Andrew Mikula

Nine months into the statewide legalization of accessory dwelling units, construction activity has certainly risen. The Executive Office of Housing and Livable Communities (EOHLC) documented 550 ADU approvals in the first half of 2025 as state preemptions were implemented and municipalities began adapting. However, Massachusetts still has a long way to go to live up to the blistering pace of ADU production exemplified by several other states.

Take California, for example. In 2024, more than 1 in 4 new homes permitted in the Golden State was an ADU, an average of more than 2,500 per month. Even on a per capita basis, that’s more than twice the 92 ADUs per month permitted in Massachusetts in the first half of 2025.

It’s true that, in many ways, California has been more aggressive at preempting local zoning and overseeing the permitting process for ADUs than Massachusetts. But California’s existing development pattern has also been a dividend for scaling up ADU production in a way that Massachusetts’ is not.

Alleys Give CA an Edge

Simply put, much of California’s urban layout is incredibly conducive to building the types of ADUs that are most valuable, versatile and popular. Residential neighborhoods are often dotted with back-loaded alleys and detached garages, providing plentiful opportunities for detached ADUs with direct frontage on a public right-of-way.

Detached ADUs are particularly desirable for homeowners because they usually maintain privacy, minimize construction-related risks and increase property values more than an attached ADU.

Meanwhile, detached ADUs have limited viability in much of urban Massachusetts because fire codes heavily favor detached ADUs with wide, non-driveway access points. While such rules are easily workable in places that maximize street frontage with narrow blocks and back-loaded alleys, many Massachusetts homes contain garages or backyards that are only accessible via driveway.

From a code perspective, a driveway is often insufficient as an access road for a fire truck in the event of a fire for the simple reason that it is likely to have a car parked there. Thus, even in sprawling suburbs with wide lots and ubiquitous driveways, trees and fences could prevent the establishment of a fire access point to a backyard ADU.

If such fire access requirements are not met, local fire chiefs have discretion to (and usually do) impose sprinkler requirements and other costly mitigations.

ADU Additions Will Be Key in MA

While other challenges to ADU production in Massachusetts largely involve (fixable) regulatory, financing and permitting concerns, longstanding development patterns are more fundamental.

Both the street network and the arrangements of homes on existing lots tend to be less conducive to detached ADU development in the Bay State relative to others, especially those on the West Coast.

This fact highlights the importance of zoning reforms to facilitate by-right ADUs via internal conversions or additions to existing structures to enable ADU production at scale, especially in Boston and other urban environments.

Massachusetts should set a statewide (and minimal) standard for setbacks and lot coverage for a prototypical ADU, rather than tying ADU dimensional standards to the local zoning code. It should also preempt parking requirements for ADUs entirely, allowing the ADU owner to evaluate the need for dedicating additional space to parking.

Each of these reforms could unlock many new opportunities for ADUs in urban areas.

Boston’s Unique Rules Could Hurt

A complicating factor is that Boston is not subject to Chapter 40A, the state Zoning Act, under which ADUs were recently legalized. Thus, it is pursuing its own set of ADU reforms that have both taken longer and may not align neatly with those of the rest of the state.

The final version of Boston’s new zoning should ensure that ADUs can be allowed by-right, without the variances currently needed to build one on 98 percent of small-scale residential properties in the city. They should also follow the state’s lead in preempting owner-occupancy requirements, which requirements make ADUs harder to build in newly constructed homes and harder to finance in general.

But even beyond variances, Boston has a host of process-based hurdles to building ADUs. Right now, layers of discretionary reviews from the city’s 53 Neighborhood Design Overlay Districts and Parks Commission could cause delays and uncertainty for homeowners that are disproportionate to the scale and impact of an ADU. ADUs should be exempted from such reviews.

Despite the general sense that Massachusetts is “behind” on ADU construction relative to California and others, it’s worth noting that the Golden State’s ADU reforms were gradual and iterative as well. Massachusetts lawmakers can learn some lessons from homeowners and contractors regarding the remaining barriers to ADU production and work to alleviate them.

A scalable, versatile and relatively low-cost housing typology is exactly what Massachusetts needs to address our harrowing housing crisis.

Andrew Mikula is a senior housing fellow at the Pioneer Institute in Boston.