Compare ADU zoning rules across 4 New Jersey cities. Average ADU score: 6.8/10.
Gathered from the 4 cities below, with the bill or ordinance each one names. Statewide measures are listed once with the cities they reached, rather than repeated for each.
Statewide law sets a floor; the numbers that decide your project are local. Here is the spread across the 4 New Jersey cities covered here, taken from each city's own ordinance rather than a state summary.
| Question | Across New Jersey |
|---|---|
| Are ADUs permitted? | 4 of 4 permit them. |
| Detached size cap | Median 800 sq ft, from 300 to 1,800 sq ft across the 4 cities publishing a numeric cap. |
| Owner-occupancy | Required in 2 of 4. |
| Build cost | $160,000 to $400,000 depending on city and unit type. |
| Easiest and hardest here | Princeton scores 8/10; Trenton scores 6/10. |
Recomputed whenever a city page changes, so this and the city pages below cannot disagree.
| City | Max Size | Setbacks | Permit Fees | Build Cost | 1BR Rent | Score |
|---|---|---|---|---|---|---|
| Jersey City | No square-footage cap in the ordinance or the city's homeowner guide ('There is no limit on square footage of an ADU'). Size is set by the 18 ft/2-story accessory height limit, the 2-3 ft side setbacks and the lot-coverage cap: R-1 lot coverage rises by 5% when an ADU is present (§345-41 E.12.c), i.e. 90% on lots up to 1,800 sq ft, 80% on 1,800-3,200 sq ft, 90% on 3,200 sq ft or more; R-2 80% (§345-42 E.11); RH-1 75% lot / 55% building coverage (§345-40 E.9-10); RH-2 75% (§345-40.1 E.10). | R-1: no zoning rear setback for accessory buildings ('No standard', §345-41 E.12.b) — the city's guide notes 2 ft applies per the building code; R-2: 3 ft (§345-42 E.16.b); RH-1: 0 ft (§345-40 E.12.a); RH-2: no standard (§345-40.1 E.13.b). Separation from the main house is governed by fire, health and safety codes (§345-60 I.1.c; 2021 IBC New Jersey Edition). | Not published as an ADU figure — UCC construction-permit fees are set by city ordinance (volume-based for a new building) plus the state permit surcharge, and no ADU fee schedule could be read on the city site; get a written fee estimate from the Division of Construction Code Official. | $200,000-$400,000 | $2,200-$3,800/mo | 7/10 |
| Montclair | Living area no more than 40% of the primary dwelling unit, minimum 300 sq ft, maximum 800 sq ft, no more than three rooms excluding kitchen and bathroom (§347-8 D(3)); a fully accessible adaptive-reuse ADU may add 5% (D(8)). Detached ADUs also follow §347-22: rear yard only, at most 25% of the rear yard (15% in R-O/R-O(a)), footprint up to 24 ft in any direction (36 ft in R-O/R-O(a)); Ord. O-25-29 (Aug 2025) changed the ADU dimension rule - confirm the current figure with the Zoning Officer. | Detached ADU in R-1: 6 ft minimum rear yard setback for accessory structures (§347-46 A(3)); R-O: 10 ft (§347-34 A(3)). R-O(a) and R-2 accessory-structure setbacks: confirm with the Zoning Officer. An accessory structure within 4 ft of the house counts as part of it and must meet principal-building setbacks (§347-22 E). | $50 ADU zoning permit fee, by check or money order (Township ADU Zoning Permit Application, unchanged since 2023). Construction permits and the ADU certificate of occupancy are charged under the township's current UCC fee schedule (§347-8 E(5)(b)) - ask the Building Department for figures. | $200,000-$400,000 | $2,400-$2,700/mo | 6/10 |
| Princeton | 800 sq ft or 25% of the principal dwelling's total floor area, whichever is greater (§T10B-255(c)(3)c); a detached ADU building may not exceed the size of the main house ((c)(3)d, Ord. 2022-14). Deed-restricted low/moderate-income (UHAC) ADU: 1,000 sq ft or 30%, whichever is greater. A fully barrier-free unit earns the property a 5% floor-area bonus usable on the house, the ADU or both. Maximum 3 habitable rooms (4 if affordable). | Detached ADU follows the accessory-structure setbacks of §T10B-256.2(b)(1): 3 ft if the building is 100 sq ft or less, one story and 10 ft tall; 5 ft if up to 1.5 stories and 20 ft; the district's principal-building setback if up to 2 stories and 25 ft. It may never be closer than 5 ft to the habitable part of the main house (§T10B-255(c)(3)l). Former Borough districts: confirm the accessory-building setback with the Zoning Office. | Zoning permit $500 for a new detached ADU or $200 for an ADU addition (Zoning Permit Application; §T10B-32 fee schedule as amended by Ord. 2025-08, April 14, 2025), plus a $300 engineering-review fee where triggered. UCC construction-permit fees are separate and scale with construction cost. | $200,000-$375,000 | $2,300-$2,800/mo | 8/10 |
| Trenton | 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). | 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). | $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. | $160,000-$300,000 | $1,400-$1,800/mo | 6/10 |
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