Every Name, One Category (Mostly)
ADU housing has accumulated terminology from at least four decades of zoning history and three regional traditions. The names are not interchangeable in legal terms, even when the structures look identical.
The federal-style and California-standard term. Defined as a self-contained second dwelling on a single-family residential lot, with its own kitchen, bathroom, and entrance. The term is now standard in most state statutes that legalized streamlined ADU permitting after 2016.
Common in Washington and Oregon. Refers specifically to a detached ADU built as a separate structure, distinguished from attached or interior ADUs. Seattle and Portland use DADU as the formal term. California uses “detached ADU” without the acronym; the legal category is identical.
California-specific category defined in Gov Code §65852.22. A JADU is up to 500 sqft, must be inside the existing house, requires a kitchenette (not a full kitchen), and triggers an owner-occupancy requirement. Most other states do not have an equivalent category, although a few municipal codes have created similar smaller-unit definitions. See the JADU deep dive.
1980s California vernacular for what we now call an ADU. The term predates streamlined state permitting and still appears in older zoning codes and real-estate listings. Legally interchangeable with ADU in California.
Spanish-language term for “little house,” used widely in the U.S. Southwest (California, Arizona, New Mexico, Texas). Refers to a small detached secondary dwelling, typically in a backyard. Legally an ADU under California law. The term carries no distinct legal status.
Vernacular term for a secondary dwelling, typically attached to or inside the main house, intended for an aging parent. If it has a full kitchen and separate entrance, it qualifies as an ADU. If it has no kitchen (just a bedroom and bathroom), it is a bedroom suite, not an ADU. The distinction matters for permits, financing, and resale.
Pacific Northwest vernacular (Seattle, Portland) for a detached ADU. The term emphasizes the structure’s separation from the main house and its backyard placement. Legally a DADU in Washington and Oregon; legally a detached ADU in California.
A tiny home built on a permanent foundation, connected to permanent utilities, and meeting California Building Code or equivalent state building code qualifies as an ADU. Most jurisdictions require minimum square footage (often 150 sqft under California efficiency-unit rules) and full plumbing, electrical, and HVAC.
Not an ADU in any state. A tiny home on a chassis with wheels is regulated as a recreational vehicle (RV) or as a manufactured home under different rules. Cannot be permitted as an ADU because it lacks a permanent foundation.
California-specific category that did legalize certain factory-built movable units as ADUs under specific conditions (foundation-required, utility-connected, code-certified). The MADU category is narrower than tiny-home-on-wheels and has limited adoption. See the MADU deep dive.
Term used in Canadian municipal codes and occasionally in the Pacific Northwest. Refers to a secondary dwelling within or attached to the main house, similar to an attached ADU.
East Coast and Midwest vernacular for a detached secondary dwelling, often built above an existing garage. Historically common in Chicago, Boston, and older East Coast cities, where many were grandfathered after mid-century downzoning. New construction in this style is now classified as an ADU or DADU under modern statutes.
Who Uses What, and When the Law Changed
ADU legalization rolled out across the U.S. unevenly between 2016 and 2026. The terminology and the rules track the local statute.
| State | Standard term | Key statute | Year of streamlining |
|---|---|---|---|
| California | ADU + JADU + MADU | Gov Code §65852.2 + §65852.22 | SB 1069 + AB 2299 (2016), AB 68 (2019), AB 670 (2019), AB 976 (2023) |
| Oregon | ADU + DADU | HB 2001 | 2019 (statewide single-family-zone reform) |
| Washington | ADU + DADU | HB 1110 + HB 1337 | 2023 |
| Massachusetts | ADU | Affordable Homes Act | 2024 (statewide ADU as of right in single-family zones) |
| Vermont | ADU | Act 47 | 2024 |
| Maine | ADU | LD 2003 | 2022 |
| Arizona | Casita / ADU | HB 2720 | 2024 |
| Montana | ADU | SB 528 | 2023 |
The legal mechanisms vary. California, Massachusetts, and Vermont preempt local zoning for ADU approvals. Oregon and Washington require cities to allow ADUs but leave more discretion at the municipal level. Arizona’s 2024 law applies primarily to larger cities.
The terminology survey above is a research-anchored summary. ADUscale represents California homeowners, and the rest of this page assumes a California reader unless flagged. If you are reading from outside California, the California pillar at /adu/what-is-an-adu covers the California-specific definitions and rules; for other states, defer to your local statute and licensed local representation.
How We Got Here — The Short History of ADU Housing
ADU housing is a 100-year category that was effectively banned for 60 of those years.
Common and unregulated. Coach houses, carriage houses, garage apartments, and granny flats were normal additions to urban and suburban lots before WWII. There was no zoning category called “ADU” because zoning had not yet evolved to ban them.
FHA suburbanization and downzoning. Federal housing finance after WWII privileged the single-family detached home on a single-use residential lot. ADUs and their predecessors were progressively zoned out of new suburban tracts. Many existing units were grandfathered; many others were retroactively reclassified or removed.
ADUs as legacy stock. Existing ADUs (the surviving granny flats, in-law suites, and garage apartments from before downzoning) continued to function as informal rental stock in many U.S. cities. New construction was rare and required variances or special-use permits.
California opens the door. SB 1069 and AB 2299 streamlined California ADU permitting at the state level, preempting many local restrictions and setting permit timelines. California issued approximately 540 ADU permits in 2016, the baseline year.
California doubles down. AB 68 strengthened state preemption. AB 670 preempted HOA restrictions on ADUs. ADU permit volume in California crossed 10,000 for the first time.
25,000+ permits, 19% of state housing production. California issued 25,000+ ADU permits in 2022, a 46x increase from 2016 (California YIMBY ADU Reform Retrospective). ADUs were 19% of all housing units produced in the state that year. LA City issued 7,160 ADU permits in 2022 alone.
Other states follow. Washington (HB 1110, 2023), Massachusetts (Affordable Homes Act, 2024), Vermont (Act 47, 2024), and Montana (SB 528, 2023) passed varying degrees of state-level ADU streamlining. Most preserve more municipal discretion than California’s framework.
The maturation phase. California ADU permit volume has stabilized at the 22,000–28,000 annual range. The conversation has shifted from “is this legal?” to “is this a good idea on my specific lot, with my specific budget, in my specific submarket?” This is the question the Reality Check is built to answer.
The terminology and the history matter for context. The decision in front of you is parcel-specific. The Reality Check returns whether your specific California property qualifies for an ADU under current state and local rules, and which of the five California ADU types the lot supports. Two minutes, no design fees, no commitment.
See if your property qualifies — free ADU Reality CheckReal Costs
The terminology does not change the math. A 600 sqft detached ADU in California in 2026 costs roughly the same whether you call it an ADU, a casita, a granny flat, or a backyard cottage.
| ADU type | 2026 all-in cost (CA) | Typical timeline |
|---|---|---|
| Garage conversion (1-car studio) | $100K–$160K | 4–6 months |
| Garage conversion (2-car 1BR) | $150K–$220K | 5–7 months |
| Attached addition (600 sqft) | $200K–$300K | 7–10 months |
| Backyard detached (600 sqft) | $220K–$330K | 8–12 months |
| Prefab / manufactured (600 sqft) | $150K–$280K (delivered + sited) | 4–8 months |
| JADU (inside existing house, up to 500 sqft) | $80K–$160K | 3–5 months |
Cost drivers that move a project from the low end to the high end of its band:
- Pre-1970 structure requiring retrofit ($5K–$25K)
- Sewer lateral upgrade ($15K–$30K)
- Electrical panel upgrade ($5K–$15K)
- Hillside soils and foundation strengthening ($20K–$60K)
The cost picture in full detail, broken out by city and by ADU type, lives on the California pillar at /adu/what-is-an-adu and on the /adu/cost hub.
Citable Factoids — ADU Housing
California issued approximately 540 ADU permits in 2016 (California YIMBY ADU Reform Retrospective). State-level streamlining under SB 1069 + AB 2299 took effect that year.
California issued 25,000+ ADU permits in 2022, a 46x increase from 2016. ADUs were 19% of California housing units produced that year.
LA City issued 7,160 ADU permits in 2022, the highest of any California city per LADBS records. Garage conversion was the most common subtype.
Massachusetts passed the Affordable Homes Act in 2024, making ADUs legal as of right in single-family residential zones statewide. The first statewide ADU streamlining in New England.
Oregon’s HB 2001 (2019) ended single-family-only zoning in cities over 25,000 population and legalized ADUs and middle housing types as of right.
California Government Code §65852.2 is the canonical state ADU statute. §65852.22 covers JADUs.