Accessory Dwelling Units are now legal in Massachusetts. With these new laws in place, you may think that building an ADU is a simple task. However, once you dive into the ADU construction process you will find this is not the case. In this guide we break down ADU construction in Boston step by step so you can make informed decisions from the start. We cover zoning laws, design, permitting and construction of your ADU. Our goal is to help you successfully build an ADU for family or rental income and maximize the value it adds to your property.
What is an ADU in construction?
An ADU (Accessory Dwelling Unit) is a small, secondary home built on the same property as an existing primary residence. It’s a fully independent living space, meaning it has its own kitchen, bathroom, entrance, and utilities, but it sits on the same lot as the main house. ADUs come in three main forms, detached, attached and internal.
A detached ADU is a stand-alone small home built on the property, offering the most privacy and flexibility since it doesn’t share walls with the main house. An attached ADU is connected to the existing home, making it a good fit for smaller lots while still functioning as an independent living space. An internal ADU, sometimes called a carve-out unit, is created entirely within the existing home’s footprint by converting space like a basement, attic, or unused rooms into a self-contained unit. This option can be the most budget-friendly but offers the least design flexibility.
How is ADU construction different?
ADU construction is different from standard residential construction for a number of reasons. The first is the blend of state and local rules that govern building an ADU. Particularly with the state ADU laws being less than a year old in Massachusetts, many towns are still determining how to best integrate them into their existing bylaws. This leads to each town having slightly different requirements and processes for building an ADU. This is why we recommend hiring a builder with local ADU expertise.
ADU construction is also different because they are infill projects, which means built within existing and previously developed areas. Typical residential new construction homes are built on clear vacant land without any structures to navigate. ADUs must be built to complement the primary home, connect to existing utilities and fit into a limited space.
The last way ADU construction can differ is the price per square foot. To the surprise of some homeowners, ADUs are typically more expensive than larger new construction projects. This is due to many of the same bigger ticket aspects of the project being applied to a smaller square foot number. Items like construction documents, permitting, excavation/foundation work, utility connections and kitchen and bathrooms all need to be completed when the project is 500 square feet or 2500 square feet. For a better understanding see our full guide to ADU construction costs here.
Can I build an ADU?
This is usually the first question a homeowner asks when thinking about ADU construction. In Massachusetts if your property is in a zoning district that allows single family homes, then according to the new state Affordable Homes Act, you are able to build an ADU. This can include single or multifamily properties, as long as the town zoning bylaws state that a single family home can be built in that zone. Therefore, most homeowners are eligible for building an ADU.
The city of Boston is its own zoning district and the new state regulations do not apply. It is still possible to build an ADU in Boston, but depending on the type (detached, attached or internal) you may have to go through a longer zoning approval process. See our full guide to ADU construction in Boston here.
Why build an ADU?
The beauty of ADUs is their flexibility. In many cases a homeowner will build an ADU for an aging parent or adult son returning from college. Then when that person moves on they keep it as a rental unit or guest house. Here are the top benefits of building an ADU:
- Housing for family: ADUs provide flexible housing for aging parents, inlaws, adult children or other guests. It is a great alternative and more cost effective than a nursing home or buying another home in the neighborhood.
- Rental income: Supplemental income from a rental unit can help to cover the primary mortgage and other living expenses and it’s less expensive because you already own the land. See our guide for ADUs as an investment here.
- Property value: ADUs in Boston are becoming more desirable as more people learn about and build them. This demand raises the value of property with ADUs
- Downsizing: Many homeowners move into their new ADU and rent out their existing home to reduce their housing costs
- Add to the housing stock: With a shortage of housing in Boston and throughout Massachusetts, ADUs help add to the supply and reduce housing prices for others. See our full guide to the benefits of ADUs here.
What are the requirements for building an ADU in Massachusetts?
The Affordable Housing Act passed in February of 2025 allows homeowners to build an ADU on their property if there is an existing dwelling and it is in a zone that allows single family homes. The state law standardizes certain aspects of ADU construction but others like setback requirements are left up to the town. A more detailed overview of the new state regulations can be found in our guide but it is also important to check with your town requirements with the local building department.
Additionally, Boston has remained its own zoning district and the state regulations do not apply. This adds another layer of complexity as most homeowners aren’t aware of this distinction. For a better understanding of the differences between Boston’s ADU program and the new state laws, take a look at our guide here. There are advantages to building an ADU in Boston like it being considered an additional legal unit that can be condoized but the permitting process can also take much longer.
What ADU construction types are there?
There are three main options for ADU construction - manufactured, modular and stick-built. Manufactured homes are built to federal HUD standards and thus do not qualify as an ADU in Massachusetts. Modular homes are partially built in an off-site warehouse or factory and delivered to the site where assembly is completed. These are built to local and state building codes and can be a viable option for ADU construction in Massachusetts.
Stick-built homes are built entirely on site in a traditional way. This offers the highest level of customization and flexibility which is imperative on tighter, more challenging lots. The remainder of this article will focus on the process for building a stick-built ADU.
Check out our guide for a deeper dive into the advantages and disadvantages of these three ADU construction methods.
Step by step process to ADU Construction?
The following is a breakdown of the ten main steps in ADU construction.
1. Feasibility and Design
The first step to building an ADU is to determine if it can be built and within your budget. To ensure all factors affecting ADU construction are taken into consideration, a feasibility study is done. This accounts for things like utilities and setbacks of the ADU but also items that may often be overlooked like topography of the lot, possible easements and access to the site. Having a full picture of the project helps to budget accurately from the get go.
Once feasibility is determined we consider the design of the ADU based on what can be built and your budget. We offer model floorpans to help with inspiration but also have the ability to create a custom design based on the space and intended use of the ADU. If you want additional storage, an office or hobby room or space to entertain these can be incorporated into the design.
This is also the period where financing for the ADU construction should be sorted out. If a bank loan or HELOC is being used, it is important to go through the qualification process prior to signing a contract and moving to the next step which involves paying for necessary construction documents. See our guide to ADU financing here.
2. Construction documents and Permitting
Once a design has been established it is time to start planning for the ADU construction by laying out the details of the build on paper. Below is a list of what the majority of towns in Massachusetts require in order to receive a building permit.
- Floorplan of the interior layout and “elevations” showing what the exterior will look like
- Structural drawings showing how building will be framed and foundation will be constructed
- Plot plan showing where the ADU will sit on the lot and how utilities will be connected
- HERS energy rating report showing how aspects like insulation, windows and HVAC will affect the energy efficiency of the building
All of these documents will need to be completed by licensed professionals and “stamped” showing accuracy and compliance with local building codes.
Once these documents are in hand they can be submitted to the town building department. The building department will most likely come back with a couple of questions or comments that will be answered by the contractor. When the building department is satisfied that the ADU construction will meet all local building codes and requirements they will issue a permit, allowing the work to begin.
During this time the permit is being obtained the homeowner will be making selections for finish materials so they can be ordered and ready when the ADU construction begins. This helps to minimize delays once the building permit is issued.
3. Site preparation
Once the building permit is received it is time to start preparing to begin the ADU construction. The project manager will do an initial walkthrough and take note of anything to consider before site work begins. This could include removing trees or bushes or taking note of trench location for utility hookups. The corners of the ADU will be marked on the property and a portable toilet will be delivered.
4. Site Work and Utility Connections
This is when the ADU construction actually begins. The excavation team will complete any tree/bush clearing the proceed to grade or level the area where the ADU will be built. This could also involve deconstructing a patio or driveway if needed. If the lot is more severely sloped and a retaining wall is needed, preparation will happen at this time.
The team will then dig trenches to where the water and sewer utilities will be connected either in the existing home’s basement or between the home and the street.
Lastly, utility pipes will be run in the trenches and connected to the existing water and sewer pipes. If the electrical service to the main house is underground, service to the ADU will also be run at this time. If it is overhead, service will be run once the ADU once it is framed.
5. Foundation
Typical foundation types for ADU construction are a slab or crawl space. Both have a wider base, or footings, of about 12 inches by 12 inches. The footings are required to be set below the frost line (the depth at which the ground freezes), which is 48 inches in Massachusetts. If the footing is not below the frost line, when the ground compacts as it freezes, pressure is applied to the bottom of the footing pushing it upwards and causing the building to be unlevel.
A slab foundation means the first floor of the ADU construction sits directly on the concrete. This is the most cost effective foundation type but requires plumbing to be built into the concrete slab and also has the chance of getting cold in winters.
If a slab foundation is used, the plumbing will be run prior to pouring the concrete.
A crawl space foundation in ADU construction has a space under the first floor of a minimum of 3 to 6 feet. This space allows for easier access to plumbing utilities and can be used for storage if prepared correctly.
6. Framing
This is the step of ADU construction where the building starts to take shape. If using a crawl space foundation, framing will start with the floor of the ADU.
Then the walls will take shape, including openings for the windows and doors.
Next the roof structure will be built, followed by plywood “sheathing” being applied on the outside walls and roof.
Lastly, the interior “partition” walls will be framed, giving a feel for the interior layout of your ADU construction.
7. Exterior finishes
Typically the next phase in ADU construction is to make the structure “weather tight”. This means the interior will be protected from rain and other weather elements.
Normally the roof will go on first as it is the main protection for the weather.
Next, windows and exterior doors will be installed followed by the exterior siding that matches your main home.
Now your ADU construction is safe to stand up to tough New England weather and ready for the interior systems to be installed.
8. Rough Mechanical
In ADU construction, rough mechanical includes HVAC, electrical and plumbing. Rough plumbing includes running water lines and drain pipes to all fixtures in the ADU. Vertical vent pipes through the roof are also needed to allow sewer gases to escape.
Electrical consists of running all wiring and installing boxes for outlets, light fixtures and all appliances. This includes all exterior lights as well.
A ductless mini split HVAC system is the most common install for ADU construction. During the rough phase, lines for refrigerant, electrical and drainage will be run in the walls to prepare for full installation after drywall.
At this point a “rough inspection” will be required and town inspectors will visit the job site to sign off on the work before proceeding to the next step. There will be a separate inspector for plumbing, electrical and framing/HVAC.
9. Insulation, Drywall and Plaster
Once the inspection is passed the ADU contractor can begin to close up the walls and ceilings. This starts with insulation. The common types of insulation in ADU construction are spray foam and fiberglass bats. Spray does a better job sealing all air gaps but fiberglass is more cost effective.
Another inspection will be required after insulation is installed to ensure the building meets energy requirements.
After the inspection, drywall will be installed on the walls and ceilings and a coat of plaster is applied to create a smooth finish.
10. Interior Finishes
This is when the ADU construction really comes together and becomes livable. Normally this will start with tiling the bathroom both floor and tub/shower area.
Next is finish carpentry where interior doors, trim/baseboard and cabinets and countertops are installed.
Flooring is the put down followed by painting in the color of your choice.
Lastly all lighting fixtures, faucets and appliances will be installed. All of the design choices that were made earlier in the process become visible at this point.
11. Final touches and punch list
Every ADU construction project will need a detailed walk through and touch ups to complete the project. This will include cleaning and any small items that need adjusting.
A final inspection by a town building inspector is also required.
At this point landscaping is often performed to complement the ADU and enhance the exterior space. This completes the ADU construction process. The unit is now ready to be lived in and enjoyed.
How Massachusetts Weather can Affect ADU Construction
We have all experienced New England winters and how they impact our lives. These conditions can have an impact on ADU construction.. Cold temperatures, snow, and frozen ground make certain phases of the project more challenging, especially excavation, foundation work, utility trenching, and concrete pours. When frost sets into the soil, excavators often need to bring in ground-thawing equipment which increases cost. Many towns throughout the state also impose a “winter moratorium” on non-emergency street openings during winter months. This can delay utility connections under mid-April if they are being run to the street.
Concrete work also becomes more complicated because it must cure within specific temperature ranges. This typically requires heated enclosures or additives to ensure the foundation sets properly.
Tasks like delivering materials or accessing the site can slow down if snow accumulates or icy conditions reduce mobility. Heavy equipment may have difficulty maneuvering, and storage space for materials can become limited. Some components, like framing or roofing, can still proceed in cold weather, but productivity naturally declines as crews work in harsher conditions.
These seasonal conditions emphasize the importance of choosing an ADU builder who is experienced in ADU construction. This ensures your project stays on schedule and is not significantly affected when challenges arise.
Contractors specializing in ADU construction are better equipped to handle not just weather related challenges but also those related to utility integration, tight space constraints and local zoning regulations.
ADU construction differs significantly from typical remodeling projects. Renovating a kitchen or building a deck is a different category of construction from building an ADU which requires utility and site design, zoning interpretation and permitting strategy.
Here at Horizon ADU, we specialize in ADU construction and are here to ensure your project moves smoothly from initial planning to final inspection - on time, on budget, and built to the highest standard.
Summary of ADU Construction Costs
The two main drivers of the cost of your ADU construction are the size and difficulty of utility connections. However there are a number of other factors contributing to cost, including local city-specific requirements and geographic considerations. Understanding all costs impacting your project can help better plan and ensure you stay with an intended budget.
Core Costs of Every ADU Project
- Construction documents
This includes architectural plans, a survey with proposed ADU construction and utility connections, structural drawings of the foundation and building and a HERS energy report. In Massachusetts, local building departments require these reports to be completed and stamped by licensed professionals. Many people do not consider this cost, but at $2k - $3k each, they add up quickly
- Permitting management
In order to receive a building permit from the town, the above documents will need to be submitted along with a contractor’s state construction license. Oftentimes followed by back and forth with the building department to answer follow up questions or potentially provide additional reports like stormwater management, fire protection or geotech/soils report.
This also includes the price for the building permit itself which is often $10 per $1,000 of cost. That means for a $300k ADU construction project the permit fee will be $4,500. This is another cost that is often overlooked by homeowners.
- Site Work
ADU construction begins with site work including grading, excavation, trenching, utility connections and foundation construction. Most of this work is not seen as part of the final product but is extremely important and contributes significantly to the overall cost of the ADU construction.
- Building Construction
This is the construction of the living space of the ADU and includes everything needed to create a livable space. This portion predictably is the cost intensive part of the process both in terms of materials and labor.
Additional Costs in ADU Construction
- Site Work
If the lot sloped this can add additional costs for grading and retaining wall
- Utilities
A significant additional cost can be the reworking of a septic system. If the system was installed prior to when Title 5 laws changed in 1995 it will most likely need to be entirely replaced. This involves both engineering and the physical installation.
Adding additional meters for water or electric can also add costs. Often the electrical service to the house will have to be upgraded from 100 amps to 200 or more. A grinder pump may be required if the property slopes from front to back and the sewer line from the ADU cannot use gravity to reach the lateral. Most towns are not yet requiring solar on ADUs in Massachusetts but this could change in the future.
- Building Costs
If you are building an ADU in Boston more than 50 feet from the street it is likely you will need to add a sprinkler system. Also depending on the distance from the main house or property line, specific fire rated ADU construction may be needed. Choosing high end or custom finishes also add to cost.
Additionally, some towns in Massachusetts have adopted the “Specialized Stretch Energy Code” for all new construction projects. For ADU construction this means the unit must be fully electric, be wired for future solar panels and meet certain energy efficiency codes regarding insulation and windows
Cost-saving Strategies
Homeowners can reduce ADU construction costs by making smart planning decisions early in the process. Choosing a smaller footprint, simplifying the layout, and limiting structural changes like complex rooflines or extensive excavation, can all lower expenses. Placing the ADU close to existing utilities helps cut down on trenching and connection costs. Opting for standard materials and finishes instead of custom selections also helps. Finally, working with a design-build team that understands local zoning and permitting can prevent costly delays or revisions, ensuring the project stays efficient from start to finish.
See our full guide to costs to build an ADU here.