Accessory dwelling unit (ADU) permits — county-by-county basics

Accessory dwelling units — sometimes called granny flats, in-law units, casitas, or backyard cottages — are one of the fastest-growing permit categories in the country. State-level laws in California, Oregon, Washington, and a growing list of others have sharply limited what counties and cities can do to block them, but local ADU rules still vary significantly. What counts as an ADU. An ADU is an independent dwelling unit on a residential lot that already has a primary single-family home. It must include a kitchen, a full bathroom, and sleeping space. Detached new construction, garage conversions, basement or attic conversions, and additions with separate entrances and kitchens can all be ADUs depending on configuration. Where ADUs are allowed. Most jurisdictions now allow at least one ADU on any lot zoned for single-family use, often by right with limited zoning review. Some allow a junior ADU (under 500 square feet, attached to the primary home) in addition to a full ADU. Maximum ADU size is usually capped — commonly 800 to 1,200 square feet for detached ADUs. Application package. Site plan showing the primary home and the ADU footprint with setbacks called out, full architectural drawings (floor plan, elevations, foundation, framing, MEP), an energy compliance form, and utility plans showing water and sewer connections. ADUs typically require their own water and sewer service or a documented connection to the primary home's service. Typical fees. ADU permit fees usually total $1,200 to $4,500 for the building permit, plus separate fees for electrical, plumbing, and mechanical permits, plus impact fees that vary widely by jurisdiction. Some states and counties cap or waive impact fees for smaller ADUs. Review timeline. Plan review typically takes 20 to 45 business days, longer for jurisdictions still scaling up to ADU volume. Inspections. Full residential sequence — footing, foundation, framing, rough-ins, insulation, final, and certificate of occupancy. Find your county's permit office to confirm local ADU size, parking, and owner-occupancy rules before you commit to a design.