Trenton, NJ Accessory Structures: ADU Rules (2026)
ADU rules in Trenton, NJ, also called accessory dwelling unit regulations or granny flat ordinances, cover setbacks, owner-occupancy, parking, and permit requirements.
Key Facts
- ADUs per lot
- 1 maximum
- Minimum lot size
- 4,000 square feet
- Max ADU floor area
- 800 square feet
- Max share of primary dwelling
- 40% of floor area
- Max bedrooms
- 2
- Detached ADU setbacks
- 3 ft side, 10 ft rear
- Added parking required
- None
Summary
Trenton allows one accessory dwelling unit per single-family lot under City Code § 315-9.4.B, capped at 800 square feet and 40% of the primary dwelling's floor area. The lot must be at least 4,000 square feet, and a detached ADU needs a 3-foot side setback and a 10-foot rear setback, with no added parking required.
1. An ADU may be located only on a lot with one single-family dwelling. ... 3. A lot must have a minimum area of 4,000 square feet to qualify for an ADU. 4. Only one ADU is permitted per lot. 5. The ADU may be within, attached to, or detached from the primary dwelling structure and may be a part of a detached accessory structure. An ADU may be developed within an existing structure or as new development. 6. A detached ADU must be set back three feet from an interior side lot line and ten feet from a rear lot line. A detached ADU is not permitted in a front yard or corner side yard. This does not apply to an ADU constructed within a detached garage. 7. An ADU is limited to a maximum gross floor area of 800 square feet. 8. In no case may an ADU exceed 40% of the primary dwelling floor area nor more than 2 bedrooms. ... 10. No additional parking is required for the ADU.
Official source re-checked September 7, 2026: no newer edition of the code had been published (publisher’s edition: rev 4853025; v21 updated 2025-12-16; through 12-16-2025).
Full Breakdown
B rather than a separate zoning district. An ADU is allowed only on a lot holding one single-family dwelling, limited to one ADU per lot, and the lot must have a minimum area of 4,000 square feet to qualify. The unit can be built within, attached to, or detached from the primary dwelling, or carved out of a detached accessory structure such as a garage, and it may be created in an existing structure or as new construction. Size is capped at 800 square feet of gross floor area, and in no case may it exceed 40% of the primary dwelling's floor area or contain more than two bedrooms.
A detached ADU must be set back three feet from an interior side lot line and ten feet from a rear lot line, and it is not permitted in a front yard or corner side yard, though that restriction does not apply to an ADU built inside a detached garage. Design standards require the primary structure to keep the outward appearance of a single house, with the ADU entrance placed so it is not obtrusive from the same vantage point as the principal dwelling's entrance.
B(10) removes the usual off-street parking requirement for the ADU itself, so adding the unit does not trigger an additional parking space on the lot.
Violations & Fines
Building an ADU without meeting the lot-size, size-cap or setback standards in § 315-9.4.B can result in denial of the required building or zoning permit under § 315-13.12, and an ADU built without permits is enforced like any other zoning violation. Where Chapter 315 sets no specific fine, the citywide general penalty in § 1-17 applies: up to $2,000, up to 90 days in jail or community service, with each day counted separately under § 1-18.
Frequently Asked Questions
Can I build an accessory dwelling unit on any lot in Trenton?
How big can a Trenton ADU be?
Does a Trenton ADU need its own parking space?
Sources & Official References
Other rules in Trenton
How Trenton compares: Easiest Cities to Build an ADU·Compare Trenton to another location·View the New Jersey accessory structures overview
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