Building an ADU in Arcadia. Rules, costs, timeline.

What you can build on your lot

Everything in this section is grounded in Arcadia Development Code § 9102.01.080 and the California Government Code it implements, with state-law citations called out where they supply or override the local rule. We won’t re-cite the Arcadia section on every line — assume it under each heading unless we tell you otherwise.

Number of ADUs per lot

• Single-family lot. The ordinance allows a conversion ADU created from existing space together with a junior ADU, and separately allows one new detached ADU of up to 800 sqft — “in addition to any JADU” — on the same lot (subsections (D)(1)(A) and (D)(1)(B)). Combined, that is up to two ADUs plus a JADU, the same stack Gov. Code § 66323(a)(1)–(2) requires.
• Multifamily lot — existing building. Conversion ADUs in non-livable space — at least one, up to 25% of the existing units — plus up to eight detached ADUs, not to exceed the number of existing units. Arcadia’s text already reflects the SB 1211 allowance (Gov. Code § 66323(a)(3)–(4)).
• Multifamily lot — proposed building. Up to two detached ADUs (subsection (D)(1)(D)).

Size limits

• Detached or attached ADU (permit track): up to 850 sqft for a studio or one-bedroom unit and 1,000 sqft for a two-or-more-bedroom unit (subsection (F)(1)(A)). An attached ADU is also held to 50% of the primary dwelling’s floor area.
• By-right detached ADU (building-permit track): up to 800 sqft (subsection (D)(1)(B)).
• The 800-sqft floor is protected. No FAR, front setback, lot-coverage, or open-space rule may shrink an ADU below 800 sqft (subsection (F)(1)(C); Gov. Code § 66321(b)(3)).
• Conversion ADU: no separate square-foot cap when created within existing permitted space; a converted accessory structure may add up to 150 sqft for ingress and egress (Gov. Code § 66323(a)(1)).
• Junior ADU: up to 500 sqft inside the single-family home, attached garage included (Gov. Code § 66313(d)).

Setbacks

Side and rear setbacks are 4 feet for a new attached or detached ADU — matching the state-law floor in Gov. Code § 66314(d)(7) — and no setback at all is required for an ADU built in the same location and to the same dimensions as an existing structure (subsection (F)(3)). The practical move on a deep Arcadia lot is to set the unit in the rear yard and let the 4-foot envelope do the work.

Maximum height — what this means for design

A detached ADU in Arcadia is capped at 16 feet — the state floor in Gov. Code § 66321(b)(4)(A) — rising to 18 feet within a half-mile walking distance of a major transit stop or high-quality transit corridor, with 2 more feet allowed to match the primary home’s roof pitch (Gov. Code § 66321(b)(4)(B)), or to a flat 18 feet on a lot with a multistory multifamily building (§ 66321(b)(4)(C)). Detached ADUs are also held to one story outright (subsection (F)(10)). An attached ADU may reach 25 feet and two stories as part of the primary structure (Gov. Code § 66321(b)(4)(D)). Read plainly: Arcadia is a single-story detached-ADU market. Our two-story Signature Homes are detached-only designs that need roughly 25 feet of envelope to permit, so they sit out on Arcadia lots. All six of our single-story models clear the 16-foot bar with conventional roof forms.

Parking

One off-street space is called for per ADU, and it can be tandem or sit in a setback. The requirement falls away in six situations the ordinance lists in subsection (F)(7)(B): within a half-mile walking distance of public transit, inside an architecturally and historically significant historic district, when the ADU is part of the existing primary or accessory structure, in an on-street-permit area where the occupant isn’t offered a permit, within a block of a car-share stop, or when the ADU is built alongside a new primary dwelling. A junior ADU never requires parking (Gov. Code § 66334(a)), and when a garage, carport, or covered space is demolished or converted for an ADU, those spaces never have to be replaced (subsection (F)(7)(C); Gov. Code § 66314(d)(11)). Arcadia’s Metro A Line station and its bus corridors put a good share of the city inside the half-mile transit radius.

Lot coverage, FAR, and open space

On Arcadia’s larger lots these rarely bind, but the numbers are worth knowing. An ADU may not push the lot’s floor-area ratio above 45%, may not push lot coverage above 45% (with a one-story primary) or 35% (with a two-story primary), and may not drop the lot’s open space below 50% (subsections (F)(2), (F)(4), (F)(5)). Every one of those limits is expressly subject to the 800-sqft floor: none of them may force an ADU below 800 square feet (subsection (F)(1)(C); Gov. Code § 66321(b)(3)). So even on a tighter lot, an 800-sqft unit is protected.

Owner-occupancy

Arcadia does not require the owner to live on the property for an ADU. Subsection (E)(6)(A) states plainly that ADUs created on or after January 1, 2020 carry no owner-occupancy requirement — matching Gov. Code § 66315, which bars it statewide. The ordinance still requires owner-occupancy for every junior ADU (subsection (E)(6)(B)), but that language predates AB 1154. Effective January 1, 2026, AB 1154 amended Gov. Code § 66333(b) so JADU owner-occupancy applies only when the JADU shares sanitation facilities with the primary dwelling. Because Arcadia last amended this section in April 2025, the local text is operatively behind the state rule — a JADU with its own dedicated bathroom carries no owner-occupancy mandate. We cover this in the state-law section below.

Impact fees and utility connections

No development impact fee may be charged on an ADU under 750 sqft, and state law exempts ADUs of 750 sqft or less outright (subsection (G)(1); Gov. Code § 66311.5(c)). Above that threshold, impact fees are charged proportionally to the primary dwelling’s square footage — the ADU’s floor area divided by the primary’s, times the usual fee — not a flat per-unit charge. Utility connections follow the same logic: a converted ADU on a single-family lot generally needs no new connection or capacity charge, while other ADUs may require a separate connection priced to the burden the unit actually adds (subsection (G)(2)).

Permitting timeline

ADU and JADU applications are reviewed ministerially — no public hearing, no discretionary design review, no neighbor sign-off (subsection (D)(3); Gov. Code § 66317). The City must approve or deny a complete application within 60 days or it is deemed approved, and SB 543 (effective January 1, 2026) adds a written completeness determination within 15 business days (Gov. Code § 66317(a)(2)). A demolition permit for a detached garage that an ADU will replace is reviewed and issued alongside the ADU (subsection (D)(3)(D)).

How California state law overrides Arcadia

Because Arcadia rebuilt its ordinance in April 2025 to answer HCD’s findings, it has fewer state-local conflicts than most cities — the unit stack, the size floors, parking, and ADU owner-occupancy already track the Government Code. But a few points still deserve the preemption flag, and under Gov. Code § 66316 the state standard controls wherever the local text reads narrower.

• Junior-ADU owner-occupancy is narrower than the local text. Subsection (E)(6)(B) requires owner-occupancy for every JADU. AB 1154 amended Gov. Code § 66333(b) effective January 1, 2026 to require it only when the JADU shares a bathroom with the main home. Arcadia’s ordinance predates that change, so the broad local rule is operatively superseded: a JADU with its own dedicated bathroom carries no owner-occupancy mandate. This is the one place Arcadia’s text is behind state law.
• The 800-sqft floor beats local coverage and FAR rules. An 800-sqft ADU at 16 feet with 4-foot setbacks must be approved regardless of lot-coverage, floor-area-ratio, or open-space limits (Gov. Code § 66321(b)(3)). Arcadia writes this protection into subsection (F)(1)(C) itself — here the ordinance and the statute agree.
• Attached ADUs get 25 feet by right. Whatever the underlying zone says, state law guarantees an attached ADU up to 25 feet or the zone’s primary-dwelling height limit, and up to two stories (Gov. Code § 66321(b)(4)(D)) — the figure subsection (E)(2)(D) publishes.
• Garage-conversion replacement parking is barred. Gov. Code § 66314(d)(11) prohibits requiring replacement parking when a garage is demolished or converted for an ADU. Subsection (F)(7)(C) reflects this in full.
• No owner-occupancy for an ADU. Gov. Code § 66315 bars an ADU owner-occupancy requirement statewide, and subsection (E)(6)(A) states the same — confirmation, not conflict.

Renting an ADU in the San Gabriel Valley

Plan an Arcadia ADU around 28-day-or-longer tenancy. The City bars renting an ADU or JADU for any term shorter than 28 days (Arcadia Development Code § 9102.01.080(E)(4)) — its own figure, slightly shorter than the 30-day minimum state law lets a city set (Gov. Code § 66323(e)). Either way, nightly and weekly short-term rental is out; the compliant model is a long-term lease or a furnished mid-term rental — corporate, traveling-medical, or academic tenants.

The benchmark worth anchoring to: HUD’s Fair Market Rents for the Los Angeles–Long Beach–Glendale metro area, revised effective May 21, 2026, put a one-bedroom at $2,328, a two-bedroom at $2,903, and a three-bedroom at $3,681 per month (FY 2026 FMRs, huduser.gov; 91 Fed. Reg. 21301). A newly built detached ADU with in-unit laundry and a private entrance typically rents at or above those figures, and Arcadia’s drivers are durable: top-rated public schools that pull families into long leases, a deep San Gabriel Valley multigenerational housing culture, and steady demand from the medical and university campuses across the central San Gabriel Valley.

For payback math keyed to your lot and financing assumptions, use the ADU ROI calculator.

Newly constructed ADUs first occupied after February 1, 1995 are generally exempt from California rent-ceiling controls under the Costa-Hawkins Rental Housing Act (Civ. Code § 1954.50 et seq.). The statewide rent-cap framework under AB 1482 (Civ. Code § 1947.12) may still apply depending on the ADU’s structure and ownership — confirm before you lease.

HUD Fair Market Rents are a federal benchmark for the metro area, not a guarantee of what your unit will rent for. Actual performance depends on finish level, lot, access, parking, and the rental market when you lease.