specific requirements.

Redevelopment in context with MDE ESC + NPDES

Redevelopment projects in MD engage multiple overlapping regulatory frameworks:

• Stormwater management (ESD-to-MEP). Post-construction water quality and quantity.
• Erosion and sediment control (ESC). Construction-period discharge controls under MD's ESC program.
• NPDES construction general permit. Federal-state framework for construction stormwater; MD's ESC program satisfies NPDES for most projects.
• Floodplain management for projects within mapped 100-year floodplain.
• Critical Area Commission review for projects within the Chesapeake Bay Critical Area.

The stormwater-design effort isn't standalone; it threads through the permit/approval stack for any site work project.

Implications for design teams

• Classify the project correctly at pre-concept. New development or redevelopment? Confirm with the local reviewer before schematic design.
• For redevelopment: establish existing-condition baseline early. Existing impervious area, existing BMPs (if any), existing drainage patterns. This is the reference point.
• For redevelopment: explore the 20% impervious reduction path. Many redevelopment projects can meet the standard more easily through impervious reduction (removing excess parking, restoring natural cover, green roofs replacing traditional roofs) than through water quality volume alone.
• Don't over-engineer new-development-style designs on redevelopment sites. The standard is different; the design should reflect that.
• Pre-submission meeting with the local reviewer pays off. MD stormwater reviewers — whether at MDE or at delegated jurisdictions — have deep site-specific knowledge and can flag classification questions early.

What to do with this

If you're designing a MD project on undisturbed or minimally-disturbed land: apply the new development ESD-to-MEP framework. Start at site layout; don't bolt BMPs on at the end.

If you're designing on previously-developed land: verify redevelopment classification. Baseline existing impervious; target 20% reduction or equivalent water quality volume. The path is different and usually more feasible on tight sites.

If you're on a brownfield or mixed-use infill project: the redevelopment framework is almost certainly applicable; confirm with the local reviewer during schematic.

For the full MD stormwater framework and primary-source links, see our Maryland Stormwater Navigator. For cross-state comparison, see Mid-Atlantic Stormwater Compared.

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