Mono County

Mono County Community Development Department issues ADU permits in unincorporated areas. Mammoth Lakes town has its own building department for parcels inside town limits. Practical permitting frictions: severe winter snow loads (design loads of 100-300+ psf at elevation; Mammoth Lakes carries the highest design snow load of any incorporated CA city, ~300 psf), very high fire severity zones across the eastern Sierra escarpment in the State Responsibility Area, short construction season (typical practical window May-October at lower elevations, July-September at June Lake / Crowley elevation), septic/well dependence outside small water-district service areas (Lee Vining, Crowley Lake CSD, June Lake PUD, Bridgeport, Walker, Coleville), STR-overlay restrictions in Mammoth-area unincorporated parcels (June Lake, Crowley Lake) where the county has established STR-zone rules, Long Valley Caldera seismic and geothermal overlays in the Mammoth-area parcels, and Mono Lake Tufa State Natural Reserve scenic / hydrologic-protection considerations on parcels near the lake.

Process overview: Standard ministerial 60-day review per Sec. 65852.2(b) for compliant ADU applications. Parcels in the State Responsibility Area Very High FHSZ require Chapter 7A ignition-resistant construction. Septic/well parcels require Environmental Health review. Long Valley Caldera-area parcels (Mammoth-area, Crowley Lake, Hilton Creek) carry geologic-hazard review for active volcanic / seismic risk; this rarely blocks ADU permits but adds geotechnical-investigation requirements on slope or fault-near parcels. STR-overlay parcels in June Lake and Crowley Lake CDPs may require a separate STR permit on top of the ADU permit if the owner intends short-term rental. Snow-load structural design above 100 psf is a non-trivial engineering cost and frequently requires stamped engineering documentation.

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 apply to ADUs over 500 sqft per Education Code Sec. 17620 (Eastern Sierra USD covers most of the county; Mammoth USD covers Mammoth Lakes town).

The Mono County Assessor's Office maintains parcel-level assessment records for the entire county. ADU additions on unincorporated parcels are captured as improvements via shared permit data with the Community Development Department. 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. Mono County's assessment roll is small (under 18,000 parcels including significant unimproved high-elevation ranching and BLM-leased grazing parcels; the federal land acreage is much larger but generates no assessment roll). LADWP land holdings in Long Valley and the Owens watershed are taxable to the City of Los Angeles as the property owner.

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.

Mono County overlays of consequence: extensive federal land coverage (Inyo National Forest, Humboldt-Toiyabe National Forest, BLM Bishop Field Office, Bodie State Historic Park, Mono Basin National Forest Scenic Area, Yosemite NP eastern strip - together covering the great majority of the county and outside county zoning entirely); CAL FIRE State Responsibility Area Very High FHSZ along the eastern Sierra escarpment; FEMA SFHA along the Walker River, Owens River headwaters, and tributary creeks; Long Valley Caldera active volcanic / seismic overlay (Mammoth-area parcels under Long Valley Volcanic Observatory monitoring); Mono Basin National Forest Scenic Area protecting Mono Lake's hydrology and tufa formations; Bodie State Historic Park (the iconic ghost town - county-zoning-exempt as state property); LADWP-owned land in Long Valley and Owens watershed; and Mammoth Lakes / June Lake / Crowley Lake STR-zone overlays.

• Inyo NF / Humboldt-Toiyabe NF / BLM Bishop / Yosemite NP / Bodie SHP / Mono Basin NFSA
• CAL FIRE State Responsibility Area / Eastern Sierra escarpment FHSZ
• Long Valley Caldera active volcanic / seismic overlay
• FEMA SFHA - Walker River, Owens River headwaters, Mammoth Creek, Convict Creek
• California Scenic Highway System - US 395 / SR 158 / SR 120 (Tioga Pass)
• Mono County STR-zone overlays - June Lake, Crowley Lake, Mammoth-area unincorporated