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

What you can build on your lot

Everything in this section is grounded in AMC § 23.22.040 and the City of Alhambra’s published ADU development standards, with state-law citations called out where they preempt or supplement the local rule. We won’t re-cite the local section on every line — assume it under each heading unless we tell you otherwise.

Number of ADUs per lot

• Single-family lot. Per Gov. Code § 66323(a), you may combine a conversion ADU created from existing space, a junior ADU (≤ 500 sqft, inside the home), and a newly built detached ADU of up to 800 sqft — up to two ADUs plus a JADU. Alhambra’s ordinance describes one ADU plus one JADU; where the local text is narrower than the state stack, state law controls (see the state-law section below).
• Multifamily lot — existing building. Up to eight detached ADUs, not to exceed the number of existing units, plus conversion ADUs in non-livable space (at least one, up to 25% of the existing units). Alhambra’s ordinance already reflects the SB 1211 eight-unit allowance.
• Multifamily lot — proposed building. Up to two detached ADUs (Gov. Code § 66323(a)(4)).

Size limits

• Detached ADU (single-family lot): up to 1,200 sqft regardless of bedroom count.
• Attached ADU: up to 50% of the main dwelling, or 850 sqft for a studio/1-bedroom and 1,000 sqft for a 2-bedroom or larger, whichever is greater.
• Statewide-exemption ADU: an 800-sqft unit at 16 feet with 4-foot side and rear setbacks must be permitted regardless of the underlying zone’s lot coverage, floor-area ratio, or open-space limits (Gov. Code § 66321(b)(3)).
• Conversion ADU: no square-foot cap when created within existing permitted space, plus up to 150 sqft of expansion for ingress and egress (Gov. Code § 66323).
• Junior ADU: up to 500 sqft inside the primary dwelling (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). No setback is required for an ADU built within existing living area, within an accessory structure, or rebuilt in the same footprint and dimensions as an existing structure. A detached ADU keeps a minimum building separation from other structures, and exterior mechanical equipment sits at least 3 feet from any property line. An ADU in front of the primary dwelling follows the front-yard setback of the underlying zone — unless that would block a statewide-exemption unit, which Gov. Code § 66321(b)(3) protects.

Maximum height — what this means for design

This is where Alhambra stands apart. The City’s own ADU standards permit detached and attached ADUs up to two stories and 25 feet on a single-family lot (AMC § 23.22.040(G)). The state floor is lower — 16 feet for a standard detached unit, 18 feet within a half-mile of transit, plus 2 feet to match a roof pitch (Gov. Code § 66321(b)(4)(A)–(B)) — but Alhambra exceeds it. Because the detached envelope reaches 25 feet, our detached two-story Signature Homes are buildable here as standalone units, not only as attached additions. Most San Gabriel Valley cities cap detached ADUs at the 16-foot floor, which forecloses a detached two-story; Alhambra does not. That makes it a two-story-friendly detached market — see the dedicated section below.

Parking

Alhambra nominally requires one parking space per ADU, but subsection (E)(4) waives it whenever any one of five conditions applies: the ADU is within a half-mile of a public transit stop (bus stops included), it is an attached ADU, it sits on a historic-listed parcel, on-street permits are required but not offered to the occupant, or a car-share hub is within one block. These mirror the state exemptions in Gov. Code § 66322 — each one is independent, and any one removes the requirement. Converting a garage, carport, or covered parking space never triggers replacement parking for the main home (Gov. Code § 66314(d)(11)), and a JADU never requires parking at all (Gov. Code § 66334(a)). Much of Alhambra’s dense, bus-served grid qualifies under the half-mile transit exemption.

Lot coverage, FAR, and the design-match rule

An ADU is generally subject to the floor-area, lot-coverage, and open-space standards of the underlying zone — with one large carve-out: an 800-sqft ADU at 16 feet with 4-foot setbacks is exempt from those standards (Gov. Code § 66321(b)(3)). Alhambra also requires an ADU to match the main dwelling’s exterior materials, roof covering, colors, and architectural details (AMC § 23.22.040(C)(2)). That design-match rule cannot be imposed on a statewide-exemption ADU under Gov. Code § 66323(b) — HCD’s Handbook (p. 16) confirms § 66323 units are exempt from local design standards. For every other ADU, design-match applies, and our Signature Homes are engineered to read naturally next to a Craftsman, Spanish, or Mid-Century Alhambra home.

Owner-occupancy

Alhambra does not require the owner to live on the property for an ADU — AMC § 23.22.040(F)(1) says so outright, matching Gov. Code § 66315. For a junior ADU, the owner must occupy either the JADU or the main dwelling (AMC § 23.22.040(F)(2)). But AB 1154 narrowed that rule effective January 1, 2026: it amended Gov. Code § 66333(b) so JADU owner-occupancy applies only when the JADU shares sanitation facilities — a bathroom — with the primary dwelling. A JADU with its own dedicated bathroom is no longer subject to the owner-occupancy mandate under state law. Alhambra’s ordinance has not yet been amended to reflect AB 1154, so its broader text is now narrower under state law and is preempted on this point.

Impact fees

No impact fees may be charged on any ADU under 750 sqft, and a JADU under 500 sqft is exempt as well (Gov. Code § 66324(c)(1); § 66311.5). For an ADU over 750 sqft, impact fees must be proportional to the primary dwelling’s size — not a flat per-unit charge. Standard building-permit and plan-check fees still apply and are based on project valuation, not on the fact that you are building an ADU.

Permitting timeline

ADU and JADU applications are reviewed ministerially — no public hearing, no design review, no neighbor sign-off (AMC § 23.22.040(C)(1); Gov. Code § 66317). The City must approve or deny a complete application within 60 days, and SB 543 (effective January 1, 2026) requires the City to determine completeness and notify you in writing within 15 business days (Gov. Code § 66317).

How California state law overrides Alhambra

Alhambra’s ordinance is recent and compliant on most standards. On a few points, though, state law gives you more than the local text describes — and under Gov. Code § 66316, the state standard controls.

• More units per single-family lot. AMC § 23.22.040(B) describes one ADU plus one JADU. State law allows more: under Gov. Code § 66323(a)(1)–(2) and the HCD ADU Handbook (pp. 17–18), a single-family lot may combine a conversion ADU, a JADU, and a new detached ADU of up to 800 sqft — up to two ADUs plus a JADU. We submit under the state stack and document the preemption.
• JADU owner-occupancy is narrower now. AB 1154 amended Gov. Code § 66333(b) effective January 1, 2026 so JADU owner-occupancy applies only when the JADU shares a bathroom with the main home. Alhambra’s text still describes the broader pre-2026 rule; a JADU with its own bathroom is exempt under state law.
• Design-match cannot reach a § 66323 unit. AMC § 23.22.040(C)(2) requires an ADU to match the main house. A statewide-exemption ADU (800 sqft / 16 ft / 4-ft setbacks) and any conversion ADU are exempt from local design standards (Gov. Code § 66323(b); HCD Handbook, p. 16), so the match rule cannot be imposed on those units.
• 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. Alhambra’s ordinance already reflects this in § 23.22.040(E)(3).
• The 800-sqft floor beats local coverage limits. An 800-sqft ADU at 16 feet with 4-foot setbacks must be allowed regardless of lot coverage, floor-area ratio, or open-space rules (Gov. Code § 66321(b)(3)).

Renting an ADU in the San Gabriel Valley

Plan an Alhambra ADU around 30-day-or-longer tenancy. State law lets a city require rental terms longer than 30 days, which rules out short-term Airbnb-style rental (Gov. Code § 66323(e) for ADUs; § 66333(g) for JADUs). The compliant income model is a long-term residential lease or a furnished mid-term rental — corporate, traveling-medical, or academic tenants — which often prices above an unfurnished 12-month rent.

That suits Alhambra. The San Gabriel Valley has deep, steady rental demand — proximity to Cal State LA, the Caltech/Pasadena employment base, downtown LA via the I-10 and Metro, and a multigenerational housing culture that keeps well-built detached units in demand. Here is what each Signature Home rents for in the area today (directional ranges):

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.

The rent ranges above are directional market estimates based on publicly available San Gabriel Valley listings for comparable 30-day-or-longer rentals. They are not a guarantee. Actual performance depends on finish level, lot, access, parking, and the rental market when you lease.