ADUs Turn One: Regulatory Barriers to production in Massachusetts and Ideas for Further Reform

By Amy Dain

May 20, 2026

Executive Summary

Massachusetts’ 2024 Affordable Homes Act legalized accessory dwelling units (ADUs) statewide, taking effect February 2025. The impact was immediate. Municipalities statewide received more than 1,600 permit applications and issued over 1,200 permits in the first year—a clear increase over prior years, but still short of meeting the state’s target pace and needs for ADU production. The shortfall does not reflect a lack of interest from homeowners or renters. High construction costs and limited financing options are real challenges—that are compounded by a regulatory system that adds unnecessary costs, delays, and uncertainty to ADU projects. The regulatory system was not designed to support small housing projects at scale.

This report looks back at the first year of implementation and recommends paths for future reform.

Massachusetts’ experience with ADU legalization offers two main lessons for housing reform generally:

• State-level legalization of housing via zoning law unlocks housing production, yielding results in a time frame that would be impossible for municipality-by-municipality reform to accomplish, even under a state mandate. State-level zoning reform accomplished more in one year than 50 years of local reform did.
• Zoning reform alone is insufficient to meet the scale of need for housing production. A comprehensive agenda is needed to address regulatory barriers to housing production, spanning building, fire, energy, septic system, wetlands, and stormwater rules. The barriers include the fragmented complexity of the regulatory system itself.

The first round of state ADU reform is already producing results, but it is not enough on its own. Sustained gains will require follow-up work on multiple fronts, which are outlined in this report:

• There should be clear, uniform state regulatory standards for ADUs, with minimal opportunities for municipal-level variation. While the Affordable Homes Act legalized ADU’s statewide, it did so in a way that preserved municipal authority to apply unique dimensional standards to ADUs, within some confusing parameters. It left in place a level of variation and haze that is beyond comprehension. There is no up-to-date inventory of local septic, wetlands, stormwater, and other requirements that apply to ADU construction, but a 2004 study showed that 109 communities in eastern Massachusetts had septic system rules that exceed the state’s Title 5 and 131 communities had adopted local wetlands standards that exceed the state’s Wetlands Protection Act. If anything, the restrictions have multiplied since 2004. Each local requirement is unique. The unmanageable complexity calls for pulling back local authority to proliferate rules that apply to ADUs.
• State regulatory standards for ADUs should be reviewed and revised to support ADU production while protecting health, safety, and the environment. Some state requirements are unnecessary barriers to ADU production; others may need strengthening as the local authority to augment them is rolled back.
• Permit review processes should be regionalized. Massachusetts has 351 cities and towns, most of them small, each with its own staffing, boards, inspection schedules, logistical demands, and informal enforcement norms. Many municipalities lack the staff and technical expertise to run the project reviews, especially if permitting activity were to pick up. Franklin County offers a model of regionalized permitting.
• Cross departmental coordination at the local and state levels should be increased. Applicants often have to navigate multiple departments, boards, and regulatory frameworks sequentially, with significant time and money committed to each step before the next constraint appears. Responses to one board or inspector’s conditions can lead to more expensive conditions downstream in the process.

The report also makes specific recommendations for state-level reforms across regulatory domains. Some of the reforms are legislative, and some are regulatory or administrative.

Zoning:

• Allow up to 1,200 square feet for any protected use ADU, regardless of the size of the principal structure.
• Define clear dimensional requirements for ADUs, such as minimum setbacks and maximum height, and allow municipalities to be more permissive with zoning requirements, but not more restrictive.
• Adopt statewide standards for site plan review.

Building and fire:

• Conduct an ADU-focused code review with the explicit goal of reducing costs and barriers to ADU production while maintaining safety. Change the threshold between residential and commercial codes so more ADU projects stay within the residential code.
• Develop and publish statewide guidance on fire code requirements for ADUs.

Energy: Develop ADU-specific guidance clarifying how energy codes apply to different ADU types and evaluate opportunities to reduce requirements for internal ADUs, as the use of existing structures for new housing has climate-emissions benefits.

Septic systems: Exempt ADUs from local septic regulations that exceed Title 5, or update Title 5 to prohibit more stringent local rules for residential septic systems statewide.

Wetlands: Strengthen the state Wetlands Protection Act and remove local authority to adopt requirements that exceed it.

Stormwater: Establish state standards to preempt municipality-by-municipality rulemaking on stormwater requirements. Include a presumptive soil drainage capacity and eliminate mandatory pre-construction soil analysis. Define thresholds below which ADU projects are exempt from full stormwater engineering requirements, absent proximity to sensitive areas.

Utilities: Consolidate utilities connection rules into a centralized guide.

Together, these changes would reduce uncertainty for property owners, enable builders to operate more efficiently across jurisdictions, and allow the ADU market to grow.