Massachusetts legalised accessory dwelling units by right in 2025, and permit records are the first place the effect would show. We found 141 ADU permits across 17 towns. The monthly counts climb steeply - 4, 9, 20, 35, 37, 33, 3 - 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 every permit from the sources reporting in that month, the rate per 10,000 permits peaks at 77.9 in 2026-05 and finishes at 32.1 - 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 that month | Sources reporting | Rate per 10,000 |
|---|---|---|---|---|
| 2026-03 | 4 | 984 | 4 | 40.7 |
| 2026-04 | 9 | 1,511 | 5 | 59.6 |
| 2026-05 | 20 | 2,567 | 9 | 77.9 |
| 2026-06 | 35 | 5,506 | 18 | 63.6 |
| 2026-07 | 37 | 8,457 | 20 | 43.8 |
| 2026-08 | 33 | 8,648 | 17 | 38.2 |
| 2026-09 | 3 | 934 | 12 | 32.1 |
The third column is the whole argument. The set of towns behind the denominator is not fixed - it grows from 4 sources to 20. That growth is us adding municipal sources, and it is what the raw ADU count is tracking. 2026-09 is a partial month, so its figures cover only part of it.
32 of 141 ADU-matching permits (22.7%) mention a garage somewhere in the description. That is worth knowing, but on its own it says nothing about whether a garage is being converted or built - and reading it as conversion is exactly the mistake this section originally made.
Matching conversion language and new-construction language separately: 60 permits describe new construction against 34 that describe a conversion. Among the garage-mentioning permits it is 13 to 15, which is too close to call. 47 of 141 permits (33%) use neither vocabulary and are left unclassified rather than assigned to whichever side would make a cleaner sentence.
So the useful signal for a framing, electrical or HVAC contractor is the opposite of the intuitive one: an ADU permit in these records is more often a new structure going up than a garage or basement being finished.
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. 141 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.
141 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 2026-09-06. Download the figures behind this page (JSON).
141 accessory-dwelling-unit permits are legible in the 20 Massachusetts municipal sources that publish work descriptions, across 17 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 4 to 37, but that tracks sources being added. Per 10,000 permits from the same sources the rate stays between 32.1 and 77.9 with no trend.
22.7% of the matching permits also mention a garage, making garage conversion the most common identifiable form in these records.
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