ADU rules in Trenton

Trenton's Land Development Ordinance allows a detached ADU as an accessory use with no district restriction of its own, but only where two objective gates are met: § 315-9.4.B(1) allows an ADU only on a lot with one single-family dwelling, and § 315-9.4.B(3) requires the lot to have a minimum area of 4,000 square feet. Single-family dwellings are a permitted use only in the seven residential districts (RL-1, RL-2, RL-3, RM-1, RM-2, RH-1, RH-2) under Table 8-1, so no ADU is possible on a two-family, rowhouse or multi-family lot, or in a commercial or industrial district. The 4,000 square foot floor is the gate that bites: RL-3's own minimum lot area is 4,000 square feet and RL-1's and RL-2's are larger, but a conforming single-family lot in RM-1, RM-2, RH-1 or RH-2 can be as small as 2,500 square feet (Table 4-1), which excludes much of Trenton's rowhouse fabric absent a C variance. A detached ADU must sit three feet from an interior side lot line and ten feet from a rear lot line, and is barred from the front yard and the corner side yard unless it is built inside a detached garage.

The same section covers interior and attached ADUs: § 315-9.4.B(5) states the ADU may be within, attached to, or detached from the primary dwelling structure and may be part of a detached accessory structure, and may be developed within an existing structure or as new development. So a basement, attic, or addition conversion is on exactly the same footing as a backyard cottage. The identical gates apply: one single-family dwelling on the lot, a lot of at least 4,000 square feet, one ADU per lot, and the design rule at § 315-9.4.B(9) that the primary structure must still read as a house with the ADU entrance unobtrusive from the view that shows the principal entrance. Note that an internal ADU is not the same thing as a residential conversion under § 315-4.5, which turns a single-family dwelling into two or more units, needs Planning Board conditional use approval, and carries its own 2,000 square foot existing-house minimum, per-unit floor area minimums and a one-space-per-new-unit parking requirement.

The headline caps sit in § 315-9.4.B(7) and (8): 800 square feet of gross floor area, which § 315-2.4.F measures as the sum of the gross horizontal areas of all floors from the exterior faces of the exterior walls, and separately no more than 40% of the primary dwelling floor area and no more than two bedrooms. Whichever of the 800 square foot and 40% tests is smaller controls, so a 1,500 square foot house caps its ADU at 600 square feet. A detached ADU also inherits the general accessory-structure rules of § 315-9.4.A: 20 feet maximum height, a footprint no larger than the principal building's, and inclusion in the district's maximum building coverage and impervious surface coverage (40% building coverage in RL-1, 35% in RL-2 and RL-3, 45% impervious in all three). An ADU inside or attached to the house is instead bounded by the district's 40 foot building height limit.

§ 315-9.4.B(2) is a single sentence: one of the dwelling units must be occupied by the owner of the property. The owner may live in either unit, the house or the ADU, and there is no carve-out for deed-restricted affordable units, unlike Asbury Park's ordinance. This reads as an ongoing occupancy condition on the use rather than a one-time test at application, but the LDO attaches no annual certificate, affidavit, or recorded-covenant machinery to it and names no enforcement route beyond the general zoning enforcement of Article 17 (Zoning Officer notice of violation, certificate of occupancy revocation under § 315-17.5). No New Jersey statute preempts a municipal owner-occupancy rule for ADUs as of 2026-09-04.

Two independent provisions land in the same place. § 315-9.4.B(10) exempts the ADU specifically, and § 315-10.4.A is a citywide rule headed 'No Minimum Vehicle Parking Spaces Required' providing that the provision of off-street vehicle parking is not required at all. The Transportation Demand Management carve-out in § 315-10.4.A applies to certain development actions and uses under § 315-10.5 and is determined during site plan review, which a single-family dwelling and its accessory buildings on one lot never undergo (§ 315-13.9.B(1)(b)), so it cannot reach an ADU. If an owner chooses to build parking anyway, Table 10-2 sets maximums rather than minimums. Contrast § 315-4.5.G, which does require one off-street space per additional unit, but that provision governs Planning Board conditional-use residential conversions, not ADUs.

The ADU is an accessory use, and § 315-8.2.D routes accessory uses to Article 9 rather than to the Table 8-1 principal use matrix, so no conditional use approval and no board hearing is involved. § 315-13.9.B(1)(b) exempts a single-family, single-family attached, or two-family dwelling and its accessory buildings on one lot from site plan review. Table 13-1 shows the resulting path is entirely administrative: a zoning permit submitted to and approved by the Zoning Officer (§ 315-13.12), then a construction permit and a certificate of occupancy from the Construction Official (§ 315-13.13, § 315-13.14). The standalone zoning permit fee is $50 where no other permit is required (Table 19-1 at § 315-19.4). Two situations flip this to a hearing: a lot smaller than 4,000 square feet or any other departure from § 315-9.4 requires a C variance from the Zoning Board of Adjustment (§ 315-13.7), and within a designated historic district a Certificate of Appropriateness under § 315-15.5 must issue before the Zoning Officer may grant zoning approval.

Nothing in § 315-9.4.B limits tenancy length, bars renting the ADU, or requires a family relationship; the only occupancy condition is the owner-occupancy rule at § 315-9.4.B(2), and lot subdivision to sell the ADU separately would need a subdivision approval under § 315-13.10 (the ADU is defined as an additional unit on the same lot as the principal single-family dwelling). Chapter 228 defines a short-term rental as any unit rented for a fee as a residential property for fewer than 30 consecutive days, requires registration with the Department of Inspections, a registration number on every listing, notice to residences within 300 feet, and insurance; § 228-7.B assumes the registrant is a primary resident of the unit and requires immediate notice if that stops. Renting long term brings Chapter 132 Article X into play: § 132-79.A(1) requires a nonoccupant owner of a rented structure to register annually and pay $110 per structure plus $35 per dwelling unit, with a family-occupancy exception at § 132-79.C, and § 132-85.A requires an inspection of a vacant unit before a new tenant moves in. Chapter 222 rent control caps increases at 4% (2% for senior citizen or disabled tenants) but its 'dwelling' definition exempts owner-occupied one- and two-family dwellings and lets a newly created unit set its own initial rent on first rental, while subjecting later increases to the chapter.