Guide
ADU Permits and Project Planning: How the Approval Path Actually Works
The cast of characters
- Building department, your primary stop. Issues the building permit, runs inspections, and issues the certificate of occupancy that makes the unit legal to live in.
- Zoning / planning, confirms how state ADU protections and the local bylaw apply to your lot, and runs site plan review where the town uses it.
- Board of health, involved when the property is on septic; an added dwelling unit affects system requirements.
- Conservation commission, involved if work falls near wetlands or other protected resources.
- Utilities, electric service capacity, water, and sewer connections where applicable.
What 'by right' changes, and what it doesn't
Since February 2025, Massachusetts law protects one ADU per single-family lot in most single-family districts: the use itself can't be denied or pushed through a special permit. What doesn't change: dimensional rules still apply, the building code still governs construction, septic capacity still has to work, and your town can still run reasonable site plan review. In short, the question is rarely 'may I?', it's 'what does it take on this lot?'
The order that protects your budget
- Feasibility first: lot constraints, septic or sewer, structure condition, utility capacity. This is observation and records, not construction drawings, and it's cheap compared to redesign.
- A building-department conversation before design is finished. Ten minutes with your town answers questions no website can.
- Septic evaluation early if you're unsewered, system capacity shapes what you can build, not the other way around.
- Design to the constraints you've confirmed, including the state's size cap (900 square feet or half your home's floor area, whichever is smaller).
- Permit application with complete drawings, incomplete applications are the most common self-inflicted delay.
- Construction with inspections at the stages your town requires.
- Certificate of occupancy, the finish line. Nobody moves in before it.
Documents worth gathering before anyone draws anything
Paperwork that speeds up every later step
- Plot plan or mortgage survey showing structures and setbacks
- Septic system records: design flow, as-built location, last inspection (unsewered properties)
- Deed and any recorded easements or restrictions
- Existing floor plans if you have them, even hand-drawn
- Photos of the space or structure you're considering converting
- Your town's ADU bylaw or a note of who you spoke to at the building department
Questions homeowners ask
How long does ADU permitting take?
Can I do the permitting myself and hire a builder after?
What happens if my town's bylaw conflicts with the state law?
Rather have one team handle the whole path?
Permitting coordination is built into every R&D ADU project. Tell us about your property and we'll walk you through what the approval path looks like for your town.
Tell us what you’re considering. A real person will review your request, and we will not sell or distribute your project information.