Unincorporated Trinity County ADU rules

Maximum size

A detached ADU may be up to 1,200 sq ft, and at least 150 sq ft or the building code's minimum for a dwelling. An attached ADU may add no more than 50% of the main home's living area, up to 1,200 sq ft. The County's rules may not block an 800 sq ft unit with 4 ft setbacks. State law also bars local size limits below 850 sq ft, or 1,000 sq ft with more than one bedroom (Gov. Code § 66321(b)(2)).

Source: Trinity County Ordinance 315-860 (adopted December 16, 2025), amending Zoning Code Section 17.30.040(C) Accessory Dwelling Units, § 17.30.040(C)(4)(b)-(c), (C)(4)(d)(i), accessed 2026-11-01.

Setbacks

The code sets a 4 ft setback for ADUs and says setbacks cannot block a unit of at least 800 sq ft. It does not say which lot lines the 4 ft figure covers. ADUs must also meet the state fire safe rules for new structures (Public Resources Code §§ 4290 and 4291).

Source: Trinity County Ordinance 315-860 (adopted December 16, 2025), amending Zoning Code Section 17.30.040(C) Accessory Dwelling Units, § 17.30.040(C)(4)(d)(i), (C)(4)(j), accessed 2026-11-01.

Local ADU rules

The County's ADU rules are in Zoning Code Section 17.30.040(C), as rewritten by Ordinance 315-860, adopted December 16, 2025 and in effect 30 days later. The County posts the signed ordinance in marked-up form (struck and added text) and a clean copy of the new text. ADU and JADU applications are reviewed ministerially, without a hearing, and the County acts within 60 days of a complete application. A demolition permit for a detached garage being replaced by an ADU is issued with the ADU permit.

Source: Trinity County Ordinance 315-860 (adopted December 16, 2025), amending Zoning Code Section 17.30.040(C) Accessory Dwelling Units, § 17.30.040(C)(3)(a)-(d); Section IV, accessed 2026-11-01.

ADU types and how many

ADUs are allowed by right in all zones that allow single-family or multifamily homes. On a single-family lot under 1 acre, up to two detached ADUs and one JADU are allowed; on 1 to 2 acres, up to three detached ADUs and one JADU; over 2 acres, up to four detached ADUs and one JADU. For these extra ADUs you must show public sewer or enough septic capacity. On a lot with an existing multifamily building: ADUs in non-livable space (such as a leasing office) up to 25% of the existing units, and at least one, plus up to eight detached ADUs, not more than the number of existing units. A lot with a planned multifamily building may have up to two detached ADUs. In multifamily zones, up to five more ADUs are allowed if half of all units are kept for lower income households for at least three years.

Source: Trinity County Ordinance 315-860 (adopted December 16, 2025), amending Zoning Code Section 17.30.040(C) Accessory Dwelling Units, § 17.30.040(C)(4)(a), (C)(4)(l), (C)(5), accessed 2026-11-01.

Height

An ADU may be up to 25 ft tall. The code also limits an ADU to the height of the main home (with a 20 ft house, a 20 ft ADU), where that does not go below state law. If a two-story detached ADU fits the state height allowances and the building code, zoning that limits the main home to one story cannot be used to deny it.

Source: Trinity County Ordinance 315-860 (adopted December 16, 2025), amending Zoning Code Section 17.30.040(C) Accessory Dwelling Units, § 17.30.040(C)(4)(d)(ii), accessed 2026-11-01.

Parking

One parking space per ADU, in addition to the main home's parking. No space is needed within a half mile of a public transit stop, in a historic district, for an ADU inside the existing home or an existing accessory structure, where on-street parking permits are required but not offered to the ADU occupant, within one block of a car share vehicle, or when the ADU is applied for together with a new home on the lot.

Source: Trinity County Ordinance 315-860 (adopted December 16, 2025), amending Zoning Code Section 17.30.040(C) Accessory Dwelling Units, § 17.30.040(C)(4)(e), accessed 2026-11-01.

Junior ADUs

The code sets no size or design standards of its own for JADUs; state law (Gov. Code §§ 66333 to 66339) applies, under which a JADU is up to 500 sq ft and inside the home. JADU applications are reviewed ministerially within 60 days, and one JADU is allowed alongside the detached ADUs listed above.

Source: Trinity County Ordinance 315-860 (adopted December 16, 2025), amending Zoning Code Section 17.30.040(C) Accessory Dwelling Units, § 17.30.040(C)(3)(a)-(b), (C)(5)(b), accessed 2026-11-01.

Sewer, water, fees and rental

A new ADU needs its own sewer hookup, unless it is inside the existing home or an existing accessory structure. Outside a sewer district, a detached ADU needs its own septic system, and Environmental Health checks the existing septic system for an attached ADU. The ADU and main home may share a water supply if it gives at least 3 gallons per minute per unit plus fire protection water. ADUs are not counted as new residential uses for utility connection fees. ADUs may not be rented for less than 30 days.

Source: Trinity County Ordinance 315-860 (adopted December 16, 2025), amending Zoning Code Section 17.30.040(C) Accessory Dwelling Units, § 17.30.040(C)(4)(f)-(h), (C)(4)(k), accessed 2026-11-01.

Movable tiny homes

Separate from the ADU rules, Chapter 15.27 (Ordinance 1375, adopted June 17, 2025) allows a movable tiny home up to 400 sq ft and one story (lofts allowed) by right in zones that allow a single-family home by right. It needs a building permit, zoning clearance, its own address, fire safe features and water and septic sign-off, must look like a house with wheels hidden, and must meet the zone's setbacks.

Source: Trinity County Ordinance 1375, Chapter 15.27 Movable Tiny Homes, §§ 15.27.040, 15.27.050, accessed 2026-11-01.