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Zoning Guide

ADU Rules in New Castle, Delaware

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ADU Rules in New Castle, Delaware

New Castle is one of Delaware's oldest and most historic cities, serving as Delaware's colonial capital. Located on the Delaware River in New Castle County, it has a nationally recognized historic district featuring 17th and 18th century architecture. The city has its own municipal government separate from New Castle County. Delaware has no statewide ADU preemption law.

Permitted ADU Types

  • Detached ADU: Standalone structure on the residential lot
  • Attached ADU: Addition with own entrance and kitchen
  • Interior Conversion: Converting existing interior space

Size Limits

ADU Type Maximum Size
Detached ADU 800 sq ft or 40% of primary dwelling
Attached ADU 40% of primary dwelling floor area
Interior Conversion Existing footprint

Setbacks

Per New Castle zoning ordinance:

  • Front: Per base zone standard
  • Side: 5 feet minimum
  • Rear: 10–15 feet minimum

Height

  • Detached ADU: Maximum 15–18 feet
  • Cannot exceed height of primary dwelling

Historic District Requirements

New Castle's historic district is one of the best-preserved colonial-era landscapes in the United States:

  • Properties within the Historic Overlay Zone require approval from the Historic District Commission
  • ADUs must be compatible with historic character — materials, massing, and design are reviewed
  • Rear structures (carriage houses, outbuildings) may have more flexibility

Flood Zone Considerations

New Castle's Delaware River location puts some areas in FEMA flood zones. Verify flood zone status before planning.

Parking

One off-street parking space required per ADU.

Permit Process

  1. Check historic overlay: Determine if property is in historic district
  2. Historic Commission review: If applicable, submit for design review
  3. Contact City Hall: Verify current ADU standards
  4. Submit plans: Site plan and architectural drawings
  5. Building permit: Issued by City of New Castle
  6. Inspections and CO: Required before occupancy

Contact

City of New Castle 220 Delaware Street, New Castle, DE 19720 Phone: (302) 322-9800 Website: newcastlecity.delaware.gov

Disclaimer: Verify current requirements with the City of New Castle, especially for properties in or near the historic district.

More about New Castle Zoning

Sources

  1. City of New Castle Code of Ordinances·library.municode.com·Accessed 2026-04-07·Direct link
  2. City of New Castle, Delaware·newcastlecity.delaware.gov·Accessed 2026-04-07·Direct link

FAQ

Are ADUs allowed in New Castle, Delaware?
Yes. New Castle is one of Delaware's oldest cities and has its own municipal zoning authority separate from New Castle County. ADUs are permitted in residential zones. Delaware has no statewide ADU preemption law.
What historic restrictions apply to ADUs in New Castle?
New Castle has a significant historic district along the Delaware River waterfront, including one of the best-preserved colonial-era townscapes in the US. Properties in historic zones may require design review approval.