Most Bay Area ADU projects take somewhere between nine and eighteen months from the first design conversation to the final inspection. Construction itself is usually the shorter part of that — often four to six months for a detached unit. The longer stretch is everything that happens before a shovel touches the ground: feasibility, design, engineering, and permit review.
That gap between what homeowners expect and what actually happens is the source of most frustration on ADU projects. The good news is that California law has tightened permit timelines considerably in recent years, and the phases are predictable enough to plan around. Here is what each one involves and where the time actually goes.
Phase One: Feasibility and Site Review
Typical duration: two to four weeks
Before any drawing is produced, someone needs to establish what your property can actually support. That means confirming setbacks, height limits, and lot coverage, locating existing utilities, checking whether your electrical panel has capacity for a second dwelling, and identifying constraints like slope, protected trees, or an easement.
This phase is short, and skipping it is expensive. A design produced without it can require substantial revision once the constraints surface.
Phase Two: Design and Engineering
Typical duration: six to twelve weeks
This covers the architectural drawings, structural engineering, and California Title 24 energy compliance calculations that a permit application requires. Depending on the site, it may also include a geotechnical report or an arborist report.
The single biggest variable here is decision speed. Layout changes, revised window placements, and reconsidered kitchen configurations each push the schedule. Homeowners who finalize decisions promptly routinely move through this phase weeks faster than those who revisit them.
Phase Three: Permit Review
Typical duration: two to four months
California law is unusually specific about ADU permitting, and the rules have improved meaningfully for homeowners.
Under state law, ADU applications receive ministerial review. That means no public hearing and no discretionary judgment — if the plans meet objective standards, the city is required to approve them. Cities must approve or deny a complete application within 60 days, and if that deadline passes without a decision, the application is deemed approved.
Recent legislation added a second protection. Cities must now determine whether an application is complete within 15 business days of submittal, and provide an itemized list of anything missing. If they do not respond, the application is treated as complete and the 60-day clock begins. This closed a long-standing gap where applications could sit untouched for months before review even started.
So why do real timelines routinely run longer than 60 days?
The answer is correction cycles. The 60-day clock applies to a complete application. When a plan checker issues corrections, the project goes back to the design team for revision and resubmittal, and each round adds weeks. A clean, complete first submittal is by far the strongest lever you have on this phase — a well-prepared package in a slower city often clears faster than an incomplete one in a fast city.
One additional option is worth knowing about. State law now requires cities to maintain programs of pre-approved ADU plans and to act on applications using those plans within 30 days. Where a pre-approved design fits your property and your needs, it can compress this phase substantially. The tradeoff is limited flexibility in layout and siting.
Phase Four: Construction
Typical duration: four to six months for a detached ADU, three to four months for a garage conversion
Construction moves in a set sequence, with city inspections at each stage:
- Site preparation, utility trenching, and foundation work
- Framing and roofing
- Rough electrical, plumbing, and mechanical, followed by inspection
- Insulation and drywall
- Interior finishes, cabinetry, flooring, and fixtures
- Exterior work, hardscape, and landscaping repair
- Final inspections and certificate of occupancy
Garage conversions run shorter because the foundation, walls, and roof already exist. Detached units on sloped or difficult-access sites run longer.
What Extends an ADU Timeline
The delays we see most often are avoidable ones:
- Incomplete permit submittals. Every correction cycle costs weeks.
- Design changes after permit submittal. A revision to permitted plans requires resubmittal and re-review.
- Utility coordination. Service upgrades depend on the utility provider schedule, which is outside any contractor control and can take months.
- Long-lead materials. Windows, custom cabinetry, and certain fixtures need to be ordered well ahead of when they are installed.
- Failed inspections. Each one adds days to weeks while work is corrected and re-scheduled.
- Discretionary review. Properties in historic districts or with special overlay zoning may face additional review beyond the standard ADU process.
How to Shorten the Overall Timeline
Start the design phase earlier than feels necessary. Every week saved in design is a week saved on the total project, and design is the phase you have the most control over.
Beyond that: make finish selections before construction begins rather than during it, respond quickly to questions from your design team, and work with people who know your specific city. Plan reviewers in different jurisdictions look for different things, and familiarity with local requirements meaningfully reduces correction cycles.
Frequently Asked Questions
Can I live in my home during ADU construction?
In most cases, yes. A detached ADU is built away from the house, and disruption is generally limited to noise, site traffic, and short utility interruptions. Garage conversions and attached ADUs involve more direct impact on the home.
What happens if the city misses the 60-day deadline?
Under state law, a complete ADU application that receives no decision within 60 days is deemed approved. In practice, most projects reach a decision within the window once the application is genuinely complete.
Is a prefab ADU faster?
Factory construction can shorten the build phase, but it does not change permitting, and it does not eliminate site work — foundation, utility connections, and installation still happen on your property. The total savings are usually smaller than the marketing suggests.
Do timelines differ across Bay Area cities?
Yes. Review capacity, submittal requirements, and correction practices vary. Cities with design review requirements generally take longer than those without.
When should I start if I want the unit finished by a specific date?
Work backward from your target and allow at least twelve months, more if the date is tied to something inflexible like a family member moving in.
Planning an ADU in the Bay Area
Design by M&M is a Bay Area design-build company working with homeowners in Palo Alto, San Francisco, and surrounding Peninsula and South Bay communities. Because we handle feasibility, design, permitting, and construction as one process, the schedule is managed as a whole rather than handed between separate firms at each stage.
If you are planning an ADU and want a realistic timeline for your property, a consultation is the place to start. We will review your lot, discuss your city requirements, and help you understand what the schedule would actually look like. Contact Design by M&M to discuss your project.
