Mariposa County

The Mariposa County Planning Department issues all ADU permits in the county because every parcel outside Yosemite National Park is unincorporated. The Building Department handles plan check and inspections. Practical permitting frictions: very high fire severity zones (CAL FIRE State Responsibility Area covers essentially all of the county), Chapter 7A ignition-resistant construction documentation requirements, septic/well design review outside the small MPUD water/sewer service area in Mariposa town, steep-slope grading review in foothill terrain, oak woodland habitat considerations under the Mariposa County Oak Woodland Conservation Ordinance, and Yosemite gateway short-term-rental (STR) conditions of approval that may apply in Wawona, Fish Camp, and El Portal census-designated places where STR demand is highest. Mariposa County has had escalating tension with STR operators since the 2018-2022 surge in Yosemite-gateway vacation rentals; the Board of Supervisors has tightened STR registration but has NOT prohibited ADUs from STR use outright.

Process overview: Standard ministerial 60-day review per Sec. 65852.2(b) for compliant ADU applications. Parcels in the very high fire severity zone (essentially all of the county) require Chapter 7A ignition-resistant construction documentation and CAL FIRE / Mariposa County Fire defensible-space conditions of approval. Septic/well parcels require Environmental Health review (well permit, percolation test, septic system design) which adds 30-60 days outside the 60-day state ministerial window because state preemption does not constrain Environmental Health timelines on health-and-safety review. STR-restricted overlay zones in Wawona, Fish Camp, and El Portal CDPs may require a separate STR permit on top of the ADU permit if the owner intends short-term rental.

Impact fees: SB 13 fee waivers apply to ADUs under 750 sqft (no impact fees). Larger ADUs are charged proportionally to the primary dwelling. School district fees (Mariposa County Unified School District) apply to ADUs over 500 sqft per Education Code Sec. 17620.

The Mariposa County Assessor's Office maintains parcel-level assessment records for the entire county. ADU additions are captured as improvements to the host parcel via shared permit data with the Planning/Building Departments. California Proposition 13 caps base-year valuation increases at 2 percent per year on the existing structure; new improvement value (the ADU) is added as a separate line item assessed at fair market value at completion. Mariposa County's assessment roll is small (under 13,000 parcels including unimproved Forest Service inholdings outside Yosemite NP), so assessor response times are typically fast. The Yosemite National Park federal inholding generates no assessment roll because federal land is not taxable.

Assessment policy: ADU improvement value is added on the next regular revaluation cycle following completion, not at permit issuance. Per Prop 13, the ADU's value is taxed at 1 percent of fair market value at completion (plus voter-approved local rates), while the existing structure remains at its base-year value plus 2 percent annual cap. Conversion ADUs (within existing structure) typically generate smaller incremental assessments than new detached ADUs. Williamson Act parcels (agricultural preserve contracts under Cal. Gov't Code Sec. 51200 et seq.) are common in the eastern foothills and Catheys Valley; ADUs on Williamson Act parcels are permitted but the parcel's overall use must remain consistent with the agricultural preserve contract.

Mariposa County overlays of consequence: CAL FIRE Very High Fire Hazard Severity Zone (FHSZ) covers essentially the entire county within the State Responsibility Area; California WUI Chapter 7A ignition-resistant construction applies broadly; FEMA SFHA along the Merced River, Chowchilla River, and tributary drainages; Williamson Act agricultural preserve contracts cover much of the eastern Catheys Valley and Hornitos foothills; the Yosemite National Park federal inholding (the dominant land-area feature) falls outside county zoning entirely; Mariposa County Oak Woodland Conservation Ordinance governs oak woodland disturbance on parcels with significant oak canopy; Highway 140 (the all-year Yosemite gateway route along the Merced River) is a designated state scenic highway corridor with a wild-and-scenic river segment along the Merced through Briceburg.

• CAL FIRE State Responsibility Area / Very High FHSZ
• FEMA SFHA - Merced and Chowchilla River drainages
• Williamson Act agricultural preserve contracts
• California Scenic Highway System - SR-140 (Yosemite Gateway)
• Mariposa County Oak Woodland Conservation Ordinance
• Yosemite National Park federal inholding