ADU Nihilism
Definitions of different housing types are arbitrary. Every productive housing reform produces conflict.
Happy Tuesday, and welcome to another edition of Rent Free. This week, we're going to jump right into it with a philosophical question: What is an accessory dwelling unit anyway?
It's a question that officials in Somerville, Massachusetts, are realizing they didn't know the answer to after learning that a state law allowing ADUs in single-family neighborhoods could also require them to approve three-story backyard mini-apartments.
Rent Free Newsletter by Christian Britschgi. Get more of Christian's urban regulation, development, and zoning coverage.
As the Boston Globe reported on Sunday, Somerville's "deep ethos in favor of new housing" is being tested by the extent of the state law. Elected officials are describing the potential for three-story ADUs as a "joke," and the area's state lawmakers, including one who voted for the state law, are demanding reforms to stop them.
The law in question was a statewide accessory dwelling unit (ADU) legalization tucked inside a major housing package of mostly new spending programs that passed in 2024.
The bill required municipalities to allow ADUs of either 900 square feet or half the size of the primary dwelling (whichever is smaller) anywhere single-family homes are allowed. A subsequent rule issued by state regulators also forbade localities from imposing height and bulk limits on ADUs that are stricter than the limits imposed on primary dwellings.
That latter rule is what's causing Somerville's freakout.
Because the town allows triple-decker apartments (a three-story triplex common in New England) in all its single-family zones, the state rule means that it must also allow ADUs to be three stories tall.
The Globe story notes that the mini-triple-deckers could still only be 900 square feet, meaning each floor of a mini-triple-decker would have to be pretty small. Thus far, no one has proposed building an ADU tower in Somerville.
Even so, the town is worried that someone will eventually propose one of these "out-of-scale" units that it'll have to approve. Its officials are now agitating for changes to the state law to prevent this.
On the one hand, one can perhaps understand why Somerville officials feel blindsided.
The text of the 2024 ADU law allows localities to set "reasonable" height and bulk limits on accessory units. It was state regulators who decided after the law's passage that requiring ADUs to be shorter than the primary unit was "unreasonable."
On the other hand, allowing a second home on a lot to be as tall as the first house sounds "reasonable" enough. Can one complain about out-of-scale development when the new unit is the same height as an existing home and a little bit smaller?
Somerville is not the first city where people's own colloquial definition of an accessory unit as "a tiny home that doesn't bother anyone" has clashed with official regulatory definitions that allow ADUs to be a fair bit bigger.
San Diego's ADU Bonus program once allowed a technically unlimited number of ADUs to be built on single-family parcels near transit stops. (Height and bulk regulations imposed practical limits on how many units could fit on a site.)
That enabled developers to build what were, in effect, ADU apartment complexes on large single-family lots throughout the city. This year, the city imposed new restrictions to stop these "monster" ADU developments, including a per-lot accessory unit cap.
The latest dust-up in Somerville, like the San Diego controversy before it, provides several lessons about the realities of housing reform.
The first is that the definition of "accessory dwelling unit" is completely arbitrary.
In Massachusetts, an ADU is a 900-square-foot home that can be one story or three stories, depending on the town and zoning district it's being built in.
In Montana, it's a 1,000-square-foot unit. In California, state law can require localities to allow ADUs as large as 1,200 square feet. Under certain conditions, these ADUs can even be sold separately from the main house.
At a certain point, an accessory dwelling unit is just a dwelling unit. A house is a house.
The second lesson is that any ADU reform that is actually intended to produce new housing units is going to cause controversy.
State and local laws allowing ADUs have been one of the more common forms of housing reforms to pass.
Supply-boosters see them as a way to get a modest number of new units. Supply skeptics don't see much to object to about allowing someone to install a kitchenette and bathroom in their garage.
If the ADU regulations don't allow much more than the garage conversion, supply-boosters will be disappointed and push for expansions of the law. California passed dozens of ADU laws before they started enabling actual units to be built.
If an ADU law actually allows more homes, the ADUs will look like homes, and the supply skeptics will be mad.
No amount of branding or planning will resolve the inherent conflict of some people wanting more development while others oppose it in their neighborhoods.
Thus far, Massachusetts' ADU law has been modestly productive. The state's ADU survey reports that 2,048 ADU permits have been issued since the state law went into effect in January 2025. About half of those units are attached ADUs (think the garage with the kitchenette).
That's a little under 10 percent of Massachusetts total housing production during that time, per the Census Bureau's building permit survey.
At 50 units permitted, Somerville has actually approved the second most ADUs of any jurisdiction in the state. (Boston has only approved 49.)
But now that it's facing the prospect of allowing ADUs that look like actual homes, it's starting to get cold feet about the new law.
Quick Links
- Speaking of Massachusetts, the town of Marblehead (of "are we kind of being pricks?" fame) has at last had its rezoning plan, which allows apartments in more districts, accepted by state officials.
- While Spain is tightening its tenant protections and rent controls in the face of housing protests, Portugal is trying to remove regulations on its rental housing industry. Either way, housing protesters are clashing with police.
- Single-family homebuilder confidence continues to drop.
- The Urban Institute has a new white paper on whether data center development is coming at the expense of housing development.
- A new report on home construction in Chicago finds that the practice of aldermanic privilege (where individual aldermen can veto projects in their district) is strangling development.
