ADUs in Massachusetts: Rules, Costs, and How to Build an ADU

Can You Build an ADU on Your Property?

An Accessory Dwelling Unit (ADU) is a smaller independent home located on the same property as a primary house. In Massachusetts they are legally allowed “by Right” or by “Special Permit” if they are under 900 square feet and meet the area requirements.

This usually depends on:

• local zoning rules
• lot size
• setbacks from property lines
• utility capacity
• parking requirements
• has a separate entrance

Each town has their own size limits and property requirements. The first step is checking the ADU regulations for your specific municipality.

What does a typical ADU Cost?

Most ADUs in Massachusetts cost between $220,000 and $400,000 to build. Typical costs per square foot range from $275–$400 per square foot, depending on size and site conditions. For example:
• 500 sq ft ADU: $150,000 – $220,000
• 750 sq ft ADU: $200,000 – $300,000
• 1,000 sq ft ADU: $275,000 – $400,000

For a detailed breakdown of ADU costs, read: How Much Does an ADU Cost in Massachusetts

In many cases, soft costs such as architecture, engineering, and permitting can add $20,000–$60,000 to a traditional ADU project. One of Dwelly’s goals is to reduce these upfront design costs by providing pre-designed ADU plans. Browse Dwelly ADU Floor Plans →

Financing an ADU
Many homeowners finance part of their ADU project. Because ADUs can increase property value and create rental income, many lenders offer financing programs designed specifically for home construction and improvements.
Learn more about ADU financing options →
Or get financing through the state with a second mortgage →

Next Steps After Choosing a Design

Once you’ve selected an ADU floor plan, the next step is preparing your project for permitting and construction. Most ADU projects in Massachusetts follow a similar process.

1. Verify Local Zoning Requirements
Confirm requirements such as setbacks, lot coverage, parking, and height limits before moving forward.

2. Prepare a Site Plan
A site plan shows where the ADU will sit on the property and how it relates to property lines, existing structures, and utilities. Some towns require a professional survey or site drawing as part of the permit application. This step helps confirm that the ADU will fit on your property before moving forward with permitting.

If you’re still evaluating placement options, the Dwelly Planning Package can help you test ADU placement and prepare a basic site layout.

Explore the Dwelly Planning Packages →

3. Confirm Utilities
The ADU will need connections to water, sewer (or septic), and electrical service. In some cases the existing home’s utilities can support the additional unit, while other projects may require upgrades.

4. Submit for Building Permit & Begin Construction
Your permit submission will typically include the architectural drawings, site plan, and any required supporting documentation. The building department reviews the plans to confirm compliance with local zoning and building codes. Once the permit is approved, construction can begin!

F.A.Q.

Can I rent out my ADU in Massachusetts?
In many municipalities, ADUs can be rented to tenants, but rules vary by town. Some communities require owner occupancy or restrict short-term rentals. Always check local zoning regulations before planning to rent your ADU.

Do ADUs increase property value?
In many cases, ADUs increase property value by adding livable space and potential rental income. However, the exact impact depends on location, construction quality, and local housing demand.

How long does it take to build an ADU?
Most ADU projects take 6–12 months from planning to completion. This timeline typically includes zoning research, permitting, and construction.

Do ADUs require separate utilities?
Not always. Some ADUs connect to the existing home’s water, sewer, and electrical systems, while others require upgraded or separate utility connections depending on the municipality and property conditions.

Do I need a separate address for an ADU?
Some municipalities assign a separate address for an ADU, while others keep the same property address with a unit designation. This varies by town and utility provider.

What size ADU can I build in Massachusetts?
Many municipalities allow ADUs up to 900 square feet, though size limits vary depending on local zoning regulations and the size of the primary home.

Do I need an architect to build an ADU?
Some homeowners hire an architect for custom designs, while others start with pre-designed ADU plans that can simplify permitting and reduce design costs.

Can I build an ADU on a small lot?
Possibly. The ability to build an ADU depends on zoning factors such as setbacks, lot coverage limits, and available space on the property.

Do ADUs require additional parking?
Some municipalities require an additional parking space for an ADU, while others waive parking requirements depending on the location or proximity to public transit.

Can an ADU be attached to the main house?
Yes. ADUs can be attached, detached, or created within existing space such as basements or garages, depending on local zoning rules.

Do ADUs have to match the main house?
Many towns require ADUs to be compatible with the primary home’s design, though exact architectural requirements vary by municipality.

Can I sell my ADU separately from my house?
In most cases, ADUs cannot be sold separately because they are legally considered part of the same property.