Yes: Trenton allows one accessory dwelling unit by right on a lot with one single-family dwelling under § 315-9.4B of its Land Development Ordinance (adopted December 21, 2023), as an accessory use approved by the Zoning Officer rather than at a Planning Board hearing. The unit is capped at 800 sq ft of gross floor area, may not exceed 40% of the main house's floor area or two bedrooms, and the lot must be at least 4,000 sq ft; one of the two units must be owner-occupied, and no extra parking is required. A detached ADU sits 3 ft from an interior side line and 10 ft from the rear line, stays under 20 ft, and cannot go in the front or corner side yard unless it is built inside a detached garage. The paperwork is a $50 zoning permit (§ 315-19.4) followed by NJ Uniform Construction Code permits from the Division of Technical Services; a lot under 4,000 sq ft needs a $300 C variance from the Zoning Board. New Jersey has no statewide ADU law: bills S1786/A3488, introduced in January 2026, had not been enacted as of September 20, 2026.
Zoning & Size Requirements [1][2][3][4]
Yes — Trenton allows ADUs. A detached ADU can typically be up to 800 sq ft under current New Jersey and Trenton zoning rules.
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| Allowed Zones | 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. |
| Max Size (Detached) | 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). |
| Max Size (Attached) | Same limits as detached: 800 sq ft GFA, capped at 40% of the primary dwelling's floor area, maximum 2 bedrooms (§ 315-9.4B.7-8). The building must keep the appearance of a house, with the ADU entrance unobtrusive from the side the main entrance faces (§ 315-9.4B.9). |
| Max Size (JADU) | 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. |
| Max Height | 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). |
| Max Units | One ADU per lot (§ 315-9.4B.4), and only on a lot with one single-family dwelling (§ 315-9.4B.1). A two-family house or a converted single-family (§ 315-4.5) is not eligible. |
Setback Requirements [1][2][3][4]
Trenton requires a rear setback of about 10 ft for a detached ADU, and no additional parking is required. Owner-occupancy is required.
| 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). |
| Parking Required | No additional parking is required for the ADU (§ 315-9.4B.10). |
| Owner Occupancy | Required: one of the two dwelling units must be occupied by the owner of the property (§ 315-9.4B.2). |
| Min Lot Size | 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. |
Permit Process & Fees [5][6][7][8]
An ADU permit in Trenton goes through a standard permit review. Review time: 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. Fees range from $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.
| Approval Process | A compliant ADU is approved administratively: the Zoning Officer issues a zoning permit (§ 315-13.12); the Division of Technical Services then issues NJ Uniform Construction Code permits (building, electrical, plumbing, fire), and no construction permit issues until the zoning permit is approved (§ 315-13.13B); a certificate of occupancy from the Construction Official closes the job (§ 315-13.14). A design that misses a § 315-9.4B standard, most often the 4,000 sq ft lot, needs a C variance from the Zoning Board of Adjustment, filed with the Development Application Packet. |
| Review Timeline | 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. |
| Permit Fees | $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. |
| Impact Fees | None in the LDO: Article 19 lists only application, escrow and administrative fees, with no development or impact fee for an added dwelling unit. Water and sewer connection charges for a new detached unit come from Trenton Water Works and the Construction Office; confirm them before pricing the job. |
| 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 | None published. The Division of Planning offers no ADU handout, pre-approved plan set or ADU-specific application form (checked September 2026). |
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Construction Costs [9][10][11][12]
Building an ADU in Trenton typically costs $160,000-$300,000 ($200-$375 per sq ft); a garage conversion runs $60,000-$170,000.
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| Average Build Cost | $160,000-$300,000 |
| Cost per Sq Ft | $200-$375 |
| Garage Conversion | $60,000-$170,000 |
| Prefab ADU (Installed) | $120,000-$250,000 installed |
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Rental Income Potential [13][14][15]
A 2-bedroom ADU in Trenton rents for around $1,750-$2,250/mo, an estimated 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.
| Avg Rent (1 Bedroom) | $1,400-$1,800/mo |
| Avg Rent (2 Bedroom) | $1,750-$2,250/mo |
| Short-Term Rental | 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. |
| Est. Annual 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. |
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Recent Law Changes
What's Changed Recently
- Re-verified September 2026 against the adopted Land Development Ordinance (December 21, 2023) posted by the Division of Planning: every § 315-9.4B standard (800 sq ft, 40% of the house, 2 bedrooms, 4,000 sq ft lot, owner-occupancy, 3 ft / 10 ft setbacks, no parking) is unchanged.
- No ADU or Chapter 315 text amendment reached the Planning Board in 2026: all 18 Planning Board agendas from January 8 to September 24, 2026 list 'Master Plan Consistency Review(s): None', and the March 12, 2026 agenda still treats the 2023 LDO as the current ordinance.
- Fees confirmed from § 315-19.4 as reproduced in the July 2025 Development Application Packet: $50 zoning permit, $75 zoning determination, $300 first C variance; construction permit fees are set by City Code under the UCC.
- Statewide: New Jersey still has no ADU mandate. S2347 expired with the 2024-25 session; successor bills S1786/A3488, introduced January 2026, had not been enacted as of September 20, 2026.
Tips for Building an ADU in Trenton
- Check your lot area first. The 4,000 sq ft minimum in § 315-9.4B.3 disqualifies a large share of Trenton's parcels (the RM districts allow single-family lots down to 2,500 sq ft), and no design work is worth doing until you have confirmed it on your survey or the tax map.
- The 800 sq ft cap is rarely the binding limit. § 315-9.4B.8 also caps the ADU at 40% of the primary dwelling's floor area: on a 1,400 sq ft house that is 560 sq ft. Work out both numbers and build to the smaller one. Two bedrooms is a hard ceiling regardless of floor area.
- Rowhouse owners: § 315-9.4B.1 requires a lot with 'one single-family dwelling', and the LDO's definitions treat a single-family (detached) dwelling, a single-family attached dwelling and a rowhouse as different things. Ask the Zoning Officer for a written zoning determination ($75) before spending on drawings.
- A detached ADU cannot go in the front yard or a corner side yard, but that restriction is waived if you build inside a detached garage (§ 315-9.4B.6). If your garage sits forward on the lot, converting it may be the only way to use that footprint; the garage rules in § 315-9.4J still apply.
- Your detached ADU's footprint cannot exceed the footprint of the main house (§ 315-9.4A.5), and it counts toward the district's building coverage and impervious surface caps (§ 315-9.4A.4, Table 4-1). A wide single-storey unit behind a narrow house is where people get caught.
- No extra parking space is required (§ 315-9.4B.10). That is worth real money on a tight Trenton lot and is not the rule in most surrounding Mercer County municipalities.
- If your lot is under 4,000 sq ft, the only route is a C (bulk) variance from the Zoning Board of Adjustment: a $300 application fee plus escrow and administrative deposits under §§ 315-19.4 to 315-19.6, a public hearing, and no guarantee. The Board's 2026 meeting calendar is on the city site.
- Because a compliant ADU is an accessory use, you should not need a Planning Board hearing. If you are told to apply for a variance, ask which specific § 315-9.4B standard your design fails.
- Cost basis: ranges are dated New Jersey industry guides, not City figures. The 5th Wall (May 27, 2026): $175-$375 per sq ft for a ground-floor addition, $90,000-$300,000 for an in-law suite/ADU. Prodigy Real Estate NJ ADU Playbook (May 9, 2026): $200,000-$400,000+ detached, with Hudson/Essex labour 15-25% above the state average (Trenton sits nearer the average). Magdev 2026 NJ guide: $95,000-$170,000 for a full garage ADU. DC Structures: $150,000-$250,000+ completed prefab. Rents: HUD FY2027 Small Area FMRs for Trenton ZIPs 08608-08638 (1BR $1,400-$1,770, 2BR $1,760-$2,250, effective October 1, 2026) and RentCafe, August 31, 2026 (1BR $1,446, 2BR $1,972, rents up 6.1% in a year).
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Sources & References
All data on this page is sourced from official government records and verified market data. View our full methodology.
- City of Trenton Land Development Ordinance, adopted December 21, 2023 (codified as Chapter 315): § 9.4 Accessory Structures and Uses incl. § 9.4B Accessory Dwelling Unit, § 9.4J Detached Garage, § 2.4F Gross Floor Area, Table 4-1 Residential Districts Dimensional Standards, Art. 13 permits, Art. 19 fees Municipal Code — Accessed 2026-09-20
- Land Development Ordinance page, City of Trenton Division of Planning (links the adopted 12/21/2023 LDO and zoning map) Planning Department — Accessed 2026-09-20
- City of Trenton Code § 315-9.4 Accessory Structures and Uses (incl. § 315-9.4B Accessory Dwelling Unit), eCode360 (Cloudflare-walled on 2026-09-20; text verified from the adopted ordinance PDF) Municipal Code — Accessed 2026-08-20
- Trenton Planning Board agendas, January 8 to September 24, 2026 (18 meetings; Master Plan Consistency Reviews: none; no Chapter 315 amendment referred) Planning Department — Accessed 2026-09-20
- Development Application Packet, City of Trenton Division of Planning (July 2025), reproducing § 315-19.4 fee schedule and § 315-19.5 escrow fees Planning Department — Accessed 2026-09-20
- Applying for Development, City of Trenton Division of Planning Planning Department — Accessed 2026-09-20
- Division of Technical Services (Uniform Construction Code permits, subcode contacts), City of Trenton Planning Department — Accessed 2026-09-20
- Construction Permit Application (GovPilot online form), City of Trenton Planning Department — Accessed 2026-09-20
- NJ Home Addition Cost 2026, The 5th Wall (May 27, 2026): $175-$375 per sq ft ground-floor addition; in-law suite/ADU $90,000-$300,000 industry — Accessed 2026-09-20
- The 2026 New Jersey ADU Playbook, Prodigy Real Estate (May 9, 2026): detached $200,000-$400,000+; garage conversion $120,000-$220,000 industry — Accessed 2026-09-20
- Garage Conversion Cost in NJ & PA (2026 Guide), Magdev Inc.: full ADU/in-law suite $95,000-$170,000 industry — Accessed 2026-09-20
- New Jersey Prefab ADUs, DC Structures: completed kit projects $150,000-$250,000+; NJ detached ADUs $160,000-$300,000 industry — Accessed 2026-09-20
- HUD FY2027 Small Area Fair Market Rents, Trenton-Princeton, NJ MSA ZIP codes 08608-08611, 08618, 08629, 08638 (1BR $1,400-$1,770; 2BR $1,760-$2,250; effective October 1, 2026) federal_data — Accessed 2026-09-20
- HUD FY2027 Fair Market Rents, Trenton-Princeton, NJ MSA (Mercer County): 1BR $1,764, 2BR $2,242 federal_data — Accessed 2026-09-20
- Average Rent in Trenton, NJ, RentCafe / Yardi Matrix (last updated August 31, 2026): 1BR $1,446, 2BR $1,972 industry — Accessed 2026-09-20
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