The real ADU timeline — month by month
Accessory dwelling units are the fastest-growing housing category in the country, but the timeline from first idea to rent check is longer than almost every homeowner expects. An 18-month schedule for a detached ADU is normal, not slow. Here's what each phase actually takes.
Months 1-3: design. Interview and hire an ADU-specialized architect or design-build firm. Pre-approved city plans (Los Angeles, San Jose, Oakland, San Diego all offer them free) can shortcut this to 4-6 weeks. Custom designs run 8-14 weeks through schematic, design development, and construction documents. During this phase, your designer should also verify setbacks, utility capacity, and HOA rules.
Months 3-6: permitting. California's 60-day statutory clock is aggressive — many cities routinely take 90-120 days for complex projects. Plan review typically produces 1-2 rounds of corrections. Running design review (for jurisdictions that require it) and building permit in parallel can compress this phase. During permitting, finalize GC selection and negotiate price based on stamped drawings.
Months 6-9: foundation and envelope. Site prep, utility trenching (the sneaky timeline eater — utility companies schedule their own work on their calendar), foundation pour and cure, framing, roofing, siding, windows, and doors. Expect this phase to slip 2-3 weeks total from weather or inspection delays.
Months 9-14: MEP and finishes. Rough-in electrical, plumbing, mechanical, inspections, insulation, drywall, tile, flooring, cabinets, countertops, paint, fixtures. This is the longest phase by calendar time because every trade works sequentially.
Months 14-18: punchlist and Certificate of Occupancy. Final inspections (electrical, plumbing, mechanical, building), energy compliance verification, landscaping and site restoration, punchlist corrections. CO typically issues 2-4 weeks after final inspection passes.
2026 ADU cost reality
Pricing varies dramatically by ADU type. Garage conversions are the cheapest at $100-$280 per sq ft ($40K-$140K for a typical 400-500 sq ft garage). Basement ADUs run $140-$280 per sq ft depending on egress window installation and ceiling height adjustments. Attached ADUs (sharing a wall with the main house) run $250-$450 per sq ft. Detached new construction is most expensive at $350-$650 per sq ft.
California metros run at the top of every range. A 600 sq ft detached ADU in Oakland or San Jose lands $280K-$430K before soft costs; the same design in Sacramento runs $210K-$310K, and in Austin or Denver $180K-$260K.
California ADU rules (2026 update)
California has the country's most permissive ADU framework, and the rules keep expanding. Current (2026) state law: every single-family lot can build one ADU plus one Junior ADU (up to 500 sq ft) by right. SB 9 additionally allows most single-family lots to split into two parcels with two units each. Cities cannot impose owner-occupancy requirements. Cities cannot charge impact fees on ADUs under 750 sq ft. Cities must approve permits within 60 days of complete submission. Setbacks are capped at 4 feet. Fire sprinklers cannot be required unless the main residence has them.
Other states are following. Oregon, Washington, Maine, and Vermont have statewide ADU legalization. Massachusetts passed a 2024 law preempting local ADU restrictions. New Hampshire allows one ADU per single-family lot statewide. If you're in any of these states, ADU is a realistic path; elsewhere, check city-specific rules first.
The rental ROI math
A $250K ADU financed at 7% over 30 years costs about $1,660/mo in principal and interest. In Oakland or San Diego, a well-built one-bedroom ADU rents for $2,400-$3,200. After property tax ($200/mo assessed only on the ADU value), insurance ($75/mo), and maintenance reserve ($150/mo), expected cash flow is $315-$1,115/mo. Beyond cash flow, the ADU typically adds 20-35% to total property value at resale — worth $75K-$120K on a $600K home.