If you’re building an ADU (Accessory Dwelling Unit), you’ve probably heard about permit costs. Maybe you’ve also heard that some of them can be waived. The reality? Permit fees are real, often unavoidable, and sometimes confusing. But they’re not impossible to understand. Let’s break it down.
What Are ADU Permit Costs?
Permit costs cover the city’s or county’s administrative work to review your plans and inspect your project. They're not just paperwork. They help pay for staff, enforcement, plan reviews, inspections, and sometimes public improvements. Think of them as the city’s “cover charge” for allowing your build.
These are the typical permit fees you might encounter:
Planning review fee – They check if your plans follow zoning laws.
Building permit fee – They review your drawings and issue the permit.
Plan check fee – Similar to a building permit fee, but specifically for reviewing your documents.
Inspection fee – For the inspections during construction.
Impact fees – These are the big ones, and we’ll get into them soon.
Fees vary city to city. Even within the same county, costs shift depending on the local agency.
So, How Much Are We Talking?
This depends on your city and the size of the ADU. But here’s a general idea:
For a 400 sq ft unit, permit costs might range from $3,000 to $5,000.
For a 750 sq ft unit, costs are often between $5,000 and $9,000.
For an 800+ sq ft unit, fees can exceed $12,000.
It’s not just size. Different cities apply different fee schedules. Some even charge based on the number of plumbing fixtures. Yes, a second toilet can raise your cost.
When Are Permit Fees Paid?
Usually, you’ll pay in stages. Some fees are due at submission (like planning review), while others are due before issuing the building permit (like school or impact fees). Your contractor or design team may front some of these early on, depending on your agreement.
Let’s Talk About Waivers
There are two big players when it comes to waivers: impact fees and school fees. These waivers aren’t about generosity. They’re legal mandates based on California state laws—specifically, SB13 and AB68.
Impact Fee Waivers
Impact fees are meant to offset the strain on public infrastructure—roads, sewer, fire departments, etc. Cities use them for new developments.
But here’s the important part: if your ADU is under 750 square feet, you are legally exempt from paying impact fees. That’s California ADU law. If your city says otherwise, they’re either uninformed or hoping you don’t know better.
This size threshold—750 sq ft—is why so many ADUs stop right there. Go over by even a foot, and you might owe $5,000 to $15,000 in extra fees.
If you're planning an 800 sq ft ADU, consider shrinking it just a bit. That small change could save thousands.
School Fee Waivers
School fees are based on the square footage of new livable space. State law allows school districts to charge fees for ADUs over 500 square feet.
If you’re under that? No school fees. Between 500 and 750 square feet? School fees are still fair game. The law doesn’t protect you there the same way it does for impact fees.
Important: School districts are separate from the city. You may have to contact them directly for their fee schedule. Don’t assume your city planner will include them.
How to Claim Your Waivers
No magic form. No secret handshake. Just code compliance.
Keep your ADU under 750 sq ft for impact fee exemption.
Keep it under 500 sq ft for school fee exemption.
Document everything. If needed, print and bring SB13 to your permit counter.
Some cities are proactive and waive fees automatically. Others may overlook it. You may need to ask. Or insist.
Are There Any Other Fee Reductions?
Sometimes, yes. Some cities offer:
Pre-approved ADU plans, which reduce plan check fees.
Flat permit fees for ADUs, simplifying the process.
Discounts or amnesty programs if converting an existing structure (like a garage).
Call your city’s building department. Ask direct questions. Don't rely on online info alone—it’s often outdated.
Are Fees Different for Detached vs. Attached ADUs?
Usually, no. The fee structure is typically based on square footage and type of construction, not whether it’s attached or detached. But some exceptions exist. Detached units sometimes trigger additional utility review fees.
If your unit shares utilities with the main home, you might avoid those. If not, expect more red tape.
How Do JADUs Compare?
Junior ADUs (JADUs) are built inside your existing home. They’re smaller—500 sq ft max—and share a bathroom. Because they use existing structure, permit fees are significantly lower. Often under $2,000.
If your budget is tight, JADUs are a smart move. But their limitations (size, layout, plumbing) make them less flexible than full ADUs.
How Do You Budget for Fees?
Use your ADU size and location to get a ballpark estimate. Then add 10-15% as a buffer. Fee structures shift. Cities adjust their rates often—usually every July.
If you’re working with a contractor or ADU builder, ask for a breakdown early. Not just labor and materials—get the city fees in writing.
What If You Already Paid a Fee That Should’ve Been Waived?
Yes, this happens. Some cities will refund. Others won’t.
You can try the following:
Submit a written refund request.
Include the specific law (SB13) and proof of square footage.
Follow up—twice, if needed.
But act quickly. Some cities have time limits on refunds (30-90 days from payment).
Can You Build First and Permit Later?
Technically, yes. But it’s risky.
Unpermitted ADUs may face fines. You may be forced to tear down work. Or pay triple fees for retroactive permits. Some cities offer “amnesty” periods—but don’t count on it. Always safer to permit before you build.
Final Thoughts
ADU permit costs aren’t the most exciting part of your project—but they’re necessary. They can range from manageable to painful, depending on size, location, and city rules.
Know the fee categories. Keep your square footage strategic. Ask the city directly—don’t assume.
And if your ADU is 750 sq ft or under, make sure you don’t pay impact fees. It’s your legal right.
