New Castle County

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.

Process overview: ADU approval in unincorporated New Castle County is a combined zoning / building review. Typical sequence: (a) owner obtains the ADU Permit Requirements handout and the ADU Property Owner Affidavit from the Department of Land Use; (b) owner submits a site plan, interior floor plans, and the signed Affidavit documenting owner-occupancy of one of the two units; (c) zoning reviews for Chapter 40 conformance (lot size >=0.5 acres for detached, setbacks matching principal dwelling, rear-of-front-building-line placement, parking, and cap availability — the department tracks the running 0.4% cap and may refuse a permit if the cap is exhausted); (d) building review for Chapter 6 (building code) and Chapter 12 (plumbing / mechanical) conformance; (e) permit issuance; (f) construction with inspections; (g) certificate of occupancy. The Property Owner Affidavit is recorded as a land-use restriction against the parcel; short-term rental and simultaneous double-rental of both units remain prohibited.

Impact fees: New Castle County's permit-fee schedule applies the standard residential permit fee to ADU construction scope; the UDC does not levy a distinct ADU impact fee. School, sewer, and water-connection fees operate under separate authorities (New Castle County Special Services Department for sewer; Artesian Water, United Water, or municipal water providers for water; Red Clay / Christina / Colonial school districts for school-related frameworks where applicable). The 0.4% countywide cap functions as a de facto quantity limit rather than a cost lever. (schedule)

The New Castle County Office of Finance — Assessment Division maintains parcel-level assessment records for all real property in New Castle County, including parcels inside incorporated municipalities (the county is the assessing authority; municipalities levy separately against the county's assessed value). New Castle County completed a general reassessment in 2024 under a court-supervised settlement; property values had not been updated since 1983. Fair market value as of 2024-07-01 is the new assessment base. Average residential assessments rose approximately 477% and non-residential approximately 233% in the reassessment. The county set a post-reassessment residential tax rate of approximately $0.1575 per $100 of assessed value (15.75 cents) and $0.2380 per $100 for non-residential, to approximate revenue-neutrality against the new base. The reassessment generated significant legislative attention — an October 2025 call for a reassessment redo and a General Assembly special session on a tax-bill reset — but the 2024 base values remain the assessment of record as of 2026-04-20.

Assessment policy: ADU improvements are captured as improvements to the host parcel on the next reassessment cycle or via a mid-cycle review triggered by building-permit data reported by the Department of Land Use. The Tyler Technologies mass-appraisal methodology used in the 2024 reassessment applies uniformly across residential properties. There is no New Castle County or Delaware state ADU-specific assessment exemption; the owner-occupancy affidavit required by the UDC does not by itself confer any tax benefit beyond the general senior / disabled property-tax-credit programs administered by the county and the school districts.

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.