The Short Answer
The Full Timeline at a Glance
The honest answer is 8 to 18 months from your first phone call to a certificate of occupancy — depending on the ADU type, the lot conditions, the city you're in, and how quickly decisions are made along the way. Here are the three phases every project moves through.
That range frustrates homeowners who want a single number. But an ADU project has three distinct phases — design, permitting, and construction — each with its own timeline and its own variables. A garage conversion on a flat Daly City lot moves faster than a detached ADU on a Pacifica hillside with a Coastal Development Permit and custom finishes on a six-week lead time.
Design Phase: 4-10 Weeks
Site Assessment
We visit the property, evaluate the lot (setbacks, slopes, access, soil conditions, existing structures), assess the primary home’s systems (electrical panel capacity, sewer lateral location, water main), and discuss the homeowner’s goals (rental income, family housing, size, budget).
Feasibility Determination
Based on the site assessment, we confirm what type of ADU the lot supports, where it should be positioned, and whether any unusual conditions (hillside foundation, long utility runs, Coastal Zone permit, tree protection) affect the scope or timeline.
If a geotechnical report is needed (hillside lots), it’s ordered now — the geotech takes 2 to 4 weeks to schedule, drill, and deliver the report, and the structural engineer needs it before finalizing the foundation design.
Floor Plan and Design Development
The floor plan is created, reflecting the site conditions, setback constraints, homeowner’s program (studio vs. one-bedroom vs. two-bedroom), and budget. For ACI’s design-build projects, the floor plan and the construction budget develop simultaneously — every design decision has a cost implication that’s visible in real time.
For pre-approved plan adaptations, this phase is shorter (2 to 3 weeks); for fully custom designs, longer (4 to 6 weeks).
Structural Engineering
The structural engineer produces calculations and details for the foundation, framing, and connections — sized for the specific soil conditions, seismic requirements, and load paths of the project. For hillside ADUs requiring a geotech report, the structural engineering can’t be finalized until the geotech report is received.
Construction Documents
The full plan set is assembled: architectural plans (floor plan, elevations, sections, roof plan, site plan), structural plans, electrical plan, plumbing plan, mechanical plan, and Title 24 energy compliance. This is the document package that gets submitted to the city for permit review.
Finish Selections
Flooring, cabinetry, countertops, tile, fixtures, appliances, and exterior materials. This process runs in parallel with the construction documents and doesn’t need to be complete before permit submission — but long-lead items (custom cabinets, specific tile, specialty fixtures) should be identified early so they can be ordered in time for installation.
What Determines Design Duration
4 weeks (fastest): Pre-approved plan on a flat lot, standard finishes, no geotech needed, decisive homeowners who confirm selections quickly.
6 to 8 weeks (typical): Custom design on a standard lot, structural engineering, moderate finish selections, one or two rounds of revision with the homeowner.
8 to 10 weeks (extended): Hillside lot requiring geotech report (adds 2–4 weeks waiting for the report), fully custom design with multiple revision rounds, complex structural conditions, homeowners who need more time for finish decisions.
In the traditional model, the homeowner hires an architect, the architect produces plans, the homeowner reviews and approves them, then the homeowner sends them to contractors for bids. The design phase takes 6 to 12 weeks — and then the bidding process adds another 4 to 8 weeks. In design-build, the team designing the ADU is the team building it.
Permitting Phase: 4-16+ Weeks
Application Submission
The plan set, application forms, and fees are submitted to the city’s building department. Most Peninsula cities accept online submissions. Some require in-person for larger projects. The plan check fee (typically 65–80% of the building permit fee) is paid at submission.
Completeness Review
Under state law (SB 543), the city must determine whether the application is complete within 15 business days. If items are missing, the city notifies the applicant in writing. An incomplete application doesn’t start the 60-day clock.
If a geotechnical report is needed (hillside lots), it’s ordered now — the geotech takes 2 to 4 weeks to schedule, drill, and deliver the report, and the structural engineer needs it before finalizing the foundation design.
Plan Check Review
The city’s plans examiner reviews the documents for code compliance — structural adequacy, energy code, fire safety, setbacks, height, and other building code requirements. The examiner produces a correction letter listing items to revise.
For pre-approved plan adaptations, this phase is shorter (2 to 3 weeks); for fully custom designs, longer (4 to 6 weeks).
Corrections and Resubmission
We revise the plans, write a formal response addressing each correction, and resubmit. Most ADU permits go through one to two correction cycles. Each cycle takes one to three weeks depending on the city’s resubmittal review turnaround. Complex projects or cities with backlogs may require three cycles.
Permit Issuance
After the plans pass review, the remaining permit fees are paid and the building permit is issued. Construction can begin.
What Causes Permitting Delays
Incomplete initial submission. Missing Title 24 calculations, incomplete structural details, or absent site plan elements trigger a “not complete” determination that delays the start of the 60-day clock. We submit thorough plan sets that anticipate each city’s requirements.
Structural engineering corrections. The plans examiner may request additional structural details — connection specifications, foundation calculations for specific soil conditions, or shear wall analysis. If the structural engineer needs to produce additional work, it adds days to the correction turnaround.
City staffing and backlog. Permitting timelines are heavily influenced by how many applications the city is processing and how many plans examiners are on staff. During high-volume periods, even the 60-day state mandate may be stretched.
Multi-agency review. Coastal Zone properties require CDP review concurrent with the building permit. Properties in historic overlay zones may require additional review. San Francisco’s two-agency process (DBI + Planning) adds a second review timeline to every project.
Design-Build Advantage: The single biggest time-saver in permitting is how fast corrections are addressed. In the traditional model, corrections go architect → contractor → resubmit, taking one to three weeks per cycle. In design-build, we receive corrections, revise internally, and resubmit in days. Over two correction cycles, this saves two to six weeks.
Construction Phase: 12-40+ Weeks
Garage Conversion: 12-20 Weeks
Garage conversions are the fastest ADU projects. The structure already exists; you’re closing in the space, installing MEP (mechanical, electrical, plumbing), insulation, drywall, finishes, and kitchen/bath.
Attached ADU: 16-28 Weeks
An attached unit requires foundation work (potentially different from the primary home’s), framing, MEP, finishes. The added complexity of tying into the existing home’s systems and managing work on the primary home’s envelope adds weeks to the schedule.
Detached ADU on Standard Lot: 18-32 Weeks
A detached unit on flat, accessible land moves relatively quickly. Clear the site, pour the foundation, frame, install MEP, insulate, drywall, finish. The variables are soil conditions and utility distances.
Hillside or Complex Foundation ADU: 28-40+ Weeks
Hillside projects require additional coordination: geotechnical testing, retaining wall design and construction, pier foundation drilling, grading, drainage infrastructure. These aren’t bottlenecks unique to ADUs — any hillside project faces similar timelines — but they’re more common in Pacifica and the peninsula foothills.
What Extends Construction Timelines
Soil conditions and foundation work. Unexpected soil conditions discovered during excavation can trigger additional geotech assessment, retaining wall revisions, or foundation redesign. On hillside lots, this is why geotech reports are ordered during design.
Utility relocations. If the utility run is longer than expected, or if the city’s sewer main is deeper than anticipated, utility coordination adds weeks. PG&E and water company scheduling can be unpredictable during high-volume seasons.
Permit inspection backlogs. Building inspectors work on their own schedule. If the city is understaffed or overburdened, framing inspection, rough MEP inspection, and final inspection can be delayed by weeks.
Supply chain delays. Appliances, custom cabinetry, and specialty fixtures ordered during design may face longer lead times during high-demand periods. Identifying long-lead items early and ordering them during the permitting phase keeps construction moving.
Finish delays and changes. Homeowners who change their minds about finishes midway through construction, or who discover they prefer a different layout during framing, can add weeks of revision work.
The construction phase is where site conditions, weather, inspector scheduling, and material availability collide. A realistic timeline acknowledges these variables rather than promising a single date.