Massachusetts legalised accessory dwelling units by right in 2025, and permit records are the first place the effect would show. We found 136 ADU permits across 18 towns. The monthly counts climb steeply - 1, 4, 15, 28, 37, 37, 14 - which looks exactly like a boom.
It is not one. Those counts rise because we were adding municipal sources over the same period, not because Massachusetts was approving more ADUs. Normalized against all permits from the same 20 sources, the rate per 10,000 permits peaks at 72.0 in 2026-04 and finishes at 41.5 - flat, noisy, and if anything declining across the months with the largest raw counts. This is the one analysis on this site that nobody outside it could reproduce, because it requires knowing our own source-onboarding history.
| Month | ADU permits | All permits, same sources | Rate per 10,000 |
|---|---|---|---|
| 2026-02 | 1 | 310 | 32.3 |
| 2026-03 | 4 | 1,300 | 30.8 |
| 2026-04 | 15 | 2,082 | 72.0 |
| 2026-05 | 28 | 4,477 | 62.5 |
| 2026-06 | 37 | 7,078 | 52.3 |
| 2026-07 | 37 | 9,814 | 37.7 |
| 2026-08 | 14 | 3,372 | 41.5 |
The left column is the story everyone would publish. The right column is the story the data actually supports.
34 of 136 ADU-matching permits (25.0%) also mention a garage - conversions of existing garage space, or units above one. That is the single most common physical form an ADU takes in these records, and it is a useful signal for anyone in framing, electrical or HVAC: the work is a conversion of existing structure far more often than new construction.
At least one legible ADU permit appears in each of these towns. Deliberately unordered, and counts are withheld. Per-town ADU counts track how richly a town describes its permits, not how many ADUs it approves - ranking them would repeat exactly the mistake this article exists to point out.
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How many accessory dwelling units appear in Massachusetts building-permit records, and is the trend real?
Rows whose description is at least 8 characters, matched against \baccessory dwelling|\badu\b|in[- ]law (apartment|suite)|accessory apartment. 136 matched. The rate denominator is every permit from the same sources in the same month, and months with fewer than 200 rows are dropped as too thin to carry a rate.
20 of 31 sources - those publishing usable free text on at least 20% of rows.
11 sources that publish category labels rather than descriptions. An ADU is invisible in those records, so including them in the denominator would deflate the rate with permits that could never match.
136 rows is a small sample, and the matching rule will catch a permit that merely mentions an in-law apartment as context. The negative result - that the normalized rate is flat - is more robust than any single monthly figure.
Computed 2026-08-18 from a dataset fetched 2026-08-18. Download the figures behind this page (JSON).
136 accessory-dwelling-unit permits are legible in the 20 Massachusetts municipal sources that publish work descriptions, across 18 towns. That is a floor, not a census: towns publishing only category labels cannot show an ADU at all.
Not measurably, in this data. Raw monthly counts rise from 1 to 37, but that tracks sources being added. Per 10,000 permits from the same sources the rate stays between 30.8 and 72.0 with no trend.
25.0% of the matching permits also mention a garage, making garage conversion the most common identifiable form in these records.
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