Everything homeowners ask about building an ADU in Trenton — answered with 2026 data.
Yes, ADUs are allowed in Any lot, in any district, that contains one single-family dwelling (§ 315-9.4B.1). An ADU is an accessory use under Article 9, not a conditional use or use-matrix item, so a compliant unit needs no Planning Board hearing. One caution: the LDO defines 'Dwelling - Single-Family' as a detached one-unit structure and lists single-family attached and rowhouse dwellings separately, so get a written zoning determination ($75, § 315-19.4) before designing an ADU behind a rowhouse. in Trenton, NJ. The maximum size for a detached ADU is 800 sq ft gross floor area (§ 315-9.4B.7), and in no case more than 40% of the primary dwelling's floor area or 2 bedrooms (§ 315-9.4B.8). GFA is measured to the exterior faces of the exterior walls (§ 315-2.4F). The footprint of a detached accessory structure also cannot exceed the footprint of the principal building (§ 315-9.4A.5). Attached ADUs can be up to Same limits as detached: 800 sq ft GFA, capped at 40% of the primary dwelling's floor area, maximum 2 bedrooms. Trenton has an ADU-friendliness score of 6/10. Check our full Trenton ADU rules page for complete zoning details.
ADU construction in Trenton typically costs $160,000-$300,000. The cost per square foot averages $200-$375. Garage conversions are more affordable at $60,000-$170,000, while prefab ADUs cost $120,000-$250,000 installed. Permit fees add $50 zoning permit (charged only when no other permit is required) and $75 for a written zoning determination (§ 315-19.4). Construction permit fees are 'as per City Code and defined under UCC' and scale with the work; get the current schedule from Technical Services. to your budget. See our detailed Trenton ADU cost guide.
Detached ADUs in Trenton can be up to 800 sq ft gross floor area. Attached ADUs are limited to Same limits as detached: 800 sq ft GFA, capped at 40% of the primary dwelling's floor area, maximum 2 bedrooms. Junior ADUs (JADUs) have a cap of Trenton has no separate JADU category. A basement or attic apartment with its own kitchen, bath and entrance is simply an ADU 'within' the primary dwelling under § 315-9.4B.5 and carries the same 800 sq ft / 40% / 2-bedroom limits and the 4,000 sq ft lot minimum. Maximum height is 20 ft for a detached accessory structure, detached ADUs included (§ 315-9.4A.2). An attached or interior ADU is part of the principal building and follows the district height limit (40 ft in RL-1 through RM-2, Table 4-1). You are allowed up to One ADU per lot on a qualifying lot.
Rear setback: 10 ft from the rear lot line for a detached ADU (§ 315-9.4B.6). Setbacks for roofed structures are measured from the edge of the roofline, eaves included (§ 315-9.4A.3.d). Side setback: 3 ft from an interior side lot line for a detached ADU (§ 315-9.4B.6); other enclosed detached accessory structures need 3 ft from any lot line, 1 ft where the line abuts an alley (§ 315-9.4A.3.a). An ADU inside a detached garage follows the garage rules in § 315-9.4J, including 20 ft from the lot line the driveway enters from unless the garage is alley-accessed (then 1 ft from the rear line). Front setback: A detached ADU is not permitted in a front yard or corner side yard (§ 315-9.4B.6). That bar does not apply to an ADU built within a detached garage; a detached garage must itself sit at least 5 ft behind the front building line and outside the front yard (§ 315-9.4J.1). Note that garage conversions may be exempt from setback requirements if the existing structure is already within the setback area.
No additional parking is required for the ADU (§ 315-9.4B.10). Many cities and states have been relaxing ADU parking requirements in recent years. In California, no additional parking is required for ADUs near transit.
Required: one of the two dwelling units must be occupied by the owner of the property (§ 315-9.4B.2). Note that owner-occupancy rules vary by state law — California and several other states have eliminated this requirement for standard ADUs.
The ADU permit review timeline in Trenton is typically Not published. Neither the Division of Planning nor the Division of Technical Services publishes a zoning-permit or ADU review time; ask both when you file. Plan check: Standard NJ Uniform Construction Code plan review by the building, electrical, plumbing and fire subcode officials. At zoning review the ADU is checked against the district's building coverage (35-50%) and impervious surface (45-70%) caps in Table 4-1, which count accessory structures (§ 315-9.4A.4), and a detached unit's footprint against the house footprint (§ 315-9.4A.5). Pre-approved plans availability: None published. The Division of Planning offers no ADU handout, pre-approved plan set or ADU-specific application form (checked September 2026).
Average ADU rental income in Trenton: 1-bedroom units rent for $1,400-$1,800/month, 2-bedroom units for $1,750-$2,250/month. Short-term rentals: The LDO does not address short-term rental of an ADU, and one unit must stay owner-occupied (§ 315-9.4B.2). Confirm rental registration and any short-term rules with the Division of Housing Inspections. Expected ROI: 7-11% gross annual return on a long-term rental before tax and financing: $1,600-$2,000/mo against a $160,000-$300,000 build. HUD FY2027 SAFMRs are 40th-percentile rents; a new unit usually rents above them.
The minimum lot size for an ADU in Trenton is 4,000 sq ft (§ 315-9.4B.3). That is above the 2,500 sq ft single-family lot minimum in the RM-1/RM-2 districts and the 1,500 sq ft rowhouse minimum (Table 4-1), so a large share of Trenton lots do not qualify without a C variance ($300 application fee, § 315-19.4). Check your parcel area before anything else. This applies to detached ADUs — garage conversions and JADUs may have different or no lot size requirements.
Garage conversions are one of the most popular and affordable ADU options. In Trenton, a garage conversion typically costs $60,000-$170,000. The key advantages are lower cost (no foundation work), potential setback exemptions, and faster permitting. Read our complete garage conversion guide.
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