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New Castle County, DE Accessory Structures: ADU Rules (2026)

Some Restrictions

Compiled from the official code textEditor Martyn O'NeillLast verified September 2026

ADU rules in New Castle County, DE, also called accessory dwelling unit regulations or granny flat ordinances, cover setbacks, owner-occupancy, parking, and permit requirements.

Key Facts

ADU limit: One ADU per single-family detached lot
Occupancy: Owner must live in home or ADU
Recording: Deed restriction filed with Recorder
Parking: One extra off-street space required
Rentals: Chapter 19 registration if rented

ADU Eligibility Quick-Check

An Accessory Dwelling Unit (ADU) is a smaller, independent living unit on the same lot as a single-family home: converted garages, detached cottages, basement or attic conversions, and attached additions all qualify. Most states have streamlined ADU permitting since 2020, but local rules still control size, height, and setback.

Is your lot zoned single-family or multi-family residential?
Does your lot have at least 1,200 sq ft of buildable space?
Is your existing home owner-occupied (in most states this is not required, but some require it for ADU permits)?
Will the ADU stay under your state's size cap (commonly 850-1,200 sq ft for detached)?
Can you provide one off-street parking space (waived in many states within 0.5 mi of transit)?

If you answered 'yes' to most of these, you likely qualify in Delaware under recent ADU streamlining laws. The New Castle County, DE rules below cover local size, height, and setback requirements.

Summary

New Castle County allows one accessory dwelling unit per single-family detached lot, but the owner must live in the main house or the ADU, record a residency restriction with the Recorder of Deeds, and get a building permit before occupying it.

These county ordinances apply to unincorporated areas of New Castle County. Incorporated cities within the county may have their own rules that take precedence over county-level regulations.

Only one (1) ADU is permitted per single-family residential detached lot. The ADU may be constructed within the home or as an attached addition to the home... The property owner shall occupy and reside in either the principal dwelling unit or the ADU as his or her principal legal residence. An affidavit (available from the Department) of owner occupancy shall accompany the application for a building permit for the ADU.

View official code

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

Chapter 40 of the New Castle County Code permits one ADU per single-family detached lot, built within the home, as an attached addition, or as a detached structure on lots of a half-acre or more meeting the same setbacks as the principal dwelling. The property owner must occupy either the principal unit or the ADU as a primary residence, file an owner-occupancy affidavit with the building permit application, and record a residency-restriction document with the New Castle County Recorder of Deeds before the permit issues. One extra off-street parking space is required, adequate sewer or septic capacity must be confirmed, and if the ADU will be rented it must be registered under Chapter 19 (Residential Rental Properties) before a certificate of occupancy is issued. This UDC provision governs unincorporated New Castle County; Wilmington and Newark are incorporated cities with their own zoning codes and are not covered by this chapter.

Violations & Fines

Building or renting an ADU without a permit, owner-occupancy affidavit, or rental registration violates the Code. Fines run $100 for a first offense, up to $1,000 and 30 days for repeats within a year, each day counted separately.

Frequently Asked Questions

Can I build a detached ADU in New Castle County?

Yes, but only on lots of a half-acre or larger, and the detached ADU must meet the same front, side, and rear setbacks as the principal dwelling.

Do I have to live on the property with the ADU?

Yes. The owner must occupy either the principal dwelling or the ADU as a primary legal residence, confirmed by an affidavit filed with the building permit.

Does this ADU rule apply inside Wilmington or Newark?

No. Wilmington and Newark are incorporated cities with their own municipal zoning codes; this UDC section governs unincorporated New Castle County only.

Sources & Official References

[1]New Castle County Code § 40.03.410.I (Municode)

Other rules in New Castle County

Shed & Carport RulesSome Restrictions ADU Owner OccupancySome Restrictions Tiny HomesSignificant Restrictions ADU Rental RestrictionsSome Restrictions Landscaping Rules: CompostingLight Restrictions Environmental Rules: Coastal DevelopmentSignificant Restrictions Noise from Specific Sources: Delivery Truck NoiseSome Restrictions Code Violation Reporting: How to ReportSome Restrictions Water Use Rules: Leak Reporting DutySome Restrictions Sign Regulations: Window SignsLight Restrictions

All New Castle County rules

How New Castle County compares: Easiest Cities to Build an ADU·Compare New Castle County to another location·View the Delaware accessory structures overview

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ADU Rules in Cities Across New Castle County

Wilmington, DE