I Processed Permits for ADUs — Here's What Nobody Tells You
Most ADU guides cover the basics: check your zoning, hire a contractor, pull a permit. What they don't cover is the stuff that kills projects quietly the small oversights that pile up into weeks of back-and-forth with a permit reviewer, missed funding windows, and sometimes a deal that falls apart entirely.
I processed ADU permits professionally. Here are four things I watched applicants get wrong on a regular basis.
1. You May Be Reading the Sq Ft Size Limit Wrong
ADU size caps sound straightforward until you actually read the code. Many jurisdictions write their maximums something like this: "900 square feet, or 60% of the primary dwelling unit's floor area, whichever is smaller."
That second condition is easy to skip over but it matters enormously. If your primary home is 1,100 square feet, 60% of that is 660 square feet, not 900. Your ADU just got 240 square feet smaller, and you may not find out until a reviewer flags your plans.
Before you finalize your design, read the full size language in your jurisdiction's ADU code. Do the math on both conditions. The smaller number is your real maximum.
2. On Septic? You Need an Approval Letter — and Probably More Than One
If your property runs on a septic system, your building department almost certainly requires a sign-off from the county or district Health Department confirming the existing system can handle the additional load. That letter is typically a required submittal item meaning your permit application is incomplete without it, and the clock doesn't start until it's in the file.
But septic is often just the beginning. Depending on your jurisdiction and site conditions, you may also need approval letters from:
Your water purveyor or water district
The sewer authority (if connection is required)
The local utility company
These agencies operate on their own timelines. Some turn letters around in a week; others take a month or more. If you don't identify these requirements early and start the outreach immediately, you can find yourself fully ready to build and stuck waiting on a letter.
3. Your Site Plans Must Match Each Other — Exactly
This one is subtle, and it's more common than you'd think.
If your property has critical areas nearby like wetlands, steep slopes, floodplain, or similar, you'll likely need an environmental or critical areas report prepared by a qualified professional. That report will include a site plan. Your building permit application will also require a site plan. And if you're adding a new access point or going through civil review, there may be additional site plans submitted as part of that process.
Every one of those site plans needs to show the same exact thing.
I once reviewed an application that had four different site plans in the submittal package. Different setbacks. Different ADU footprints. Different driveway configurations. The review had to stop, I had to flag the inconsistencies, and request clarification before anything could move forward. That delay is baked in and needs to be addressed before the permit can move forward.
When you're compiling your submittal package, lay all your site plans side by side. Verify that setbacks, dimensions, structures, access points, and parcel boundaries match across every sheet.
4. Access and Parking: Show It, Prove It, or Cite Why You Don't Need It
Parking and access requirements vary widely by jurisdiction, and this is an area where vague or inconsistent site plans create real problems.
If your jurisdiction requires only one point of access, your site plan should show only one. A second driveway curb cut drawn in, even casually, even as an option, can trigger a flag and require a redesign response.
If onsite parking is required, it needs to appear on every site plan in your package, correctly dimensioned. And here's a detail that gets missed constantly: check the scale of your site plan. A site plan that's drawn at the wrong scale will show parking stalls or a building footprint that appear too small or too large when a reviewer measures them. That's a correction request, and another delay.
If your jurisdiction does not require parking for your ADU as many have eliminated this requirement in recent years, don't just leave it off and hope the reviewer knows. Call it out explicitly. Reference the specific code section in your permit narrative and note it on the site plan itself. Reviewers are processing dozens of applications. Make it easy for them to confirm you've done the work.
Why This All Matters: Delays Kill Projects
Each of these issues on its own might cost you a week or two. Together, they can push a permit timeline out by months. And ADU projects are often time-sensitive in ways that aren't obvious from the outside like construction financing with rate locks, sellers on tight timelines, accessory income that's already been factored into a purchase decision.
One way applicants are protecting themselves against timeline risk is by choosing modern building systems that compress the construction schedule. Panelized modular systems like MOMO and SteelBuilt are engineered to go up in a fraction of the time of a conventional stick-frame build. When permits are finally in hand, these systems move fast which means a delay in permitting doesn't automatically cascade into a delay in income or occupancy.
Getting the permit right the first time is still the goal. But pairing a clean application with a system designed for speed gives your project two layers of protection against the delays that derail ADU projects every day.
Visit www.theaduwizard.com to learn more.