Half of MA ADUs Are Detached. That Splits the Job List

TL;DR

• Massachusetts approved 1,224 ADUs in 2025 across 217 communities.
• 48% were detached structures. The rest were attached or interior units.
• Detached and attached ADUs share a permit type and almost no trades.
• 415 applications did not become permits in 2025. Those stalls are workable.

Massachusetts cities and towns permitted or approved 1,224 accessory dwelling units in 2025, and 48% of them were detached structures rather than conversions inside the existing house. That split is the most useful field in the whole dataset, because a detached ADU permit is a from-scratch building with excavation, a separate utility run, and a probable septic redesign, while an attached or interior ADU permit is an envelope-in remodel with none of those things. Same permit category. Two entirely different bid lists.

Almost everything written about Massachusetts ADUs is aimed at the homeowner deciding whether to build one. Very little of it is aimed at the businesses that get the work, and none of it treats the detached-versus-attached field as a routing decision. It is the fastest filter you have.

What the 2025 ADU numbers actually say

The Executive Office of Housing and Livable Communities surveyed municipalities twice, in July 2025 and again in January 2026, and published the results as a statewide ADU tracker. The headline figures: 1,639 applications, 1,224 permits issued or approved, 217 communities with at least one approval.

Two caveats worth carrying. First, 293 of the state's 351 municipalities responded to at least one survey, so 1,224 is a floor, not a ceiling. Fifty-eight cities and towns are missing from the count entirely. Second, these are permits and approvals, not finished buildings. A 2025 permit is frequently a 2026 or 2027 construction project, which is exactly why the record is worth having early.

This is year one of a by-right regime. The Affordable Homes Act provisions took effect February 2, 2025, and 760 CMR 71.00 now allows a protected-use ADU of up to 900 square feet, or half the principal dwelling's floor area if that is smaller, in any zone that permits single-family homes. No zoning board, no special permit, no discretionary hearing for a qualifying unit. That is the structural change that produced the volume, and it is not going away.

For the broader case on why an ADU filing is one of the richest records in the dataset, what an ADU permit means as a lead category covers the ground. This piece is about what to do once you can see the composition.

Detached or attached: the field that resets your bid list

Here is the routing table. Read the permit, place it in a column, and you know your call list before you know anything else about the homeowner.

	Detached ADU (48%)	Attached / interior ADU (52%)
What it is	Separate structure on the lot, including a converted garage or barn	Unit inside or connected to the existing dwelling
Site work	Excavation, foundation, grading, often a new walkway or drive	None, or minimal
Utility service	New runs out to the structure; electrical service upgrade common	Usually extends the existing service; upgrade sometimes needed
Envelope	Full exterior: framing, roofing, siding, windows, doors	Existing envelope; interior partitions and insulation only
Mechanical	Its own heating and cooling system, often a mini-split	May extend the existing system, or add a zone
Title 5 exposure	Design flow increases; septic redesign likely on non-sewered lots	Same design-flow math, same exposure, if bedrooms are added
Kitchen and bath	Full new kitchen and bath, from scratch	Full new kitchen and bath, from scratch
Front-of-window trades	Excavator, foundation, framer, roofer, siding, electrician, well and septic	Framer, electrician, plumber
Later-window trades	Flooring, cabinetry, appliances, paint, landscaping, driveway	Flooring, cabinetry, appliances, paint
Realistic job count	Eight to twelve separate trade engagements	Three to six

Notice what survives both columns. Every ADU gets a kitchen and a bath, because that is what makes it a dwelling. If you sell cabinetry, countertops, flooring, or appliances, the detached-versus-attached distinction changes your timing but not your eligibility, and you should be working both columns. If you sell excavation, foundations, roofing, siding, or driveways, half the file is noise and you want the detached column only.

That is the practical payoff. A landscaping company chasing all 1,224 records is wasting more than half its outreach. Filtering to detached first roughly doubles the hit rate on the same list.

The Title 5 fork: when an ADU forces a septic redesign

On a property served by a septic system, an ADU with bedrooms is a Title 5 event. MassDEP's April 2025 guidance on Title 5 compliance for accessory dwelling units calculates ADU design flow at 110 gallons per day per bedroom, the same rate applied to any dwelling. If adding the ADU pushes the property's total design flow above what the existing system was approved to handle, Title 5, codified at 310 CMR 15.000, treats the project as New Construction.

The part people miss: this holds whether the ADU discharges into the existing system or gets a brand-new system of its own. Building a separate leaching field does not sidestep the calculation.

Run the arithmetic on a common case. A three-bedroom house sits on a system designed for 330 gallons per day. The owner adds a two-bedroom ADU. Total design flow becomes five bedrooms, or 550 gallons per day. Unless that original system was oversized, it does not carry the load, and the project now needs a soil evaluation, a designer, a local board of health approval, and an installer before a single stud goes up.

This is why the septic trades should treat the ADU file as a primary lead source rather than an adjacent one, and why everyone else should read it as a schedule warning. A non-sewered ADU is a longer project with a real gate in front of it. The finish trades that call in month one are calling before the homeowner knows whether the thing is buildable. How Title 5 permits work and what they signal has the fuller procedural picture.

The inverse is just as useful. On municipal sewer, none of this applies, the project skips a multi-month gate, and the build window compresses. Dense communities on sewer produce faster ADU projects than Cape and rural towns on septic, and your follow-up cadence should reflect that.

Why 415 applications never turned into permits

Subtract 1,224 approvals from 1,639 applications and 415 filings are left over. Some are ordinary timing, filed in December and approved in January. But a meaningful share are stalls, and stalls are a lead category that almost nobody works.

The usual reasons an ADU application sits: septic capacity came back short, the design exceeded 900 square feet or half the principal dwelling and lost by-right protection, setbacks or lot coverage forced a redesign, or the number came in above what the owner could fund.

Every one of those has a business on the other side of it. A failed design-flow calculation is a septic designer's lead. A unit that overshot the size cap needs a redraw. A financing gap has an answer now that MassHousing offers second-mortgage products specifically for ADUs, which the 2026 ADU financing programs walks through.

The operating rule: an application without a matching permit sixty days later is worth a call, not a delete. The homeowner has already spent money on drawings and has not gotten what they wanted. That is a warmer conversation than a cold prospect who has never thought about it. Reading application dates against issued dates is the mechanic for spotting these systematically.

How to work a detached ADU lead versus an attached one

Detached first. The build sequence is long and strictly ordered, so your slot in it is fixed. Site and structural trades have a window of days to weeks after the permit issues. Electricians sit near the front too, because a detached structure usually needs service run to it and often a panel upgrade at the house, which is its own permit and its own confirming signal. Electrical service upgrade permits filed within a few weeks of an ADU permit at the same address are close to a guarantee that the detached build is real and funded.

Finish trades on a detached ADU should not call in week one. The homeowner is thinking about foundations and inspections and has no cabinet opinion yet. Two to four months out is the slot, and a mailer that lands then reads as timely rather than premature.

Attached and interior ADUs compress all of this. No site work, no separate service run, no exterior trades. The project goes from permit to finish selections faster, sometimes in six to eight weeks, so the finish trades that waited three months on a detached job will be late here. Same permit category, different clock.

One thing that applies to both. A permit is a signal about the homeowner, not the contractor who filed it. An ADU permit says a household decided to add a second dwelling to their property, which is a decision about aging parents, adult children, or rental income. That person is going to make thirty more purchasing decisions over the next year. Selling them one of those and being forgettable about it is the actual mistake.

Common mistakes reading ADU permit data

Treating 1,224 as the market. It is 217 communities out of 351, from 293 respondents. Your town may have ADU activity that never reached the state tally. Work the permit record itself, not the survey summary.

Assuming detached means expensive and attached means small. Attached ADUs get full kitchens and full baths. The finish scope is comparable. What differs is the site and shell work, not the interior.

Ignoring the sewer status. It is the difference between a project that starts in eight weeks and one that starts in eight months. Two identical ADU permits in Newton and in a Barnstable County town on septic are not the same lead.

Calling once. ADU builds run months. A single touch at the permit date reaches a homeowner who is not buying what you sell yet. The Weeks 1–12 window exists because the buying decisions are staggered across it.

Skipping the applications. The 415 gap is the least competitive segment in the whole file, because working it requires noticing that a permit is missing rather than noticing that one appeared.

How permits.llc fits in

permits.llc aggregates 167,000+ Massachusetts permit records across 92 cities and 11 counties, refreshed daily from official municipal portals. Every record carries the permit type, description, filed date, and property address, which is what lets you sort ADU filings into the detached and attached columns and cross-reference the electrical and septic permits landing at the same address. The state tracker tells you the shape of the market once a year. The permit feed tells you which specific house in your county filed yesterday.

Start with the free 2026 dataset: download every 2025 Massachusetts permit and pull your county's ADU records to see your own detached-versus-attached split at the free MA permit download. When you want the filings as they happen rather than a year later, set up daily alerts for your trade and county so an ADU permit reaches you inside the window where your trade actually gets hired.