Accessory Dwelling Unit FAQs

An accessory dwelling unit (ADU) is a small dwelling that is “accessory” to the main house on
your property that must include:

• Its own entrance and parking
• A bathroom and kitchen
• At least one, but not more than two bedrooms

• You can add one ADU to your home “by right”, that is without any special permit
• You cannot have more than one ADU
• Board of Health and Building regulations apply
• Site plan review by the Planning Board is required (parking, lighting, screening)
• See Procedures/permits/regulations below

To create an ADU, you can:

• Divide/remodel your existing residence (for example, remodel a basement)
• Expand your residence (as long as it retains the appearance of a single-family home)
• Convert an existing accessory building (garage, barn, studio)
• Build a detached ADU

• ADUs are restricted to 900 square feet of living space or one third of the combined square footage of both units, whichever is smaller
• For very large (4,000 sq. ft. and up) houses, a larger ADU may be permitted if approved by the Zoning Board of Appeals and Planning Board

Yes! Part of the incentive for the new rules about ADUs is to increase the number of rental homes in Massachusetts.

Anyone. You are not restricted to family.

You will need to comply with the usual departments and boards:

• Board of Health – must verify your septic can support the ADU’s bedroom(s)
• Building Department  – you or your contractor need to complete applications and receive the applicable (building, wiring, plumbing) permits
• Planning Board – you will need to apply for a site plan review (Site Plan Review Application)

Here’s some online help:

• Guidelines and Requirements for Accessory Dwelling Units

Yes. A fee is required when you submit your application for site review to the Planning Board. See the Planning Board fee schedule.

The Planning Board highly recommends that you start with an informal visit to them as early in the process as possible:

• They have a lot of experience with ADUs, and they will have to sign off on your site plan anyway
• They are very supportive of ADUs and can answer many of your questions
• You don’t need a detailed design, or any design at all
• Many Princeton ADU builders made multiple informal visits to the Planning Board before they hired an architect or contractor or applied for a site plan review
• You can contact the Board’s chair and ask for time at a meeting or simply show up at a meeting (in person or online) and ask your questions during the time slot set aside for that