Ask ten people what an ADU is and most will describe one correctly — the small home behind the house, the converted garage, the apartment over the workshop — even if they have never heard the initials. ADUs are at once one of the oldest forms of housing in America and one of the newest categories in California law. This page answers the three questions we hear most: what ADU stands for, where the term came from, and how a housing type that spent half a century being zoned out of existence became roughly one in five new homes permitted in California.
What does ADU stand for?
ADU stands for accessory dwelling unit. Each word does specific work, and each comes from the language of zoning codes rather than everyday speech.
Accessory means subordinate. In zoning law, an accessory use or accessory structure is one that is incidental to the main use of a lot — the way a garage or a garden shed serves the house. An ADU is legally accessory to the primary home: it shares the lot, it cannot normally be sold separately from it, and the main house remains the principal dwelling.
Dwelling unit is the planning term for a complete, independent place to live: sleeping, living, cooking, eating, and sanitation facilities in one self-contained unit. A spare bedroom is not a dwelling unit. A converted garage with its own kitchen, bathroom, and entrance is.
Put together, an accessory dwelling unit is a complete second home on the same lot as a primary residence — smaller than the main house, legally tied to it, and fully livable on its own. California law also recognizes a junior ADU, or JADU: a unit of up to 500 square feet created inside the walls of an existing single-family home.
Where did the term ADU come from?
The housing is centuries old. The name is not. Long before anyone said "ADU," American homes came with secondary dwellings as a matter of course: the carriage house behind a Victorian, the coach house over the stable, the alley flat in Washington and Baltimore, the garage apartment, the servants' quarters, and later the granny flat or in-law unit built for family. Nobody needed a special term, because nobody had banned them yet.
The term arrived with zoning. When American cities began writing zoning codes in the early twentieth century, planners needed a word for uses that served a main use without being one, and they settled on "accessory." A second home on a single-family lot was, in code language, an accessory dwelling. Through the middle of the century the more common phrases were "accessory apartment" and, in California's first statute, "second unit." Housing advocates in the 1980s — AARP prominent among them, promoting accessory apartments and temporary "ECHO" cottages (Elder Cottage Housing Opportunity) so older people could live beside family — helped standardize the fuller, more neutral "accessory dwelling unit." Planners liked it precisely because it sounded technical rather than familial: a unit for anyone, not just granny.
California made the term official in 2016. The state's ADU statutes, Government Code section 65852.2, had spoken of "second units" since 1982. Senate Bill 1069 and Assembly Bill 2299, effective January 1, 2017, rewrote the statute around "accessory dwelling unit" — and "ADU" has been the standard term in law, lending, and real estate ever since. The older names never died: granny flat, in-law unit, mother-in-law suite, backyard cottage, casita, carriage house, and (in Canada) secondary suite all describe the same idea, and most now appear in California marketing and even in city ordinances as synonyms for the legal ADU.
A short history of ADUs
Before zoning, second homes on a lot were unremarkable. Nineteenth and early-twentieth-century neighborhoods were full of them — carriage houses, alley dwellings, and small rear cottages that housed family, employees, and renters, and quietly supplied much of a city's affordable housing.
The suburban era pushed them out. After the Second World War, the new model was one house on one lot, and most suburban zoning codes either banned second units outright or buried them under discretionary approvals, owner-occupancy rules, and parking demands. Existing units did not disappear; they went informal. California accumulated tens of thousands of unpermitted "bootleg" in-law units that everyone knew about and no code acknowledged.
California's first ADU law came in 1982, when the state required cities to allow second units under Government Code section 65852.2. It was a landmark and, for two decades, mostly a paper one: cities kept wide discretion, fees and hearings stayed heavy, and few units were built. A handful of cities showed what was possible — Santa Cruz became an early model with its 2003 ADU program and design manual — and Portland, Oregon later proved the demand was real when it waived development charges in 2010 and ADU permits multiplied.
The modern era starts in 2003, when Assembly Bill 1866 required California cities to approve compliant second units ministerially — by right, without discretionary hearings. The bigger break came with the 2016 laws (SB 1069 and AB 2299, effective 2017): the ADU name, sharp limits on parking requirements, and relief from utility connection fees. Statewide ADU permits jumped from roughly 1,300 in 2016 to several thousand within two years.
Then the floodgates. The 2019 package — AB 68, AB 881, and SB 13, effective January 1, 2020 — let most single-family lots add both an ADU and a junior ADU, barred cities from requiring owner-occupancy, imposed a 60-day decision clock, and killed impact fees on units under 750 square feet. AB 2221 and SB 897 (effective 2023) cleaned up height and setback abuses. AB 1033 (effective 2024) let cities allow ADUs to be sold separately as condominiums, and SB 1211 (effective 2025) widened ADU rights on multifamily lots. The California Department of Housing and Community Development now counts more than 20,000 ADU permits a year — roughly one in five new homes permitted in the state. Laws taking effect in 2027 (AB 956 and SB 1117) are covered in our guide to California's 2027 ADU laws.
Why the history matters if you own a home today
Two lessons run through the whole story. First, the direction of California law has moved one way for forty years: every few years the state removes more of the local barriers that kept second units rare, and nothing in Sacramento suggests a reversal. The rights a homeowner has today — to build by right, with four-foot setbacks, without replacing garage parking — are the accumulated product of that history, and they are broader than they were even five years ago.
Second, yesterday's informal unit is today's legal project. The same history that produced thousands of unpermitted in-law units also produced the legalization pathways that now bring them into compliance; our ADU permit process guide walks through both new permits and legalizing an existing unit. And if the terminology is still fuzzy — ADU, JADU, second unit, granny flat — our California Accessory Dwelling Unit Handbook and ADU FAQs translate all of it into plain decisions about your lot.
Quick answers
What does ADU stand for? Accessory dwelling unit — a complete, self-contained second home on the same lot as a primary residence.
Where did the term ADU come from? From zoning language: "accessory" is the code word for a use subordinate to a lot's main use. California's 1982 law called them "second units"; the 2016 laws (SB 1069 and AB 2299, effective January 1, 2017) renamed them accessory dwelling units, and ADU stuck.
When did ADUs start in California? The housing type is as old as carriage houses, but legally ADUs start with California's 1982 second-unit law. The modern boom starts with the 2016–2020 state reforms that made ADUs buildable by right on most residential lots.
Ready to put a piece of that history in your own backyard? ADU Cities designs and builds ADUs across California — feasibility, plans, permits, and construction under one roof. Call 888.627.1131 for a free consultation. CSLB #1138550.
