California ADU Height Rules: 16, 18, and 25 Feet

The four height tiers California cities must allow

Government Code Section 66321(b)(4) lists the height limits a local agency must allow for an ADU, written as four separate subparagraphs. The four subparagraphs together set a state protected floor below which no city can fall, and a city ordinance that is more restrictive than any of them is preempted by Section 66325.

Subparagraph (A) sets the default floor for a detached ADU on a lot with an existing or proposed single family or multifamily dwelling at 16 feet. Subparagraph (B) raises the same floor to 18 feet for a detached ADU on a lot that is within one half mile walking distance of a major transit stop or a high quality transit corridor, and adds an additional two feet to accommodate a roof pitch on the ADU that is aligned with the roof pitch of the primary dwelling. Subparagraph (C) sets an 18 foot floor for a detached ADU on a lot with an existing or proposed multifamily multistory dwelling. Subparagraph (D) sets a 25 foot floor, or the local height limit that applies to the primary dwelling, whichever is lower, for an ADU that is attached to a primary dwelling, and adds that the subparagraph does not require a local agency to allow an ADU to exceed two stories.

L Square Design & Build (CSLB #1095476) designs, permits and builds ADUs and junior ADUs across Orange County and Los Angeles County. For a free estimate on your property, call (949) 374-7980, or start with what an ADU costs in Orange County and ADU rules by city.

• Section 66321(b)(4)(A): 16 feet for a detached ADU on a lot with an existing or proposed single family or multifamily dwelling.
• Section 66321(b)(4)(B): 18 feet for a detached ADU within one half mile walking distance of a major transit stop or a high quality transit corridor, plus two feet to match the primary roof pitch.
• Section 66321(b)(4)(C): 18 feet for a detached ADU on a lot with an existing or proposed multifamily multistory dwelling.
• Section 66321(b)(4)(D): 25 feet or the local primary dwelling height limit, whichever is lower, for an ADU attached to a primary dwelling, with no requirement to allow more than two stories.

What the 18 foot transit tier actually means on the lot

Section 66321(b)(4)(B) ties the 18 foot detached floor to two defined terms that come from Public Resources Code Section 21155, namely a major transit stop and a high quality transit corridor. The same half mile walking distance test appears in Section 66322 for the ADU parking exemption and in Section 21155 for transit oriented development, so the same physical location of the lot tends to drive both outcomes. The 18 foot floor plus the two foot roof pitch bonus is the state protected envelope, not a discretionary ceiling, and a city that tries to set a lower detached ADU height in this transit zone conflicts with state law and is preempted.

Cities are allowed to be more permissive. A city that adopts a higher detached ADU height on its own authority is allowed under Section 66325(a) as a less restrictive requirement, but a city cannot use a stricter local height rule to push the same detached ADU below the 18 foot plus two foot roof pitch line on a lot that meets the half mile test. A two story detached ADU is the practical target of this tier, and a stacked one story ADU with a low slope roof is the configuration that fits inside the 18 foot plus two foot envelope without a local bump.

The 25 foot attached ADU tier and how it stacks with the rest of the ADU

Section 66321(b)(4)(D) is the most permissive state tier. The text reads a height of 25 feet or the height limitation in the local zoning ordinance that applies to the primary dwelling, whichever is lower, for an accessory dwelling unit that is attached to a primary dwelling. The phrase whichever is lower matters: if the local zone caps a primary dwelling at 20 feet, the attached ADU caps at 20 feet, not 25 feet. The subparagraph then writes that it does not require a local agency to allow an accessory dwelling unit to exceed two stories, which keeps the two story ceiling even when the numerical height of 25 feet is higher than two stories at typical floor to floor heights.

An ADU that is attached to a primary dwelling is read against the 25 foot tier regardless of whether the primary is one story or two, and the same ADU that is detached from the primary is read against the 16 foot or 18 foot tier in Section 66321(b)(4)(A) through (C). The two tiers are separate. Section 66314(d)(4) separately limits the total floor area of an attached ADU to 50 percent of the existing primary dwelling floor area, so an attached ADU is constrained by both the 50 percent floor area cap in Section 66314(d)(4) and the 25 foot or local limit, whichever is lower, height cap in Section 66321(b)(4)(D).

What the height floor does not cover

Section 66321(b)(4) writes the four height tiers as a state floor, not a state ceiling. A city may allow more height than the four figures, and a city may also require a height lower than the four figures for an ADU that does not qualify for any of the four subparagraphs, such as an ADU on a lot that has no existing or proposed single family or multifamily dwelling or an ADU that does not meet the half mile walking distance test for subparagraph (B). Section 66325(a) still preempts a local ordinance that conflicts with Article 2 (Sections 66314 to 66331) except for less restrictive requirements, and Section 66325(c) repeats the same rule for Article 3 on JADUs.

Section 66321(b)(4) does not by its terms create a separate height rule for an ADU built on top of an existing garage. The garage topped configuration is read against the same detached ADU height tier in Section 66321(b)(4)(A) through (C) that applies to the lot. Section 66314(d)(7) separately bars a new setback on the existing garage footprint when the garage is converted, and the existing structure carve out does not change the height tier; it changes the setback tier. Cities that allow a garage topped ADU up to 25 feet total do so under their own authority, not under state law.

How L Square works within the height tiers on real California lots

L Square Design & Build's published 2026 model lineup runs from the 600 sq ft two bedroom Model 600 at $180,000, to the 800 sq ft Models 800A and 800B at $240,000 each, to the 1,000 sq ft Models 1000A and 1000B at $295,000 each, up to the 1,200 sq ft Model 1200 at $354,000. The two additional feet for roof pitch under Section 66321(b)(4)(B) only attach when the lot is within the half mile walking distance of a major transit stop or a high quality transit corridor.

A 700 sq ft detached ADU with a vaulted ceiling built by L Square in Pasadena is documented on the project walkthrough page, with the finished great room, kitchen, bath, and bedroom shown. That unit is a single story under the 16 foot floor in Section 66321(b)(4)(A). A completed 1,188 sq ft two story ADU in Rosemead is documented on the second project walkthrough page, and the two story stack is read against the local zone height limit for the primary dwelling in Section 66321(b)(4)(D) on the attached tier; a detached two story ADU on a standard lot outside the transit half mile needs a city specific bump above the 16 foot floor and is a separate conversation with the permitting agency.

For a homeowner comparing the height rule across California, the right questions for the local agency are: (1) is this a detached or an attached ADU, (2) is the lot within one half mile walking distance of a major transit stop or a high quality transit corridor, and (3) does the local zoning ordinance set a primary dwelling height lower than 25 feet. Those three answers drive the Section 66321(b)(4) tier. L Square Design & Build can be reached at (949) 374-7980 to discuss a specific lot in Orange County or Los Angeles County.