The Baker House Newsletter Issue V
The Apartment Hidden in the Backyard
Zoning has quietly changed across much of the country, and property many people already own can now legally hold a second home. What the rules allow, what to check, and where the risks are.
THE LEAD
Accessory Dwelling Units and Hidden Density
An accessory dwelling unit, or ADU, is a second small home on a lot that already has one: a garage apartment, a basement unit with its own entrance, a cottage in the backyard. For most of the last half century, zoning made them illegal on the majority of American residential land, which is largely restricted to one detached house per lot. That is the rule that has been quietly changing.
Massachusetts made ADUs under 900 square feet legal by-right statewide in August 2024, and in the first year municipalities approved more than 1,200 permits, nearly half for detached backyard units, per the state’s own tracker. Washington now requires its cities to allow two ADUs per residential lot in urban growth areas. California is a decade into the same reform, where ADUs have become a major share of new housing permits. A growing list of states has followed with laws of their own.
The opportunity in plain words: one property can now legally become two or three income-producing units, and many current owners do not know it. A house bought and priced as a single home may sit on a lot where the rules already allow a second one. The upside lives in the municipal code, and the listing never mentions it. The caveat belongs in the same sentence: this is highly location-specific. Roughly a third of jurisdictions allow ADUs by-right, many still require hearings or owner occupancy, and the same lot can be an opportunity in one town and a dead end one town over.
KEEP READING
The rest of this issue is yours for an email
Enter your email to finish reading — and get each new issue the morning it goes out. Free, and one issue a week.
No pitch. Unsubscribe whenever you like.
CHART
Source: Massachusetts ADU Tracker, mass.gov, survey of 293 municipalities. First-year counts under the Affordable Homes Act.
THE TURN
Why the reform wave now? The housing shortage made hidden density politically possible. Adding a unit behind an existing house requires no new land, no new roads, and no rezoning fight over a tower, so states short on housing have found ADUs the reform that passes. The result is a rare situation in real estate: the rules changed faster than the market’s understanding of them. Owners who bought under the old code are often sitting on rights they never priced and do not know they hold.
Hidden density takes more forms than the backyard cottage. A garage with ceiling height can become an apartment. A basement with egress can become a unit. A large house on a corner lot may convert to a legal duplex. An oversized parcel may split into two lots, or hold an additional home outright. Each is the same move: recognizing that one property can legally become two or three income-producing units, then checking whether the local code agrees.
The rules changed. Most properties are still priced as if they did not.
THE FRAME
One Property, Two Ways to See It
| WHAT THE MARKET SEES | WHAT A DISCIPLINED BUYER SEES |
|---|---|
| A single-family house | A lot the code now allows to hold two or three units |
| An oversized backyard | A buildable site with utilities already at the line |
| A dated garage or basement | The least expensive apartment shell in town |
WHAT TO LOOK FOR
The code before the property. Read your municipality’s ADU ordinance before walking a single lot: by-right or special permit, size caps, owner-occupancy requirements, parking rules, setbacks. State law may override local rules, and the two disagree often. One hour on the town website decides whether the rest of this list matters.
The property. Alley access or a corner lot, an oversized parcel, a garage with ceiling height, a basement with real egress potential. Then the unglamorous gatekeepers: the sewer lateral, the water line, and the electrical panel, because utility capacity decides more ADU projects than design does.
The cheapest unit wins. Converting existing space, a garage or basement, generally costs far less than building a detached cottage from the ground up. The best first project is usually the one where the walls already exist.
Below-market signals. Listings priced as pure single-family in towns where a second unit recently became legal, sellers who have never read the new ordinance, and neighborhoods where the first few backyard cottages are already appearing. The market reprices these rights street by street, and it has not finished.
HOW THE MATH WORKS
A homeowner converts a detached two-car garage into a 600 square foot apartment for 120,000 dollars, using round numbers. It rents for 1,400 a month, or 16,800 a year, a gross yield on cost near 14 percent before expenses, on land already owned and taxes already partly paid. Some mortgage programs now count expected ADU rental income toward loan qualification, per Fannie Mae guidelines, which can help finance the build itself. The same logic scales up: a buyer who pays single-family price for a house on a lot where the code allows a second unit is buying the apartment nobody else underwrote. The value is created by knowing the rules, building legally, and adding housing a town has already said it wants.
THE RISKS, TOLD STRAIGHT
Every strength here has a matching risk, and the first is the one from the top: location. Only about a third of jurisdictions allow ADUs by-right, owner-occupancy rules can bar renting both units, and some ADUs cannot be sold separately from the main house. Construction is the second: utility connections, foundations, and code upgrades routinely push conversions past their budgets, and an unpermitted unit is a liability with plumbing. The third is lifestyle: an ADU landlord often lives thirty feet from the tenant, which is not for everyone. Add tax reassessment, insurance changes, and the risk of over-improving for the street, and the discipline is the same as every issue in this series: the category is attractive, and the individual project is earned through diligence.
THE WIRE
The reform is still spreading. More states are adopting statewide ADU laws each session, Washington’s mandate took full effect in 2025, and Massachusetts posted four-figure permit counts in its first year. Financing is following, with ADU rental income now countable toward qualification in some federal loan programs. It is early days in most of the country: the towns that changed their rules last year have not yet seen their prices adjust, and the gap between what the code allows and what the market has noticed is where the opportunity lives. It closes one informed buyer at a time.
OFFICE HOURS
This issue covered the case, the checklist, the math, and the risks. The judgment, whether your town’s rules make this real and whether your lot is the right one, is where the real questions start. Richard takes them every Wednesday at 2pm ET in his free office hours. No slides, no pitch. He answers whatever attendees ask, live, for an hour. If this issue raised a question for you, bring it.
SOURCES
- Massachusetts ADU Tracker and Affordable Homes Act materials, mass.gov, 2025 survey of 293 municipalities.
- Washington State RCW 36.70A.680 and Municipal Research and Services Center ADU guidance, 2026.
- Mercatus Center, A Taxonomy of State Accessory Dwelling Unit Laws, 2025: single-unit zoning prevalence, owner-occupancy and discretionary review constraints; share of jurisdictions permitting ADUs by-right.
- California HCD ADU program materials; Fannie Mae selling guide provisions on ADU rental income.