Almost every homeowner we meet asks the same question before anything else: what actually happens between "I want an ADU" and "I have keys"? The permitting stage is the part people find most opaque, so here is the honest version: the sequence, who is involved at each step, and where projects genuinely get stuck.
Stage 1: Feasibility and site review
Before any drawing is worth paying for, someone needs to confirm what your lot can support. That means looking at setbacks, existing structures, easements, utility locations, slope, tree protections, and whether your property sits in a special overlay such as a fire zone, a flood zone, or a historic district. A site that looks identical to your neighbour's from the street can have a completely different set of constraints once the survey comes back.
This is also the stage where the honest conversations happen. If a detached unit will not fit the rear yard the way you imagined, it is far cheaper to learn that now than after a full design package is drawn.
Stage 2: Design and construction documents
Once feasibility is settled, the design work begins: floor plans, elevations, the site plan, and then the technical set the city reviews: structural calculations, Title 24 energy compliance, and the mechanical, electrical, and plumbing layouts.
A complete, coordinated set is the single biggest lever you have on permit speed. Thin drawing sets do not slip through; they come back with comments.
Stage 3: Plan check
Your package is submitted to the local building department. Reviewers from building, planning, and often fire and public works each look at it and return written comments. You respond, resubmit, and the cycle repeats until the set is approved.
This is where timelines are won or lost. A well-prepared set typically clears in fewer rounds; an incomplete one can cycle several times, with a queue wait between each round. Every jurisdiction (San Mateo, Burlingame, Redwood City, Millbrae, San Carlos) runs its own queue at its own pace.
Stage 4: Permit issuance and fees
When the plans are approved, the permit is issued once the applicable fees are paid. These vary by jurisdiction and by the size of the unit, and they are a real line item, worth building into your budget from the start rather than discovering at issuance.
Stage 5: Construction and inspections
Construction proceeds against a schedule of inspections: foundation, framing, rough mechanical/electrical/plumbing, insulation, drywall, and final. Each one has to pass before the next phase of work can be covered up. A missed or failed inspection does not just cost that day. It can stall the trades queued behind it.
The last one is the one that matters: final inspection and the certificate of occupancy. That is the point at which the unit is legally habitable.
The three things that actually cause delays
- Incomplete submittals. Missing structural calculations or an uncoordinated MEP set guarantees another review round.
- Utility capacity. If the existing electrical service cannot carry a second dwelling, a service upgrade brings the utility company into your timeline, and their schedule is not yours.
- Changing the design mid-review. A revision after submittal usually restarts the clock on the affected disciplines.
How we handle it
We keep design, engineering, permitting, and construction under one roof, which means the person answering a plan-check comment is on the same team as the person who will build the detail. There is no gap between the designer, the engineer, and the crew, and no finger-pointing when a comment comes back.
Requirements and fees change, and they differ from city to city within the county. Before you budget around any specific number or timeline, confirm the current rules with your local building department, or talk to our team and we will walk your property with you.
You can see how we sequence the whole build on our process page.