ADU rules in Princeton

An accessory dwelling unit is a permitted accessory use in the Township residence districts R-A through R-H, and the code's definition covers a unit that is 'located in an accessory building on the same lot as a principal, one-family dwelling,' so a backyard cottage or a unit over a detached garage is allowed. A detached ADU must comply with the accessory-structure bulk requirements for its zoning district, must 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. Detached units may also be built on undersized lots that existed as of July 1, 2020 as long as the district's other bulk requirements are met. The same standards reach the former Borough side by cross-reference from Borough Code § B17A-228(c)(7).

An ADU may instead be a group of interrelated rooms that constitutes an entirely self-contained portion of the principal one-family dwelling, which covers basement, attic, and in-law style conversions. An attached unit must use the same exterior materials and colors as the principal dwelling, stairways leading to a unit above the ground floor must be enclosed or placed at the rear of the house, and balconies and decks must face the interior of the lot. There may be no external entrance facing a street that is separate from any other external entrance to a building on the same lot facing that street.

These are the codified standards, last amended 6-28-2022 by Ordinance 2022-14, which added the UHAC affordable-unit allowances, the barrier-free floor-area bonus, and the rule that a detached structure containing an ADU may not exceed the principal dwelling. The section's later 9-23-2024 amendment (Ordinance 2024-32) changed only the roof-mounted solar panel paragraph, not the ADU standards. Floor area is measured under the Township Code's floor-area definition, which exempts basements and, for units without a basement, up to 100 square feet of mechanical and utility storage plus 48 square feet of bicycle and miscellaneous storage.

Princeton removed owner occupancy from its accessory-unit rules in 2020 and has not reinstated it. Ordinance 2020-16, which made ADUs a permitted accessory use, says in its statement of purpose that the ordinance 'also eliminates any homeowner occupancy requirement,' and the codified standards at § T10B-255(c)(3) impose no occupancy condition on either unit. Ordinance 2020-28, adopted 8-24-2020, went one step further and struck the clause requiring the ADU to be 'owned by the owner of the principal, one-family dwelling' from the definition in both the Township and Borough codes; its stated purpose is 'to eliminate the requirement that the principal dwelling and accessory dwelling be in common ownership.' Neither unit has to be owner-occupied for ordinary long-term renting. Owner or long-term-lessee principal residence in the main dwelling is required only to use the ADU as a short-term rental of under 30 days.

Parking scales with habitable room count rather than bedrooms or unit size, so a two-room ADU adds no parking obligation at all and only a three-room unit triggers one extra space. The rule also protects the main house: an ADU may not be built in a way that removes the parking the principal single-family dwelling is required to have.

ADUs are listed as a permitted accessory use in the Township residence districts R-A through R-H and, by cross-reference from Borough Code § B17A-228(c)(7), in the former Borough residence districts, subject only to the objective standards in subsection (c)(3). No conditional-use approval, variance, or board hearing is needed for a conforming unit; Ordinance 2020-16 was titled an ordinance 'making accessory dwelling units a permitted accessory use in all residential zones.' The ordinary zoning permit and construction permits still apply, and the ADU standards expressly control over any conflicting provision of either the Borough or Township Code.

The short-term rental scheme Princeton adopted as Ordinance 2025-20 on 12-8-2025 is now codified as Borough Code Chapter B16, Article VIII. Section B16-80(a)(4) reads: 'If the dwelling unit is an accessory dwelling unit, then the accessory dwelling may be used as a short-term rental when the principal, one-family dwelling is the owner's or long-term lessee's principal residence.' An application needs a floor plan with room measurements, parking details, documentary proof of principal residence, at least $500,000 of liability insurance, and a $200 fee, with $100 renewals; a 3 percent occupancy tax applies to stays that qualify as transient accommodations. Nothing in the zoning standards sets a minimum lease term or otherwise limits ordinary long-term renting of an ADU.