New Castle County, DE Environmental Rules: Grading & Drainage (2026)

Grading and drainage in New Castle County is governed by several overlapping authorities: the NCC building code under Chapter 6 (which adopts the International Building Code and International Residential Code with Delaware amendments), the Delaware Sediment and Stormwater Program (7 Del.C. Chapter 40) administered through the New Castle Conservation District, and UDC Article 40 for site-plan-level review. Grading permits are required for earth-moving activities exceeding specified thresholds, typically 100 cubic yards of excavation or fill, or any grading that changes drainage patterns affecting off-site conditions. Delaware common law recognizes the Modified Civil Law Rule for surface water drainage: upper landowners may discharge natural drainage to lower properties, but they cannot artificially concentrate, accelerate, or redirect water onto neighbors in ways that cause damage. Diverting drainage onto a neighbor can result in injunction and damages. Retaining walls over 4 feet (measured from bottom of footing) require engineered plans sealed by a Delaware-licensed engineer and a separate permit. Walls must not impede natural drainage and must provide adequate weep holes or subsurface drainage. Site grading must direct drainage away from primary structures (minimum 5% slope for 10 feet per IRC Section R401.3) and into approved conveyance systems, not onto neighbors. Compaction testing required for structural fill. Floodplain rules (NCC floodplain ordinance consistent with FEMA NFIP) impose additional controls on fill in floodways.