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

What you can build on your lot

Everything in this section is grounded in the California Government Code — the rules that govern a Cerritos ADU directly, since the City has no compliant local ordinance — cross-checked against the HCD ADU Handbook. We won’t re-cite the code on every line; assume it under each heading unless we tell you otherwise.

Number of ADUs per lot

• Single-family lot. State law requires the City to allow a conversion ADU created from existing space, plus a junior ADU, plus a new detached ADU of up to 800 sqft — up to two ADUs and a JADU on one lot (Gov. Code § 66323(a)(1)–(2)). There is no minimum lot size: Gov. Code § 66314(b)(1)(A) bars one, so the old local 10,000 sqft threshold does not apply.
• 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 on a lot with an existing multifamily building, not to exceed the number of existing units (Gov. Code § 66323(a)(3)–(4), as amended by SB 1211).
• Existing accessory structures. A detached accessory structure may be converted to an ADU — an old workshop or oversized garage can become a unit without being held to the new-construction size limits.

Size limits

• Detached ADU: buildable by right up to 800 sqft, and a city must allow at least 850 sqft for a studio or one-bedroom and 1,000 sqft for two or more bedrooms (Gov. Code §§ 66321(b)(2), 66323(a)(2)) — roomy enough for our 1,000 sqft single-story flagship, the Lincoln.
• Attached ADU: up to 50% of the existing primary dwelling under the usual state framework — but never forced below the 800 sqft floor or the 850/1,000 sqft size floors (Gov. Code § 66321(b)(2)–(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 (Gov. Code § 66313(d)).

Setbacks

Side and rear setbacks are 4 feet for a new single-story ADU — the state-law floor in Gov. Code § 66314(d)(7) — and the front setback follows the underlying zoning district. No added setback is required for an ADU built within an existing structure, whether a conversion or a same-footprint replacement. On a typical Cerritos parcel the practical move is to set the unit in the rear yard and let the 4-foot envelope do the work.

Maximum height — single story by right

A detached ADU is capped at 16 feet under the state floor that governs here (Gov. Code § 66321(b)(4)(A)). A lot within a half-mile of a major transit stop or a high-quality transit corridor may reach 18 feet, plus 2 feet to match the primary roof pitch (Gov. Code § 66321(b)(4)(B)). An attached ADU may reach 25 feet and is not limited to one story (Gov. Code § 66321(b)(4)(D)). In plain terms: a standard by-right backyard ADU in Cerritos is a single-story project. Our three two-story Signature Homes — the Fairfax, Venice, and Culver — are detached designs that need roughly a 25-foot detached envelope, so they generally do not fit a Cerritos lot. We say that plainly because it shapes the plan: on a Cerritos lot the right move is one of our six single-story models, and the 1,000-square-foot state size floor is roomy enough to put a full three-bedroom single-level home in the back yard.

Parking

State law lets a city require at most one off-street parking space per ADU, and waives even that in a list of situations — a lot within one-half mile of public transit, an ADU within an existing structure, a converted garage, and a historic district among them (Gov. Code § 66322). 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 (Gov. Code § 66314(d)(11)). The old local code’s flat ban on garage conversions does not survive that rule.

Lot coverage, FAR, and open space

The underlying zone’s floor-area-ratio, lot-coverage, and open-space limits still apply to an ADU, but every one of them is expressly subject to the 800-sqft floor: none may force an ADU below 800 square feet (Gov. Code § 66321(b)(3)). On Cerritos’s generous planned-community lots these rarely bind a backyard unit, and where a tighter parcel comes into play, an 800-sqft unit is still protected.

Owner-occupancy

You do not have to live on the property to build an ADU in Cerritos. Gov. Code § 66315 bars an owner-occupancy requirement for an ADU statewide, and with no local ordinance, that rule applies directly. The old local code does not address junior ADUs at all, so the state junior-ADU rule governs: Gov. Code § 66333 requires owner-occupancy for a JADU, and AB 1154 — effective January 1, 2026 — narrowed that to JADUs that share sanitation facilities with the primary home (Gov. Code § 66333(b)). A junior ADU with its own dedicated bathroom carries no owner-occupancy mandate.

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 (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. An ADU generally relies on the primary dwelling’s existing utility connections; where a separate connection is required, the connection and capacity charge are priced to the burden the unit actually adds.

Permitting timeline

ADU and JADU applications are reviewed ministerially — no public hearing, no discretionary design review, no neighbor sign-off (Gov. Code § 66317). The City must approve or deny a complete application within 60 days or it is deemed approved. SB 543 (effective January 1, 2026) adds a written completeness determination within 15 business days (Gov. Code § 66317(a)(2)).

How state law governs your Cerritos ADU

Because Cerritos has no compliant ADU ordinance, state law does not just override the local rule on a few points — it supplies the entire rulebook (Gov. Code § 66316). Here is where that matters most against the old “second unit” code, and where it gives you more than the 2006 text ever did.

• No minimum lot size. The old code allowed a second unit only on a lot of 10,000 sqft or larger. Gov. Code § 66314(b)(1)(A) bars any minimum-lot-size requirement for an ADU, so a standard Cerritos parcel qualifies.
• You can build more than one unit. The old code allowed a single second unit. State law requires the City to allow the full stack: a conversion ADU from existing space, plus a junior ADU, plus a new detached ADU of up to 800 sqft — up to two ADUs and a JADU on one single-family lot (Gov. Code § 66323(a)(1)–(2); HCD ADU Handbook, March 2026, pp. 17–18).
• Garage conversions are protected. The old code banned converting a garage to a second unit. That ban is preempted — a garage conversion is a by-right ADU type (Gov. Code § 66323(a)(1)), and no replacement parking may be required (Gov. Code § 66314(d)(11)).
• Bigger than 750 square feet, more than one bedroom. The old 750 sqft / one-bedroom / two-occupant caps are preempted by the state size floors — at least 850 sqft for a studio or one-bedroom and 1,000 sqft for two or more bedrooms (Gov. Code § 66321(b)(2)) — with no bedroom or occupancy cap.
• Ministerial review, no design board. The old discretionary “second unit permit” and its subjective design review are preempted by the 60-day ministerial standard (Gov. Code § 66317) — no hearing, no neighbor appeal, objective standards only.
• No owner-occupancy for an ADU. Gov. Code § 66315 bars an ADU owner-occupancy requirement statewide; for a junior ADU, AB 1154 narrowed it to shared-sanitation cases as of January 1, 2026 (Gov. Code § 66333(b)).

Renting an ADU in the Gateway Cities

Plan a Cerritos ADU around 30-day-or-longer tenancy. State law lets a city require a rental term longer than 30 days (Gov. Code § 66323(e) for ADUs; § 66333(g) for JADUs), which rules out nightly and weekly short-term rental. The compliant model is a long-term residential lease or a furnished mid-term rental of 30 days or longer — 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 ADU with in-unit laundry and a private entrance typically rents at or above those figures — and the 1,000-square-foot state size floor reaches a full three-bedroom single-level home, which lands in the three-bedroom band. Cerritos’s rental drivers are durable: well-regarded ABC Unified schools, a deep multigenerational housing culture, a strong retail and employment base around the Los Cerritos Center and Cerritos Towne Center, and freeway access via the 91, 605, and 5.

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.