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

What you can build on your lot

Everything in this section is grounded in Monrovia Municipal Code § 17.44.005 and the California Government Code that supersedes it on the few points where the two disagree. Because the 2020 ordinance is mostly aligned with state law, we name the local rule first and call out the state rule only where it changes the answer. Assume § 17.44.005 under each heading unless we tell you otherwise.

Number of ADUs per lot

• Single-family lot. The ordinance describes one ADU plus one junior ADU (§ 17.44.005(B)(3), (D)(1)(c)). State law goes further: it 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); HCD ADU Handbook, March 2026, pp. 17–18).
• Multifamily lot — existing building. The ordinance allows conversion ADUs in non-habitable space (at least one, up to 25% of existing units) and two detached ADUs (§ 17.44.005(B)(4)). State law now allows up to eight detached ADUs on a lot with an existing multifamily building under SB 1211, not to exceed the number of existing units (Gov. Code § 66323(a)(4)) — so the local two-unit cap is superseded.
• Existing accessory structures. A detached accessory structure may be converted to an ADU, with up to 150 sqft added for ingress and egress (§ 17.44.005(B)(2); Gov. Code § 66323(a)(1)) — an old workshop or oversized garage can become a unit without being held to the new-construction size limits.

Size limits

• Detached: up to 1,000 sqft (§ 17.44.005(D)(2)(a)); held to 800 sqft when it is the by-right detached unit built alongside a JADU (§ 17.44.005(D)(1)(c); Gov. Code § 66323(a)(2)).
• Attached: up to 50% of the primary dwelling’s gross floor area (§ 17.44.005(D)(2)(a)) — but that percentage may never push a unit below the state floor of 850 sqft for a studio or one-bedroom and 1,000 sqft for two or more bedrooms (Gov. Code § 66321(b)(2)).
• The 800 sqft unit is protected outright. An 800 sqft ADU must be allowed regardless of any FAR, lot-coverage, or open-space limit (Gov. Code § 66321(b)(3)).
• Conversion ADU: no separate square-foot cap when created within existing permitted space (Gov. Code § 66323(a)(1)). Junior ADU: up to 500 sqft inside the home (Gov. Code § 66313(d)). Minimum unit size is 220 sqft (§ 17.44.005(D)(2)(a)).

Setbacks

Monrovia requires a four-foot side and rear setback for a new detached ADU (§ 17.44.005(D)(2)(b)) — the same number state law caps the requirement at (Gov. Code § 66314(d)(7)) — and no added setback for an ADU built within an existing structure, whether a conversion or a same-footprint replacement. The front setback follows the underlying zone, and an ADU generally may not sit between the primary home and the street except where the standards would otherwise block an 800 sqft, 16-foot, four-foot-setback unit. On a typical Monrovia parcel the practical move is to set the unit in the rear yard and let the four-foot envelope do the work.

Maximum height — single story by right

A detached ADU in Monrovia is limited to 16 feet and one story (§ 17.44.005(D)(2)(c)), which matches the state floor (Gov. Code § 66321(b)(4)(A)) — so this is a single-story market for a standalone backyard unit. Within a half mile of a major transit stop, state law raises the detached floor to 18 feet, plus up to 2 feet to match the primary roof pitch (§ 66321(b)(4)(B)). An attached ADU may reach 25 feet or the height of the primary dwelling (Gov. Code § 66321(b)(4)(D)) — the local 16-foot cap is preempted for attached units. For a standalone detached home, our three two-story Signature Homes — the Fairfax, Venice, and Culver — are detached designs that need roughly a 25-foot standalone envelope, and we do not sell them as attached units, so on a Monrovia lot the right move is one of our six single-story models.

Parking

Monrovia requires one off-street parking space for an ADU (§ 17.44.005(D)(2)(g)), waived in five situations: within one-half mile of transit, in a historic district, when the ADU is a conversion or part of a proposed home, in an on-street-permit area where the occupant is not offered a permit, or within one block of a car-share vehicle. That list tracks the state framework in Gov. Code § 66322. When a garage is converted or demolished to build the ADU, no replacement parking may be required (Gov. Code § 66314(d)(11), consistent with § 17.44.005(D)(2)(g)), and a junior ADU never requires parking (Gov. Code § 66334(a)). Where a space is required, it may be tandem or in the setback area, capped at 25% of the front yard.

Lot coverage, FAR, and open space

The ordinance applies the underlying zone’s development standards, but any FAR, lot-coverage, or open-space limit is expressly subject to the 800-sqft floor: none may force an ADU below 800 square feet (Gov. Code § 66321(b)(3)). On Monrovia’s generous foothill single-family lots these rarely bind a backyard unit, and where a tighter parcel comes into play, an 800-sqft unit is still protected.

Owner-occupancy

Monrovia does not require owner-occupancy for a standard ADU — consistent with Gov. Code § 66315, which bars an ADU owner-occupancy mandate statewide. For a junior ADU, the ordinance requires the owner to live in either the main house or the JADU (§ 17.44.005(B)(3), (C)), but AB 1154 amended Gov. Code § 66333(b) effective January 1, 2026 to narrow that requirement to junior ADUs that share a bathroom with the primary dwelling. A junior ADU with its own dedicated bathroom no longer carries an owner-occupancy mandate, even though the 2020 ordinance still describes the broader rule.

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 — not a flat per-unit charge. Monrovia’s ordinance goes a step further and expressly charges no traffic impact fees on ADUs, and routes an ADU’s water and sewer through the primary dwelling’s points of connection (§ 17.44.005(E)); where a separate connection is required, the connection and capacity charge are priced to the burden the unit actually adds.

Permitting timeline

A qualifying ADU runs as a ministerial “building permit only” approval through the Planning Division (§ 17.44.005(D)(1)) — 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)(A)).

How California state law overrides the local ordinance

Monrovia’s 2020 ordinance is mostly current, so the list of overrides is short — but it matters, because every item on it gives you more than the local text alone suggests. Under Gov. Code § 66316 the state standard controls wherever the local rule is narrower.

• You can build more than one ADU. The ordinance frames the lot as one ADU plus one JADU (§ 17.44.005(B)(3), (D)(1)(c)). State law requires the full stack: a conversion ADU, 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).
• Attached ADUs can be taller. The local 16-foot cap is preempted for attached units, which may reach 25 feet or the height of the primary dwelling (Gov. Code § 66321(b)(4)(D)). A standalone detached unit stays at the 16-foot floor.
• Up to eight detached ADUs on a multifamily lot. The ordinance caps multifamily detached ADUs at two (§ 17.44.005(B)(4)); SB 1211 raised that to as many as eight on a lot with an existing multifamily building (Gov. Code § 66323(a)(4)).
• No owner-occupancy for an ADU; narrowed for a JADU. Monrovia already imposes no ADU owner-occupancy requirement, matching Gov. Code § 66315. For a junior ADU, AB 1154 narrows owner-occupancy to shared-bathroom cases as of January 1, 2026 (Gov. Code § 66333(b)), superseding the broader 2020 language.
• The 800-sqft unit is guaranteed. An 800-sqft ADU at 16 feet with four-foot setbacks must be approved regardless of lot-coverage, FAR, or open-space limits (Gov. Code § 66321(b)(3)), and a 50%-of- primary attached rule may never drop a unit below the 850/1,000 sqft floors (§ 66321(b)(2)).

Renting an ADU in the San Gabriel Valley

Plan a Monrovia ADU around 30-day-or-longer tenancy. The City’s recorded covenant prohibits a rental term shorter than 30 consecutive days (§ 17.44.005(C)) — the 30-day minimum state law lets a city set (Gov. Code § 66323(e)). 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.

For a sense of the ceiling, the federal Fair Market Rent benchmark for the Los Angeles–Long Beach–Glendale area — which includes Monrovia — runs about $2,328 for a one-bedroom, $2,903 for a two-bedroom, and $3,681 for a three-bedroom (HUD Fair Market Rents, FY 2026, effective May 21, 2026). A new, detached, never-lived-in ADU typically rents at or above those figures, because FMR is a market-wide midpoint that includes older stock. The Melrose (800 sqft, two-bedroom) and the Lincoln (1,000 sqft, three-bedroom) reach the upper end of that range — demand helped by Monrovia’s Gold Line station, its walkable Old Town, and the foothill setting that draws long-term tenants.