Lancaster Ave

New Castle County regulates ADUs on parcels in unincorporated areas under Chapter 40 of the County Code — the Unified Development Code (UDC). ADUs entered the UDC through Ordinance 07-001, adopted 2007-04-10 and effective 2007-04-20, which added an ADU permit pathway and a countywide cap. Ordinance 16-117 (adopted 2017-02-28, effective 2017-03-03) raised that cap from 0.2% to 0.4% of single-family detached dwellings in the unincorporated area. Ordinance 24-087 (adopted 2024-09) reduced the minimum detached-ADU lot size from 2 acres to 0.5 acres. Through December 2024 a cumulative 357 ADU building permits had been issued since 2007 (244 north of the C&D Canal, 113 south of the canal; 85 of the 357 were detached). The UDC was last consolidated 2025-07-25 with a subsequent amendment on 2026-03-10.

• New Castle County Code Chapter 40 — Unified Development Code (Article 3 Use Regulations, ADU provisions)
• ADU Permit Requirements handout — New Castle County Department of Land Use
• Ordinance 07-001 — original ADU ordinance
• Ordinance 16-117 — ADU cap raised to 0.4%
• Ordinance 24-087 — detached-ADU minimum lot size reduced to 0.5 acres

State-floor overlay: Delaware has NOT enacted a statewide ADU preemption law. SS1 for SB 23 (152nd General Assembly) died in committee when the session ended 2024-06-30; SB 87 (153rd General Assembly, chief sponsor Sen. Russ Huxtable) was reported favorably with Senate Amendment 1 on 2025-05-07 but was tabled at the full Senate floor and has not been taken up in the 2026 portion of the session as of 2026-04-20. In the absence of state preemption, New Castle County UDC Chapter 40 is the authoritative ADU framework for unincorporated New Castle County. Any SEO-content-site description of Delaware SB 23 as enacted law with an April 2024 effective date is incorrect (SB 23 is a bill number that existed under the 152nd General Assembly and died in committee, not an adopted Delaware law).

New Castle County administers floodplain, historic, and environmental overlays that cut across multiple municipalities and its unincorporated areas. The county's floodplain-management provisions are in Chapter 40 Article 10 (Environmental Standards) and implement the FEMA National Flood Insurance Program; the county contains 2,431 structures in the 100-year floodplain (Delaware's largest share), with the Christina River accounting for 1,007 of those structures and 44 floodplain road-miles. The Delaware Piedmont in northern New Castle and the tidal Delaware River / C&D Canal frontage in the south are the two principal flood-hazard geographies. Delaware does not have a CalFire-equivalent statewide WUI wildfire regime; seismic hazard is low.

• New Castle County Floodplain Regulations (UDC Chapter 40 Article 10) — Implements the FEMA National Flood Insurance Program for unincorporated New Castle County. ADUs proposed in a Special Flood Hazard Area (Zone A, AE) face elevation, venting, anchoring, and lowest-floor-elevation-certificate requirements. Coastal A and V zones apply along the tidal Delaware River reaches; the city of New Castle maintains a levee but the county-level overlay still applies outside municipal levee districts.
• FEMA Flood Insurance Rate Maps — New Castle County and Incorporated Areas — FEMA's preliminary flood risk maps provide 100-year flood information for several New Castle County watersheds that had not been previously mapped, including tributaries of the Christina, Brandywine, and Red Clay Creeks. County hosted informational meetings in May 2025 on the preliminary map updates.
• Delaware Coastal Zone Act jurisdiction — Delaware's Coastal Zone Act restricts heavy industrial uses within a defined coastal strip along the Delaware River and Bay; it does not substantively regulate single-family residential or ADU development. Parcels near the C&D Canal, Delaware River, or tidal wetlands should confirm wetlands-delineation compliance via DNREC before siting a detached ADU.
• DNREC Wellhead Protection / Source Water Protection Areas — DNREC administers source-water protection overlays that constrain on-site sewage design within delineated wellhead protection areas. New Castle County ADUs on parcels outside public-sewer service areas require DNREC on-site-wastewater permitting per Title 7 before building-permit issuance.
• Local historic districts — New Castle County does not operate a county-wide historic-preservation overlay for unincorporated parcels, although several unincorporated communities (Arden, Ardentown, Ardencroft) have leasehold / architectural review traditions with de facto historic character. Historic-district review operates at the municipal level inside incorporated cities; the City of New Castle's colonial core is within a federally designated National Historic Landmark district.

Permits for ADUs on parcels in unincorporated New Castle County are issued by the New Castle County Department of Land Use, which combines planning / zoning review, building-code plan review, and inspections. The department serves unincorporated parcels. Wilmington, Newark, New Castle, Middletown, Delaware City, Odessa, Port Penn, and Townsend are incorporated municipalities with their own municipal permitting pathways; Bear, Claymont, Hockessin, Montchanin, Rockland, Saint Georges, and Yorklyn are unincorporated CDPs / communities permitted through the county.