An accessory dwelling unit is one small building on a lot that already has a house. It should be one of the simplest permits you pull all year.
In practice, ADUs are where permit surprises happen. The building permit itself is rarely the problem. The delays come from the approvals nobody listed at kickoff: the electrical service upgrade the utility takes four months to schedule, the protected oak 12 feet from the new foundation, the fire hose-pull distance to a backyard unit, the school district fee that has to be paid before the city will issue anything.
This checklist walks through every permit and approval an ADU can trigger, grouped by phase, so you can confirm scope before the design is locked and set a schedule your client can actually rely on.
đź’ˇ Requirements vary a lot by jurisdiction. Treat this as the list of questions to ask your city and county, not a list of permits every project needs. For California projects, the state's ADU Handbook summarizes the rules cited below.
Phase 1: Zoning and Land Use
Confirm these before schematic design. Any one of them can change the footprint.
1. Zoning Clearance and ADU Eligibility
Confirm the lot allows an ADU at all, and under what rules: maximum size, height, number of units, whether a junior ADU (JADU) is allowed alongside it, and lot coverage or floor-area limits. Many cities publish an ADU-specific handbook that overrides the base zoning code, so check which one governs.
2. Setback and Height Relief
If the design needs to encroach on a setback or exceed a height limit, you may need a variance or an administrative adjustment. In California, state law caps side and rear setbacks for new-construction ADUs at 4 feet, and a conversion of an existing structure can't be required to add any setback. Other states and cities are far less generous.
3. Overlay District Reviews
Overlays are the most common source of "we didn't know that applied." Check for:
- Coastal zone: a coastal development permit, often with its own hearing timeline
- Historic district: design review for visible changes
- Hillside or slope ordinances: extra geotechnical and grading scrutiny
- Wildfire hazard zones: ignition-resistant construction and defensible space requirements
- Flood zones: a floodplain development permit and elevation requirements
4. Tree Removal or Protection Permit
Protected trees are defined by species and trunk diameter, and protection zones usually extend to the dripline. A backyard ADU foundation or new utility trench often lands inside one. You may need an arborist report, a tree protection plan, or a removal permit with replacement requirements.
5. Private Approvals: HOA Review and Easements
These aren't city permits, but they stop projects just as effectively. Pull the title report early to check for utility or access easements across the building area. If there's an HOA, find out whether it requires architectural review. In California, HOAs can't prohibit ADUs but may still impose reasonable restrictions.
Phase 2: Plan Review
These run through the building department once the design is set.
6. Building Permit
The core permit. Many jurisdictions issue a combination permit that covers building, electrical, plumbing and mechanical for small residential work. Confirm whether yours does. In California, the permitting agency must tell you within 15 business days whether your application is complete, then approve or deny it within 60 days, or it's deemed approved. That gives you real dates to hold them to.
7. Trade Permits (Electrical, Plumbing, Mechanical)
Where the building permit isn't combined, each trade needs its own permit, usually pulled by the licensed contractor. Note who is responsible for each one so none of them falls between the design team and the builder.
8. Energy Compliance
Expect an energy code review (Title 24 in California, the IECC or a local amendment elsewhere). In California, newly constructed detached ADUs generally need new solar PV under the Energy Code, while attached ADUs and garage conversions are treated as additions and don't. The panels can go anywhere on the lot, including as an expansion of an existing system on the main house, but either way it affects electrical design, so find out early.
9. Fire Department Review
Three things to check:
- Sprinklers: some jurisdictions require them in the ADU. In California, they can't be required unless the primary house is sprinklered.
- Fire access: fire codes typically require every part of the building's exterior walls to be within a set hose-pull distance of a fire access road, commonly 150 feet measured around the building. A unit at the back of a deep lot can exceed it and trigger sprinklers or other mitigation.
- Wildfire zones: ignition-resistant materials and vents.
10. Grading and Drainage
Cut and fill above a threshold volume, work on slopes, or new impervious area can require a grading permit, a drainage plan, or stormwater controls. Retaining walls often carry their own permit.
11. Demolition Permit
A garage conversion or the removal of an existing shed or accessory structure usually needs a demolition permit, sometimes with an asbestos or lead survey for older structures.
Utilities and Site Access
These are the items most likely to set your critical path, because they involve agencies other than the building department.
12. Electrical Service Upgrade
A new unit often means a larger service or panel for the main house. That's a request to the utility, not the city, and service planning plus scheduling can take months. Submit it as early as the load calculation allows.
13. Sewer Connection
The sewer agency may need to confirm capacity, inspect or replace the existing lateral, or require a new connection with its own fees. Some jurisdictions require a separate lateral for detached units.
14. Water Service
Adding fixtures, and especially adding fire sprinklers, can mean upsizing the water service or meter. A separate meter is usually optional but changes the fee picture.
15. Septic Permit
On lots without sewer, county environmental health will need to confirm the existing septic system can handle the added unit, or approve a new or expanded system. This can be the single longest approval on a rural ADU.
16. Right-of-Way, Driveway and Encroachment Permits
A new curb cut, sidewalk repair, work in the street for a utility connection, or even a dumpster on the street can each require a public works permit.
Fees and Closeout
17. Impact and School District Fees
Budget for these early, because some must be paid before the building permit is issued. In California, impact fees are waived for ADUs of 750 square feet of interior livable space or less, but school district fees can apply once a unit reaches 500 square feet. Water and sewer connection fees are a separate category and may still apply to a detached unit. The school district typically has to issue a payment certificate before the city will issue the permit.
18. Address Assignment
The new unit needs its own address for utilities, mail and emergency services. It's a small step, but it can hold up utility accounts at the end of the job.
19. Final Inspections and Certificate of Occupancy
Final sign-off from building, fire and any other agency that reviewed the project, followed by a certificate of occupancy or final permit. The unit can't be legally occupied or rented until this is done.
What Changes by ADU Type
Not every ADU triggers the whole list. As a rough guide:
- Junior ADU: within the existing house footprint (up to 500 square feet in California). Usually just a building permit, plus fire separation and egress review.
- Garage conversion: building permit plus demolition of garage elements, fire separation, and often an electrical upgrade. In California, the lost garage parking doesn't have to be replaced.
- Attached addition: treated much like a residential addition, with setbacks, height and energy compliance all in play.
- Detached new construction: the full list. Expect utility connections, fire access, grading, trees and solar to be the long poles.
How to Use This Checklist
- Confirm the list during due diligence, not after schematic design. Overlays, trees and easements are the items most likely to force a redesign.
- Sort by lead time, not by phase. Utility service upgrades, septic approvals and coastal permits often take longer than plan review. Start them in parallel.
- Put the permit scope in your proposal. Listing the expected permits up front sets client expectations and protects your fee if an unexpected overlay shows up.
- Track each approval with its own dates. A single "permit" line on a project schedule hides the five approvals that actually determine when construction starts. For the dates worth recording on each one, see our guide to the permit milestones every architect should track.
Build the List Automatically With Permitful
Researching all of this by hand for every new address takes hours. Permit Advisor does the first pass for you: create a project, set the project type to Accessory Dwelling Unit (ADU), enter the address, and it searches official city, county and state sources to suggest the permits your project likely needs, sorted into zoning/land use and plan review phases.
From there, Permitful keeps every permit, fee, milestone and due date for the ADU in one place, and exports deadlines to Google Calendar or Outlook so the reminders show up where you already plan your day. You can also share a live dashboard with your client, so the homeowner can see where their permits stand without emailing you to ask.
Your first project is free, including a Permit Advisor run, so you can check an ADU you're working on right now against this list.