Fees
Estimated city fees
Baseline for a simple permitted adu: $650–$1,900 estimated combined fees (building permit + plan review + state surcharge) for a typical new ADU in Boston; excludes any ZBA variance or historic preservation review costs
| Fee | Amount | Notes |
|---|---|---|
| Building permit fee | Valuation-based (estimated $500–$1,500 for a typical ADU) | Boston ISD calculates residential permit fees based on declared construction valuation. A new detached ADU or substantial addition will generate higher fees than a simple internal conversion. Verify current fee schedule with Boston ISD at 1010 Massachusetts Avenue. |
| Plan review fee | $150–$400 (estimated) | Boston ISD charges a plan review fee in addition to the building permit fee. Combined permit and plan review fees for a new ADU typically run $650–$1,900 depending on scope and valuation. |
| State building surcharge | $10–$40 | Massachusetts assesses a state building surcharge on all permitted construction projects, collected by ISD at permit issuance and remitted to the Commonwealth. |
| Zoning review / variance (if required) | Varies | If the ADU design requires a setback variance or other zoning relief, a hearing before the Boston Zoning Board of Appeal (ZBA) is required and adds cost and significant time before ISD can issue a building permit. |
| Historic review (if applicable) | Varies by district and scope | Properties in Boston Landmarks Commission, Back Bay Architectural Commission, or other local preservation districts may require a Certificate of Appropriateness review before ISD will issue a building permit. Costs and timing vary by body. |
Documents
Required documents
- Boston ISD building permit application, submitted in person at ISD offices at 1010 Massachusetts Avenue, Boston, MA 02118.
- Zoning review documentation confirming the project complies with Article 25 allowances and the applicable zoning subdistrict's dimensional standards (setbacks, lot coverage, open space).
- Site plan drawn to scale showing property lines, existing structures, proposed ADU footprint, setback dimensions, and off-street parking location.
- Construction drawings including floor plans, elevations, cross sections, and roof plan for the ADU, complying with Massachusetts State Building Code 9th Edition (780 CMR).
- Structural footing and framing details including footing depth below the Massachusetts frost line (minimum 48 inches in Boston), post/beam/joist sizing, and all connection hardware.
- Construction Supervisor License (CSL) number for the supervising contractor, as required by 780 CMR and OCABR. If the homeowner is self-contracting under the homeowner exemption (owner-occupied only), a completed Homeowner Exemption Affidavit must be submitted instead.
- Certificate of Appropriateness from the relevant preservation body (Boston Landmarks Commission, Back Bay Architectural Commission, or similar), if the property is in a historic or architectural review district and the work is subject to their jurisdiction.
- Owner-occupancy affidavit or documentation confirming the property owner resides or will reside in the principal dwelling or the ADU, as required by Article 25.
Timeline
Typical timing
- Plan review
- 5–15 business days for standard residential plan review
- Total cycle
- 6–16 weeks from application to final inspection for a straightforward ADU project
Historic preservation review (Certificate of Appropriateness) adds 4–12 weeks before the building permit can even be applied for. ZBA variance proceedings, if required, can add 3–5 months. Correction rounds add additional review cycles.
Affiliate slot
Need a contractor?
Contextual referral placement for Angi / HomeAdvisor style contractor matching.
Process
How the permit process works
- Confirm Article 25 eligibility and zoning subdistrict Use the City of Boston Zoning Viewer (bostonplans.org) to identify the applicable zoning subdistrict. Article 25 allowances apply to the 1F (single-family), 2F (two-family), and 3F residential subdistricts. Confirm the lot meets the dimensional standards for the proposed ADU type and that no variance is needed.
- Check for historic or architectural review district overlay Determine whether the property is in a Boston Landmarks Commission, Back Bay Architectural Commission, or other local preservation district. If so, obtain a Certificate of Appropriateness (COA) from the relevant body before applying for the building permit. This step is required before ISD will issue the permit and can add weeks to months to the timeline.
- Confirm owner-occupancy and parking requirements Verify that the property owner will reside in the principal dwelling or the ADU as required by Article 25. Check whether the lot is in a Boston Transit Overlay District (which waives the ADU parking requirement) or whether one off-street space must be designated for the ADU.
- Engage a licensed contractor and prepare permit package All permitted construction in Massachusetts must be supervised by a contractor holding a valid Construction Supervisor License (CSL) from OCABR, or the homeowner must qualify for and file the Homeowner Exemption. Prepare the full permit package including site plan, construction drawings, structural details, and energy code compliance documentation per 780 CMR.
- Submit in person at Boston ISD File the building permit application and all supporting documents in person at Boston ISD, 1010 Massachusetts Avenue, Boston, MA 02118. Boston ISD does not currently offer full online permit submission for residential building permits. Bring: permit application, zoning review documentation, site plan, construction drawings, structural details, CSL or Homeowner Exemption Affidavit, owner-occupancy affidavit, and any COA from a preservation body.
- Pay fees and complete plan review After intake review, pay the plan review fee. Boston ISD reviewers will check the submission for compliance with 780 CMR and Boston Zoning Code. Address any correction notices and resubmit corrected materials as needed to advance the application.
- Receive the permit and begin construction After plan approval, pay the building permit fee and state surcharge. Post the permit card visibly at the job site before construction begins. Keep approved drawings on site for inspector review throughout construction.
- Schedule ISD inspections and close out the permit Contact Boston ISD to schedule required inspections (typically footing before concrete pour, rough framing, and final inspection). After all inspections pass, ISD closes the permit. The ADU may not be lawfully occupied until the permit is closed.
Code basis
What Boston reviews against
Boston Zoning Code Article 25 (Accessory Dwelling Units, 2023), Massachusetts State Building Code 9th Edition (780 CMR, based on 2015 IRC with Massachusetts amendments), and Massachusetts Stretch Energy Code (Article 115.AA of 780 CMR) — Boston enforces the Stretch Energy Code as a designated Green Community.
If you skip the permit
What can go wrong
- Boston ISD can issue a stop-work order requiring all construction to cease immediately and post a notice of violation on the property.
- ISD can require removal or demolition of unpermitted ADU work at the owner's expense if the structure cannot be brought into compliance after the fact.
- Massachusetts law requires sellers to disclose known building code violations and unpermitted construction in residential real estate transactions — an unpermitted ADU creates material disclosure obligations that can affect sale price, require remediation, or complicate financing.
- An unpermitted ADU in a Boston historic district may trigger separate Historic Preservation enforcement in addition to ISD building code enforcement.
- A non-permitted ADU cannot lawfully be occupied or rented; it lacks a final inspection and closed permit from ISD.
- Lenders and title companies increasingly flag unpermitted construction in Greater Boston real estate transactions, creating complications during refinancing or home equity applications.
Affiliate slot
What you’ll need for the project
Contextual Amazon-style tools and materials block for adu projects.
FAQ
Common Boston adu permit questions
Do I need a permit for an ADU in Boston, MA?
Yes. Any ADU in Boston — whether an internal conversion, an addition to the primary structure, or a new detached structure — requires a building permit from Boston Inspectional Services Department (ISD) under the Massachusetts State Building Code (780 CMR). Boston ISD does not process ADUs as simple accessory structure permits.
What is Article 25 and how does it affect Boston ADUs?
Article 25 is Boston's city-specific ADU zoning provision, adopted in 2023. It added accessory dwelling unit allowances to Boston's 1F, 2F, and 3F residential zoning subdistricts. Unlike states such as California, Washington, and Oregon, Massachusetts has no statewide ADU preemption law — so Boston's rules, including the owner-occupancy requirement and parking standards, are enforceable local regulations rather than remnants being preempted by a state mandate.
Does Boston require the owner to live on site for an ADU?
Yes. Article 25 requires the property owner to reside in either the principal dwelling or the ADU. This is a meaningful difference from cities in preemption states like Seattle, Portland, and Denver, where state law prohibits owner-occupancy requirements. Verify current enforcement with Boston ISD before relying on Article 25 for a non-owner-occupied ADU.
What is the maximum ADU size in Boston?
Under Article 25, the ADU may not exceed 900 square feet of gross floor area or the gross floor area of the principal dwelling unit, whichever is smaller. If your primary home is 700 square feet, the ADU is capped at 700 square feet.
Do I need extra parking for an ADU in Boston?
Generally yes — Article 25 requires one off-street parking space for the ADU. However, if the property is within a Boston Transit Overlay District, the parking requirement is waived. Use the Boston Zoning Viewer at bostonplans.org to check whether your lot falls in a Transit Overlay District.
What happens if my Boston property is in a historic district?
If your property is in a Boston Landmarks Commission area, the Back Bay Architectural Commission area, or another local preservation district, any ADU work that is visible from the public way or otherwise triggers their jurisdiction will require a Certificate of Appropriateness (COA) before Boston ISD will issue a building permit. Historic review is on a separate timeline from ISD plan review and can add months to your project. Engage the relevant preservation body early in the process.
Can a homeowner pull their own ADU permit in Boston?
Yes, under limited conditions. The Massachusetts Homeowner Exemption allows an owner-occupant of a one- or two-family dwelling to supervise permitted construction personally without a contractor's CSL on file. You must complete the Homeowner Exemption Affidavit and submit it with your permit application. The exemption does not apply to rental properties, and the homeowner assumes full code compliance responsibility.
How long does it take to get an ADU permit in Boston?
Plan review at ISD typically takes 5–15 business days. A complete project — from first application to final inspection — commonly takes 6–16 weeks for a straightforward ADU with no complications. If your project requires a Certificate of Appropriateness from a historic preservation body, add 4–12 weeks before the building permit application can even be submitted. ZBA variance proceedings can add 3–5 months on top of that.
Sources
Official links and freshness
- https://www.boston.gov/departments/inspectional-services
- https://www.boston.gov/departments/inspectional-services/how-get-building-permit
- https://www.bostonplans.org/zoning/zoning-code
- https://www.mass.gov/780cmr
- https://www.mass.gov/orgs/office-of-consumer-affairs-and-business-regulation/csl
- https://www.boston.gov/departments/landmarks-commission
Related permits
More permits for Boston, MA
Related tools
Other free homeowner tools
Disclaimer: This page is informational, not legal advice. Permit rules, fees, and processes change. Verify your project with Boston permitting staff before building.