Outwith ADUs

Notes

Can I build an ADU in Massachusetts? Which laws are relevant to ADUs?

3 min read

Building an Accessory Dwelling Unit (ADU) can be more complicated than it might seem. When designing an ADU, you have to consider:

  • Local zoning
  • The statewide ADU zoning preemption law
  • Building and Fire Codes (state-level law that's interpreted locally)
  • Wetlands and other conservation regulations
  • Wastewater regulations (especially Title V septic regulations if you don't have a sewer line)
  • Energy use and GHG emissions standards
  • Historic Districts
  • Design Standards
  • Possibly more, depending on where you live

Some of these these (local zoning, statewide ADU law, access rules in building/fire codes, conservation regulations, and wastewater regulations) are mission-critical. They can take your ADU dream and make it impossible to build. Other regulations (some building/fire code rules, historic districts, and design guidelines) may add cost and time, but are generally workable on most lots.

Local zoning and the statwide preemption

Local zoning bylaws in Massachusetts have been able to allow or disallow ADUs. Many towns and cities have allowed ADUs in the past, but some of them placed restrictions on those ADUs that made actual ADU development infeasible. The 2024 Affordable Homes Act changed all that. Accessory dwelling units are now allowed by-right in any zoning district that permits single-family homes across the state. The one exception is Boston, which gets its zoning authority from a different law than other MA cities and towns. Under this statewide preemption, an ADU on a single-family property is allowed if it's 900sf in floor area or 50% of the floor area of the primary home (whichever is less). These "protected use ADUs" can be of any type, parking requirements are limited, and zoning's dimensional regulations for ADUs must match those for single-family homes and accessory structures. There's a lot of nuance in the law, which is covered in more detail in what the Massachusetts ADU law actually says. Local zoning regulations that are more restrictive than the state law are now unenforceable, and many towns/cities are passing new ADU laws to conform to the new baseline. Local zoning can be more permissive than the state law, but it can't be less.

Building and Fire codes

While zoning is more about the form of new development, the Building and Fire Codes are all about life safety and wellbeing. They dictate the minimum size of different room types, access to light and running water, resident access to entry/exit points, fire safety measures, and the ability of firefighters to reach a home. Most of these standards are set internationally, but the law is adopted at the state level in Massachusetts, and interpretation of the law is down to local building officials and local fire departments. On most properties looking to add an ADU (like a single-family home that wants to convert the basement, build an addition, or build a backyard building for their ADU), new development has to conform to the International Residential Code (IRC). This is relatively inexpensive to work with. By contrast, a new ADU may need to conform to the International Building Code (IBC) on denser residential parcels, which increases the cost and complexity. The most common killer on ADU projects is firefighter access paths on relatively narrow lots.

Wastewater regulations

The tail wagging the dog of ADUs in Massachusetts is wastewater. If your home is served by sewer, your ADU is most likely fine. If your home is on a septic system, that could be a major constraint. You may need to do a new perc test and install an auxilliary system, which would need additional space on your lot. Otherwise, you may need a whole new septic system. Excess capacity in your existing septic system could be used for an ADU, though. Getting wastewater right without blowing up your budget is site-specific and will require testing and engineering. All that said, towns can't impose more stringent wastewater rules for ADUs than they would for other homes, and local regulations must enable low-cost methods of meeting septic requirements.