ADU comparisons
ADU vs duplex: two ways to get two homes on one lot
By the HyreADU research desk. Last checked .
The short answer
ADU vs duplex is the difference between adding a smaller home to your house and building two primary homes.
An ADU is accessory to a main house, is size-limited, and in California gets a ministerial permit.
A duplex is two primary units, needs zoning that allows two units (or a law such as SB 9), and usually means new or rebuilt construction.
| Factor | House plus ADU | Duplex |
|---|---|---|
| Units | One main house plus one accessory unit | Two primary units |
| Zoning in California | Allowed on residential lots under state ADU law | Needs two-unit zoning or SB 9 |
| Permit path | Ministerial (Gov. Code § 66317) | Local rules; SB 9 is ministerial if it qualifies |
| Size | Capped by ADU rules | Set by zoning |
| Keep your current house | Yes | Often a rebuild or major conversion |
When an ADU wins
An ADU wins when you want to keep your house and add a second home without a rezone. The permit path is narrower and faster in California, and you can live in the main house while the ADU is built.
When a duplex wins
A duplex wins when you plan to rebuild anyway and want two full-size homes. It can also suit buyers who want both units to be equal. Lot splits are a different route; the SB 9 comparison is held for review, but our SB 9 and ADU research covers the data.
Selling and financing
Selling the ADU apart from the house is limited in California and usually needs a lot split or a condominium route. Lender treatment also differs between a house with an ADU and a two-unit property. Ask your lender before you design, and read ADU financing.
A mistake to avoid
The mistake to avoid is assuming a duplex is allowed because an ADU is.
State ADU laws, such as California’s, override local zoning for accessory units, but a second primary unit still needs zoning that allows it or a separate state law.
Ask your planning department which path your lot qualifies for before paying for drawings of either one.
What to do next: check your lot with the ADU feasibility checker, read the questions to ask an ADU builder, then tell us about your project. Matching is not live yet, so we reply with information, not a quote.
Get ADU optionsQuestions
Can I add an ADU to a duplex?
California allows ADUs on lots with multifamily buildings under specific rules in § 66323. Ask your city.
Is a duplex the same as a house with an ADU?
No. A duplex has two primary units; an ADU is accessory to a main house.
Which is easier to permit?
In California, an ADU usually, because the law requires ministerial review.
Can I sell an ADU separately?
Only in limited cases in California. That page is held for legal review.
Sources
- Gov. Code § 66317 (ministerial review, completeness and the 60-day decision). Checked September 5, 2026.
- Gov. Code § 66323 (ADUs a city must approve ministerially). Checked September 5, 2026.
- Gov. Code § 66313 (ADU and junior ADU definitions). Checked October 7, 2026.
HyreADU is a publisher. It does not design, permit or build ADUs, has no partner builders, and gives no legal, tax, lending or engineering advice.
Rules vary by state and city; California rules are labeled as such. Spotted an error? Tell us through our corrections page. Licensed professionals can also contribute as named sources.