Grease Trap Installation for Restaurants: Cost & Code Guide

A grease trap installation is the plumbing fixture that keeps fats, oil and grease (FOG) out of the public sewer system, and King County will not approve a commercial kitchen without one sized and installed to code. Every restaurant, café or bar that cooks or washes greasy dishware needs one, whether it sits under a three-compartment sink or outside the building as a large in-ground interceptor.

We size and install grease traps as part of nearly every kitchen build-out we handle. Here is what one actually costs in 2026, how the two main types differ, and the code requirements your plumbing inspector will check before you open.

Grease trap installation cost in 2026

Pricing depends mainly on flow rate and whether the unit sits indoors under a sink or outdoors as a full interceptor. For planning purposes, these installed ranges are realistic across Seattle & King County:

• Under-sink hydromechanical trap (10–50 GPM): $800–$2,500 installed
• Outdoor grease interceptor (500–1,000 gallon): $8,000–$18,000 installed
• Large multi-compartment interceptor (1,500+ gallon): $18,000–$35,000+ installed
• Routine pumping & maintenance: $200–$450 per service visit

Those figures cover the unit, piping tie-ins, permit fees and the plumbing inspector’s sign-off. However, a full-service kitchen with a high seat count, or a building without existing exterior sewer access, can push the number higher. Our commercial kitchen hood installation guide covers where grease management fits inside the rest of the mechanical budget.

How a grease trap protects your kitchen and your permit

An under-sink hydromechanical trap uses baffles to slow water flow so grease cools, floats and separates before wastewater reaches the sewer line. It works well for smaller kitchens with limited fixture counts and a modest cooking volume.

A full-service restaurant with a fryer line, wok station or high dish volume usually needs a larger outdoor gravity interceptor instead. These buried concrete or fiberglass tanks hold far more wastewater, so they need pumping less often, but they also require more excavation and a longer permit timeline.

Grease trap code and permit requirements in King County

Local sewer authorities enforce FOG (fats, oils and grease) control programs that require any food service facility to install and maintain a grease trap sized to its fixture units. The EPA outlines why FOG control matters for public sewer systems, and King County’s wastewater program sets the local sizing and inspection rules your contractor must follow.

A licensed plumbing contractor pulls the permit, sizes the trap to your fixture count, and schedules the inspection before your health permit is finalized. Washington State also requires that contractor to hold an active license through the Department of Labor and Industries, so always confirm that credential before signing a contract.

Grease trap maintenance and pumping schedule

Once installed, an under-sink trap typically needs pumping every one to three months, depending on volume, while a large outdoor interceptor may only need service every three to six months. Your local sewer authority sets the actual schedule and expects a maintenance log on file.

Skipping service is one of the fastest ways to trigger a sewer backup, fail a health inspection, or draw a fine from the local FOG program. Therefore we recommend setting a standing pumping contract before your grand opening, not after the first overflow.

Common grease trap mistakes we see

• Undersizing the trap for the kitchen’s actual fixture count and meal volume
• Choosing an under-sink trap for a full-service kitchen that really needs an outdoor interceptor
• Skipping the maintenance log that health and sewer inspectors ask to see
• Installing the trap somewhere the pumping truck cannot reach
• Leaving grease management off the plumbing permit set, which stalls inspection before opening

Every one of these is avoidable when the grease trap is sized and placed early, alongside the rest of the kitchen plumbing, instead of as an afterthought.

Frequently asked questions