Massachusetts Home Buyer Guide

An accessory dwelling unit (ADU) is not a new concept.  ADUs have been around in some form in the United States for nearly as long as housing. 

Once known as servant's quarters, they morphed over the years to "mother-in-law houses," garage apartments, and detached secondary living units. The simplest definition is that they are secondary dwellings on a primary residential property.  Some also call an ADU an accessory apartment, guest apartment, family apartment, or in-law apartment. 

For a time, ADUs fell out of fashion.  In recent years, ADUs are coming back into focus as an effective avenue for homeowners to increase the value or utility of their property. 

Massachusetts Governor Maura Healey signed a $5.16 billion housing bond bill on Tuesday, August 6, 2024. A key provision allows ADUs by right in single-family zoned neighborhoods statewide. These ADUs can be basement and attic apartments, home additions, or small detached dwellings on the property. There are still rules and regulations, but the new law allows more flexibility and options. 

Whether a homebuyer is looking for a property with an existing ADU or is considering building one in the future, it's essential to understand available options and how regulations might impact decisions. 

What Constitutes an Accessory Dwelling Unit?

Accessory dwelling units fall into three categories, including internal, attached, and detached. 

Internal: Internal ADUs are fully-contained within the original structure, including modifications to basements or attics. 

Attached: In this category, you will find converted garages, apartments built above garages, and additions and build-outs. 

Detached: A detached unit is a free-standing, independent structure on the same lot as the primary home.  This type of ADU may include a "tiny house" on the lot. Apartments built above a free-standing garage also would be considered detached. 

Who Benefits From Accessory Dwelling Units?

Local governments: Increasing the home's assessed value generates additional tax revenues for cities and towns, typically without increased student enrollment in public schools.  Cities also benefit from moderately priced, smaller-sized housing that does not usually require government funding or subsidies. 

Senior citizens: People with fixed incomes can afford the lower prices accessory dwelling units generally command.  In addition, an ADU also is an opportunity to live independently but with family. 

Families: A family may want to accommodate the privacy needs of a young adult who needs the financial safety net of living at home. An aging parent might need some assistance but wants to retain the dignity of their own home.  An accessory dwelling unit may offer the perfect solution for a child or a family with a disability. 

Renters: Renters cover a range of people, from those who wish to save money to buy their own home to temporary workers, such as traveling nurses and professionals who may already be maintaining a home elsewhere.  College students often find such housing far superior to living in a dormitory, expensive apartment, or other student housing options. They can all benefit from a small, affordable, private living space.

Homeowners: By renting out an accessory dwelling unit, homeowners can supplement their mortgage costs by paying a lower monthly amount or paying off the mortgage earlier.  Even modest rent creates an additional revenue stream to boost savings or offset daily living costs.  Flexibility is key to a thoughtful investment. Small multi-purpose spaces available for use as a home office, home gym, or dedicated to homeschooling will help ensure continued added value. 

Employers: Moderately priced housing in a community may attract additional employees to the local labor pool.

Society: Accessory units may help concentrate development, using land, energy, water, and materials more efficiently.  Accessory apartments ease development pressure on farmland and open space, and ADUs increase energy efficiency with a more compact living space.

What Regulations Govern Accessory Dwelling Units?

Local zoning regulations typically dictate the parameters a homeowner must abide by to build an ADU. Such laws will include stipulations on the size of the unit, number of bedrooms, parking requirements, and the number of occupants permitted.  Some jurisdictions have specific guidelines regarding the circumstances when a homeowner can add an ADU, often requiring a special permit. 

As mentioned above, Governor Healey signed a law in August 2024, that provided homeowners statewide with more flexibility to add ADUs to their property. 

Beyond local regulations, homeowner associations may also have strict policies, likely prohibiting ADUs.  Before investing any money into an ADU, homeowners or home buyers should closely review all state laws, municipal bylaws or ordinances governing ADUs. It is equally essential for a homebuyer who purchases a property with an existing ADU to research local building codes before the closing to ensure the unit meets current zoning regulations.  The current homeowner may have built the secondary dwelling without obtaining the required building permits or follow-up inspections.

It is also important to note that the accessory dwelling is a permanent part of the property – even if it is detached – meaning you cannot sell it separately from the primary residence.  As an additional living space, an ADU must comply with all safety and health requirements, including sprinkler systems or smoke detectors, carbon monoxide detectors, and septic systems, which apply to the main house. Once again, local ordinances or bylaws will apply. 

What are Some of the Barriers to Building an Accessory Dwelling Unit?

The most significant hurdles the homeowners face are cumbersome regulations and a lack of lending instruments to fund building or upgrading existing ADUs. While diversity in housing options is beneficial to most communities, not all local governments are convinced.  Most Massachusetts municipalities haven't yet addressed the building and zoning regulations that must be written or changed.  

Some cities and towns have led the way in creating a favorable environment for the creation ADUs; however, many ordinances and bylaws require that the ADU sit within the existing structure.  Limiting the size of the secondary unit to 600 or 800 square feet or a percentage of the existing structure's square feet – whichever is less – appears common in Massachusetts. In some instances, municipalities require a minimum lot size or minimum square footage for the existing dwelling.  Other communities prohibit structures not connected to the public water and sanitary sewer systems from having a secondary dwelling unit.  

The Housing Choice Initiative signed into law in January of 2021 made several amendments to Chapter 40A of the Massachusetts General Laws, commonly known as the Zoning Act.  Among the amendments were two sections of the Zoning Act, which reduced the number of votes required to enact certain types of zoning ordinances, bylaws, and special permits from a two-thirds supermajority to a simple majority.

Interested homebuyers should research and review a community's latest building and zoning regulations before purchasing a property, which you intend to add an accessory dwelling unit.