New Castle County, DE Building Setbacks & Zoning: Lot Coverage Limits (2026)
Key Facts
- Impervious cap in WRPAs
- 20% of base site area
- To exceed cap
- EIA report plus RPATAC recommendation required
- Outside WRPAs
- set by each zoning district's Open Space Ratio
- Geographic scope
- unincorporated New Castle County only
- Max administrative fine
- $100 per day of violation
Summary
New Castle County caps impervious surface at 20 percent of the base site area inside Water Resource Protection Areas, the Cockeysville Formation, wellheads and recharge areas, without a certified environmental study and RPATAC sign-off; elsewhere lot coverage is set district by district through Open Space Ratio standards, not one flat number.
No development shall be permitted to have more than twenty (20) percent impervious surface ratio without an environmental impact assessment report and a recommendation from RPATAC supporting a greater than twenty (20) percent impervious surface ratio. The environmental impact assessment report shall be certified by a State-registered professional geologist or professional engineer with technical expertise in hydrogeology. The impervious surface ratio and open space ratio operate independently and are based on the base site area.
Official source re-checked September 7, 2026: no newer edition of the code had been published (publisher’s edition: Code of Ordinances: Supplement 5).
Full Breakdown
The Unified Development Code caps impervious surface, combined roofs, driveways and pavement, at 20 percent of the base site area within Water Resource Protection Areas: the Cockeysville Formation, its drainage area, wellheads and groundwater recharge areas. To build beyond that 20 percent ceiling, an applicant needs an environmental impact assessment report certified by a State-registered geologist or engineer plus a supporting recommendation from the Resource Protection Area Technical Advisory Committee (RPATAC) before the Department of Land Use will approve it. Outside WRPAs, the County does not set one flat lot-coverage percentage; Table 40.04.110 assigns each of its roughly two dozen zoning districts its own minimum Open Space Ratio, so allowable building and paving coverage varies by district. The UDC applies only in unincorporated New Castle County, Wilmington and Newark enforce their own separate zoning codes, not this chapter.
Violations & Fines
Exceeding the 20 percent WRPA impervious cap without an approved environmental impact assessment and RPATAC recommendation is a UDC violation, enforceable by a stop-work order, administrative fines up to $100 per day, or a $500 criminal fine if no other penalty applies.
Frequently Asked Questions
Does New Castle County limit impervious cover on every lot?
Can I pave or build beyond 20 percent impervious cover in a WRPA?
Sources & Official References
Other rules in New Castle County
Compare New Castle County to another location·View the Delaware building setbacks & zoning overview
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