ADUs & Detached Cottages

The right ADU type depends on available space, existing construction, and how independently the occupants need to live. Attached units share a building connection; detached cottages provide greater physical separation but require their own foundation and site connections. Garage conversions reuse an enclosure, not necessarily a compliant residential structure.

A JADU is a smaller option, up to 500 square feet within the existing home, with an efficiency kitchen and potentially shared sanitation facilities. Its rules differ from a full ADU, including owner-occupancy requirements. Single-family properties can typically accommodate one ADU plus one JADU; confirm the applicable pathway before planning multiple dwellings.

California requires ministerial ADU approval for qualifying applications, but cities still administer objective standards and building safety requirements. The agency must act within 60 days of a complete application, subject to applicable statutory conditions; this is not a construction completion deadline.

State law protects an allowance for at least an 800-square-foot ADU meeting four-foot side and rear setbacks and applicable height limits, notwithstanding floor-area ratio or lot-coverage restrictions. Required detached ADU side and rear setbacks generally cannot exceed four feet. Qualifying existing-space conversions may follow different setback rules.

Detached height is generally 16 feet, with 18-foot allowances in specified circumstances and an additional roof-pitch allowance where applicable. Attached ADUs may reach 25 feet or the zone limit, whichever is lower. Confirm the precise pathway with the city: objective design standards, fire-zone requirements, historic protections, and tree ordinances may still affect siting and construction, but cannot simply override state protections.

SB 9 creates a pathway for two primary homes and qualifying urban lot splits, rather than an accessory dwelling subordinate to a primary residence. Eligible single-family properties may pursue two units, a split into two lots, or both, subject to statutory conditions and objective standards.

Historic districts, fire-hazard conditions, and other exclusions require screening; some fire-zone exceptions apply. The lot-split pathway carries a three-year owner-occupancy affidavit. Do not assume SB 9 and ADU allowances stack into a particular unit count: establish eligibility, ownership goals, access, and utility requirements before choosing a development strategy.

Utility capacity and ground conditions can determine an ADU's practical layout before architectural details do. Evaluate sewer lateral sizing, condition, slope, and whether a shared or separate connection is appropriate. Confirm water-meter requirements and electrical service capacity with the serving utilities; Palo Alto electrical upgrades go through CPAU rather than PG&E.

Coordinate panel upgrades, load management, trench routes, easements, and drainage together. Fire sprinklers are not required for an ADU when they are not required for the primary home. Where required, verify water-service implications before fixing the utility plan.

New ADUs permitted under the 2025 Title 24 Energy Code, effective January 1, 2026, need a documented compliance strategy. Heat pumps for space and water heating are central options. Newly constructed detached ADUs generally require solar PV, subject to applicable exceptions; conversions and attached configurations need separate evaluation. Confirm current local reach codes rather than assuming a blanket gas prohibition.

Foundation design must address seismic loads and soil behavior. Expansive clay, slopes, or questionable existing garage slabs may require geotechnical investigation and engineered foundations. Coordinate insulation, glazing, ventilation, and any required field verification with the energy documentation.

An ADU supports long-term independence when everyday movement, privacy, and maintenance are designed together. Plan a step-free route where feasible, generous door clearances, a low-threshold shower, slip-resistant surfaces, and reinforcement for future grab bars. Locate controls and storage within comfortable reach.

Separate entries and screened outdoor space let relatives remain close without sharing every routine. Acoustic separation, bedroom daylight, nighttime lighting, and room for a visiting caregiver make compact living more adaptable as family needs change.