“How big does our grease interceptor actually need to be?” sounds like a question with a single, straightforward answer. Increasingly, at Northbay Restaurant Design, we’re finding it isn’t — particularly for clients building in Sonoma and Marin counties, where two different authorities can arrive at two different sizing conclusions for the same restaurant, based on two different sizing methodologies applied to the same project.
As we’ve covered in our grease trap requirements guide, grease interceptor requirements in California are enforced locally by wastewater agencies, not through a single unified state formula. What creates the specific conflict we’re seeing more often in Sonoma and Marin is that two different professionals are sometimes calculating your required interceptor size using two different accepted methodologies — and they don’t always agree.
The Plumbing Code Fixture-Based Method Your plumbing engineer or plumbing contractor often sizes your grease interceptor using the fixture-unit method outlined in the Uniform Plumbing Code — a calculation based on the number and type of drainage fixtures connected to the interceptor (sinks, dishwashers, floor drains), each assigned a fixture unit value that totals into a required interceptor capacity.
The Local Wastewater Agency’s Volume-Based Method Your local wastewater agency or municipal FOG Control Program, meanwhile, may apply its own sizing methodology based on projected wastewater flow, meal volume, or a formula specific to that jurisdiction’s local ordinance — which can produce a different required capacity than the plumbing code fixture-unit calculation, sometimes significantly different.
Sonoma and Marin counties, as we’ve noted in prior guides, maintain some of the more rigorous and locally specific wastewater and FOG compliance programs in the NorCal market — a reflection of both counties’ strong environmental and public health regulatory culture. As these local programs have become more detailed and specific over recent years — including the tightened inspection cadence we’ve covered in Sonoma’s case — the local wastewater agency’s own sizing expectations have become more explicitly documented and more actively enforced, sometimes diverging further from a straightforward plumbing code calculation than in counties with lighter local wastewater oversight.
Operators building in counties with less locally specific FOG enforcement often don’t encounter this conflict at all, because the plumbing code calculation and the local agency’s expectations align closely enough that nobody notices a discrepancy. In Sonoma and Marin, that alignment is less reliable — which is exactly why we’re fielding this question more frequently from clients in those specific counties.
If your plumbing engineer’s fixture-unit calculation produces a smaller required interceptor than your local wastewater agency’s own methodology requires, your project faces a real decision point: which number do you actually build to? Building to the smaller plumbing code calculation risks a wastewater agency rejection or required upsizing once your project reaches their specific review — a correction that can be significantly more expensive to implement after installation than before it. Building to the larger local agency requirement from the start avoids that risk but may mean a larger, more expensive interceptor than the plumbing code technically requires.
In nearly every case we’ve encountered, the safer and ultimately more cost-effective path is designing to whichever methodology produces the larger, more conservative sizing — particularly in a jurisdiction like Sonoma or Marin where local wastewater agency enforcement has real teeth and where under-sizing risk carries genuine downstream consequences, including the accumulation and inspection issues we’ve detailed in our Sonoma grease trap guide.
Discovering this discrepancy after your grease interceptor is already installed based on a plumbing code calculation alone — only to have your local wastewater agency require a larger unit during their own review — means real excavation, re-installation, and civil work cost that a coordinated design process would have avoided. This is exactly the kind of conflict that needs to be identified and resolved before your civil and plumbing drawings are finalized, not discovered as a correction during a separate wastewater agency review that runs on its own timeline.
At Northbay Restaurant Design, when we’re working on a project in Sonoma, Marin, or any jurisdiction where we know local wastewater agency requirements can diverge from a straightforward plumbing code calculation, we proactively coordinate between your plumbing engineer and the local wastewater agency’s specific FOG program requirements during the design phase — rather than letting each calculation happen in isolation and discovering a conflict later. This means your interceptor sizing satisfies both standards from the start, avoiding the costly reconciliation that happens when the conflict surfaces after installation.
The “how big does our grease interceptor need to be” question genuinely has two possible answers in counties like Sonoma and Marin — and building to the wrong one is an expensive mistake to correct later. Northbay Restaurant Design coordinates both sizing methodologies during design, so your interceptor is right the first time.
Contact us today for a free consultation and let’s make sure your grease interceptor sizing satisfies every authority reviewing it, not just one.
