Unincorporated Humboldt County ADU rules

Maximum size (non-coastal areas only)

Outside the Coastal Zone only (does not apply to Coastal Zone parcels): A detached ADU may be up to 1,200 sq ft. An attached ADU may be up to 50% of the existing main home, and no more than 1,200 sq ft. Larger ADUs may be allowed with a Special Permit. The minimum is 150 sq ft. A new detached ADU built together with a JADU is limited to 800 sq ft. State law still allows an 800 sq ft ADU with 4 ft side and rear setbacks.

Source: Humboldt County Code § 314-69.05 Accessory Dwelling Units (Chapter 4, Inland Zoning Regulations), § 314-69.05.4.3, .4.3.1, .3.8.2.1, accessed 2026-11-01.

Setbacks (non-coastal areas only)

Outside the Coastal Zone only (does not apply to Coastal Zone parcels): A new ADU needs side and rear setbacks of no more than 4 ft. No setback is needed for an ADU converted from existing living area or an existing structure. An ADU inside an existing home needs side and rear setbacks that the fire authority finds sufficient for fire safety. The ADU section sets no front setback figure.

Source: Humboldt County Code § 314-69.05 Accessory Dwelling Units (Chapter 4, Inland Zoning Regulations), § 314-69.05.4.5, .3.8.1.3, .3.8.2, accessed 2026-11-01.

Local regulations (non-coastal areas only)

Outside the Coastal Zone only (does not apply to Coastal Zone parcels): Humboldt County's ADU rules outside the Coastal Zone are in County Code § 314-69.05 (Inland Zoning Regulations), adopted in 2020 (Ordinance 2650) and last amended by Ordinance 2678 (July 13, 2021). Land along the coast and Humboldt Bay is in the Coastal Zone, where § 313-69.05 applies instead. This lookup cannot tell whether a parcel is in the Coastal Zone; check with the Planning and Building Department.

Source: Humboldt County Ordinance No. 2678 amending § 314-69.05 (Board of Supervisors, July 13, 2021), § 314-69.05, accessed 2026-11-01.

Coastal Zone rules

In the Coastal Zone, ADUs follow County Code § 313-69.05. ADUs in the RS, RM, R-2, RA, AE, TC and TPZ zones need a coastal development permit; converting space inside a home generally does not, unless the home gets bigger. The figures are much like the inland rules: a detached ADU up to 1,200 sq ft, an attached ADU up to 50% of the main home and 1,200 sq ft, 4 ft side and rear setbacks for new ADUs (none for conversions), a new detached ADU up to 800 sq ft and 16 ft alongside a JADU, and one parking space per ADU (tandem allowed) with the same exemptions. Coastal policies on public access, views, habitat, farmland and timberland also apply.

Source: Humboldt County Code § 313-69.05 Accessory Dwelling Units (Chapter 3, Coastal Zoning Regulations), § 313-69.05.2, .3.8.2, .4.3, .4.5, .4.6, accessed 2026-11-01.

Local ADU rules (non-coastal areas only)

Outside the Coastal Zone only (does not apply to Coastal Zone parcels): ADUs are a principal permitted use in any zone that allows single-family or multifamily homes, and need only a building permit unless the lot is in an ADU Special Permit Area or the ADU goes beyond the code's standards. The configurations listed in § 314-69.05.3.8 get a ministerial building permit, and the County acts on a complete application within 60 days when a home already exists on the lot.

Source: Humboldt County Code § 314-69.05 Accessory Dwelling Units (Chapter 4, Inland Zoning Regulations), § 314-69.05.2, .3.8, accessed 2026-11-01.

ADU types and how many (non-coastal areas only)

Outside the Coastal Zone only (does not apply to Coastal Zone parcels): On a lot with a single-family home, the code lists one ADU inside a proposed or existing home or accessory structure (with up to 150 sq ft added for entry and exit), or one new detached ADU up to 800 sq ft and 16 ft with 4 ft side and rear setbacks together with one JADU. On a lot with a multifamily building: ADUs in non-livable space, at least one and up to 25% of the existing units. On AE lots of 40 acres or more, an ADU of any size may be allowed. An ADU may be site-built, a tiny house, a moveable tiny house or a manufactured home.

Source: Humboldt County Code § 314-69.05 Accessory Dwelling Units (Chapter 4, Inland Zoning Regulations), § 314-69.05.3.1, .3.5, .3.8.1-.3.8.3, accessed 2026-11-01.

ADU Special Permit Area (non-coastal areas only)

Outside the Coastal Zone only (does not apply to Coastal Zone parcels): Lots in the mapped ADU Special Permit Area are presumed to have limits on services or safety, and an ADU there may need a Special Permit or may not be allowed. The areas include lots outside a fire protection district, airport safety zones (where a Special Permit may not be issued above the allowed density), active or historic landslide and liquefaction areas, flood and tsunami hazard zones, and sites near toxic cleanup sites. Check with the Planning and Building Department before you apply. State law may still allow some ADUs here (Gov. Code § 66323).

Source: Humboldt County Code § 314-69.05 Accessory Dwelling Units (Chapter 4, Inland Zoning Regulations), § 314-69.05.6, accessed 2026-11-01.

Height (non-coastal areas only)

Outside the Coastal Zone only (does not apply to Coastal Zone parcels): Height limits for ADUs and JADUs must allow at least 16 ft, in spite of the general 15 ft limit in § 314-102.1. A new detached ADU built together with a JADU may be no more than 16 ft tall.

Source: Humboldt County Code § 314-69.05 Accessory Dwelling Units (Chapter 4, Inland Zoning Regulations), § 314-69.05.4.8, .3.8.2.1, accessed 2026-11-01.

Parking (non-coastal areas only)

Outside the Coastal Zone only (does not apply to Coastal Zone parcels): Each ADU needs one parking space, which may be tandem on a driveway or in a setback area unless the County finds that unsafe. No space is needed within a half mile walk of public transit, in an architecturally and historically significant district, for an ADU that is part of the home or an existing accessory structure, where on-street permits are required but not offered, or within one block of a car share vehicle. In mapped Housing Opportunity Zones, ADUs under 1,000 sq ft need no parking. Under state law, garage or carport spaces removed for an ADU need not be replaced.

Source: Humboldt County Code § 314-69.05 Accessory Dwelling Units (Chapter 4, Inland Zoning Regulations), § 314-69.05.4.6, .4.6.1, accessed 2026-11-01.

Junior ADUs (non-coastal areas only)

Outside the Coastal Zone only (does not apply to Coastal Zone parcels): A JADU may be up to 500 sq ft and must be inside a proposed or existing single-family home, with its own exterior entrance. It must meet state JADU law, which the code cites. One JADU may be added together with a new detached ADU up to 800 sq ft and 16 ft. No parking is needed for a JADU.

Source: Humboldt County Code § 314-69.05 Accessory Dwelling Units (Chapter 4, Inland Zoning Regulations), § 314-69.05.1, .3.8.1.2, .3.8.1.4, .3.8.2, accessed 2026-11-01.

Fees, utilities and rental (non-coastal areas only)

Outside the Coastal Zone only (does not apply to Coastal Zone parcels): No impact fees for an ADU under 750 sq ft; larger ADUs pay in proportion to the main home's size. An ADU inside existing space needs no new utility connection; a new detached ADU may need one, with a fee in proportion to its burden on the system. Fire sprinklers are needed only if the main home needs them. In an Urban Service Area with public sewer, the ADU must connect. An ADU may be rented, but not for 30 days or less, and may not be sold separately from the main home (a moveable tiny house may be sold once removed).

Source: Humboldt County Code § 314-69.05 Accessory Dwelling Units (Chapter 4, Inland Zoning Regulations), § 314-69.05.3.2-.3.4, .3.6, .4.1, .4.4, accessed 2026-11-01.