Building an Accessory Dwelling Unit (ADU) in Boston can feel complicated. If you live outside of Mattapan and want to build an attached or detached ADU, the city requires homeowners to seek approval through the Boston Zoning Board of Appeal (ZBA). Unlike other parts of Massachusetts, Boston hasn’t fully adopted the statewide ADU law, so attached and detached projects must go through a formal zoning and permitting process. This means working with city departments, engaging your neighbors, and presenting your case at public hearings before you can break ground.
To make the journey more approachable, we’ve put together a step-by-step outline of the process based on our experiences. This guide explains what to expect at each stage so you can avoid surprises and proceed with confidence.
If you have specific questions or would like Horizon ADU to handle the entire process, please reach out to us here.
- Apply for a Long Form Permit
- In Boston, attached and detached ADUs (Accessory Dwelling Units) require a long form building permit application through the city’s online portal.
- Unlike a short form (minor alterations), a long form covers new construction, additions, or major changes.
- You’ll need architectural drawings, site plans, and property details ready.
- Receive Denial Letter from Inspectional Services Department (ISD)
- Your application for an attached or detached ADU will be denied since it is not currently allowed under Boston Zoning bylaws.
- However, this denial isn’t a dead end, it’s the trigger that allows you to appeal to the Zoning Board of Appeal (ZBA).
- Appeal the Denial through the Zoning Board of Appeal (ZBA)
- You file an appeal with the Board of Appeal to request a variance (permission to build something not automatically allowed by zoning).
- This involves additional paperwork, filing fees, and scheduling a hearing date.
- Your architect, attorney or Horizon ADU can help prepare the appeal package. It is also possible to do yourself as the homeowner.
- Inform Neighbors of Your Project
- Boston zoning requires community review for variances.
- You’ll need to send notices to abutters (neighbors within 300 feet) informing them of your project.
- This step is critical, since neighbor support (or opposition) can strongly influence the ZBA’s decision.
- Attend an Online Abutters Meeting
- The Mayor’s Office of Neighborhood Services (ONS) coordinates an online meeting where you present your ADU plans to neighbors.
- This is your chance to explain the design, answer questions, and address concerns.
- The ONS representative reports feedback to the ZBA.
- Attend an In-Person Neighborhood Association Meeting
- Many neighborhoods in Boston (e.g., Dorchester, Jamaica Plain, South Boston) have civic associations that review zoning appeals.
- These meetings are often community-run and carry weight with the ZBA.
- Present your ADU project, show drawings, and expect Q&A. Community approval makes approval much more likely.
- Attend an Online Zoning Board of Appeal Meeting
- Your appeal goes before the ZBA in a public hearing, typically online.
- You (or your architect, attorney or Horizon ADU) present the case, ONS and neighborhood groups give input, and board members ask questions.
- The ZBA votes to approve or deny your ADU variance request.
- Receive Approval from ZBA
- If approved, you’ll receive a decision letter from the ZBA.
- Approvals often come with conditions, such as specific design requirements or usage restrictions.
- Send for Design Review
- Many projects must undergo design review with the Boston Planning & Development Agency (BPDA).
- A design reviewer ensures the ADU matches neighborhood character and city guidelines.
- You may need to revise plans (façade, landscaping, materials) before moving forward.
- Submit to Plans Examiner within ISD
- After zoning approval, your ADU plans go to a Plans Examiner at Inspectional Services Department (ISD).
- They check for building code compliance (structural safety, egress, accessibility, energy code, fire safety).
- This is separate from zoning, both must align before you get a permit.
- Submit All Required Paperwork (especially for Detached ADUs)
- Detached ADUs (a unit in the backyard or above a garage) require more detailed paperwork:
- Full site plan showing setbacks, lot coverage, utilities.
- Foundation and structural drawings.
- Energy code compliance documentation (HERS rating, insulation, etc.).
- Stormwater management plan if applicable.
- Waiting Period for Feedback
- The Plans Examiner has 30 days to make comments or request revisions.
- If more information is required you will upload these documents to the ISD portal.
- This back-and-forth can take weeks to months depending on complexity and workload.
- Pay Permit Fee and Start Construction
- Once your Plans Examiner has approved your building permit, you pay your permit fee (calculated based on project cost - $10 for every $1k of project cost).
- You’ll receive an official building permit placard to post at your property.
- At this point, construction can legally begin.
- ISD will inspect the work at milestones (foundation, framing, final occupancy).