City · New Jersey
ADU rules in Princeton
These are the primary sources for accessory dwelling unit regulations in Princeton: the city's own zoning code, planning department pages, and permit guides. Start here before talking to anyone: the rules below are what your permit reviewer will apply.
Rules at a glance
A permitted accessory use in residence districts R-A through R-H includes an accessory dwelling unit, and the ADU definition allows the unit to be located in an accessory building on the same lot as a principal one-family dwelling. A detached ADU must comply with the accessory-structure bulk requirements for its zoning district, sit at least five feet from the habitable portion of the principal dwelling, and no detached structure containing an ADU may exceed the size of the principal dwelling.
Township Code § T10B-255(c)(3)a, l and dThe ADU definition covers a group of interrelated rooms that constitutes an entirely self-contained portion of a principal one-family dwelling; attached ADUs must use the same exterior materials and colors as the principal dwelling, stairways to an ADU above the ground floor must be enclosed or at the rear, and there may be no separate external entrance facing a street.
Township Code § T10B-255(c)(3)f, h, k; ADU definition per Ordinance 2020-16 § 2The base 800 sq ft or 25 percent standard dates to Ordinance 2020-16; the affordable-unit allowances and the detached-structure size cap appear in the current codified text following amendments in 2022 and 2024.
Township Code § T10B-255(c)(3)c, d, eOrdinance 2020-16 expressly eliminated any homeowner occupancy requirement for ADUs; its statement of purpose says the ordinance replaces the old owner-occupied flat and secondary-residence rules and 'eliminates any homeowner occupancy requirement.' The ADU must be owned by the owner of the principal one-family dwelling, but neither unit must be owner-occupied for long-term use. Owner or long-term-lessee principal residence in the main dwelling IS required to use the ADU as a short-term rental.
Ordinance 2020-16 (adopted 6-29-2020), Statement of Purpose and § 2; Township Code § T10B-255(c)(3)Parking scales with habitable room count rather than bedrooms or unit size.
Township Code § T10B-255(c)(3)gADUs are listed as a permitted accessory use in residence districts R-A through R-H, subject to the objective standards of subsection (c)(3); no conditional-use or variance approval is required for a conforming ADU. Ordinance 2020-16 was titled 'making accessory dwelling units a permitted accessory use in all residential zones.'
Township Code § T10B-255(c)(3)The town's Rental Housing Inspections page states for eligibility: 'Accessory dwelling unit: Allowed if the main dwelling is the principal residence.' STR permits require a floor plan, parking information, proof of principal residence, and at least 500,000 dollars of liability insurance.
Ordinance 2025-20; Township Code § T10B-255(c)(3)bThe fine print: 7 caveats that can change the picture
- ADUs are permitted in districts where single-family dwellings are permitted; the uniform standards are codified in Township Code § T10B-255(c)(3) for districts R-A through R-H, and Ordinance 2020-16 also amended the former Borough Code (§ 17A-228(c)(7) and definitions) to point Borough-side districts to the same standards, since Princeton still maintains separate former Township and former Borough codes
- One ADU per lot, maximum three habitable rooms (four if deed-restricted to low- and moderate-income occupancy under UHAC), and no external entrance that faces a street separate from other entrances on the same street frontage
- The ADU must be owned by the owner of the principal one-family dwelling; it cannot be sold separately
- A detached ADU must meet accessory-structure bulk requirements for its district and be at least five feet from the habitable portion of the principal dwelling, and the detached structure may not exceed the size of the principal dwelling
- ADUs may be built on undersized lots that existed as of July 1, 2020 if all other bulk requirements for the district are met; the former Borough's 125 percent lot-size premium for secondary residences was repealed
- ADUs existing as of July 1, 2020 that do not meet the standards are grandfathered and may be altered or reconstructed if no new nonconformities are created
- Short-term rental of an ADU (under 30 days) is only allowed when the main dwelling is the operator's principal residence, under a permit from the Office of Rental Housing Inspections
Summarized from Princeton's ordinance text, read July 2026; every row links the code section it came from. Rules often vary by zone and lot, so treat this as orientation, not a permit determination, and confirm with the planning department before you commit to a design.
ADU regulations
Short-term rental rules
Planning to rent your ADU on Airbnb or VRBO? Princeton regulates short-term rentals separately; check these before counting on nightly rental income.
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