Building Your Accessory Dwelling Unit

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Learn more about what to expect during construction of your ADU.

Still have questions? Contact:
Housing

617-635-3880

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12 Channel Street
9th Floor

Boston, MA 02210
United States

Construction Overview

ADU Phase timeline showing the Build Phase highlighted.

Once you have secured the necessary permits, your next step is to begin construction on your ADU.

During the construction phase, you'll work with your contractor and building team to bring your approved ADU design to life. While the construction timeline will depend on the type of ADU you are building and the scope of your project, you should anticipate that construction will take anywhere from 6 - 12 months. However, there are some major milestones and general tips that you should keep in mind before you launch into the building phase.

Before You Start: ensure you have all the necessary permits to build

You must receive an approved Building Permit from the City's Inspectional Services Department before you can start any construction work for your ADU. For more information about how permit your ADU, please see our page Permitting Your ADU.

ADU Questions? We can help!

We'll answer your questions at any stage of the process. Email ADU@boston.gov

Construction

BEFORE CONSTRUCTION STARTS

Before work begins, establish a preferred method of communication with your contractor. Review the project scope, anticipated milestones, and scheduled payment plan and raise any questions that you have as soon as possible. Get all major project decisions in writing.
Double check all permits and ensure that you know how to access all permit application documents, if your contractor managed the permitting process.
Take your time and do your own research before making any decisions, especially ones that will change the scope or projected cost of the project.

DURING CONSTRUCTION

Throughout the building process, your general contractor will manage the project schedule and be the main point of contact for any subcontractors, inspectors, and material suppliers.
You should be around during construction. This will allow you to quickly bring up issues or questions with your building team.
If something goes wrong, know that you have options. Contact the City's Consumer Affairs office for mediation help and/or the Massachusetts Office of Consumer Affairs and Business Regulation's (OCABR) office for arbitration. Learn more about the state's arbitration program here.

construction phases

Below are the general construction phases you can expect, although there may be additional phases or some differences necessary for your project. The exact steps and timeline of building your ADU will depend on the type of ADU you are building, the scope of your project, and unique characteristics of your property.

Site preparation - During this phase, your contractor will focus on clearing and preparing the site -- be it your basement, backyard, or elsewhere on your property -- for construction. They may complete demolition, excavation, and/or remove debris.
Foundation & framing - Once the site is cleared and prepped, your building team will lay foundation, if needed, and commence the framing process, which outlines the skeleton of your ADU. This includes framing the walls, floors, and roof.
Mechanicals, electric, plumbing - Your unit will be outfitted with all necessary utilities systems.
Interior & exterior details - Exterior finishes, such as siding and roofing, are completed. Your building team also works inside the unit to install insulation, flooring, appliances, and other finishing touches.

Inspections

Your ADU is required to undergo several inspections at different points during construction.

All ADUs will need to undergo several partial and final inspections over the course of construction. Your contractor will coordinate with ISD inspectors to schedule inspections and collect the documents and signatures that are needed. Once construction is finalized, you must apply for and receive a Certificate of Occupancy before anyone can move into the ADU.