A commercial kitchen fit-out has a long list of decisions competing for attention, layout, equipment, exhaust, cool room placement, and the grease trap is easy to leave until late in the process because it sits underground or under a bench rather than somewhere anyone walks past every day. That’s exactly the mistake worth avoiding. Grease trap sizing and the associated council approval sit on the critical path to opening, not somewhere alongside it, and getting the size or the paperwork wrong partway through a fit-out is one of the more expensive and time-consuming corrections a new venue can face.
Why does grease trap sizing need to happen early, not at the end of the fit-out?
A grease trap is sized around the kitchen’s peak hydraulic flow rate, not its floor area or seating count alone, and that figure depends on decisions made throughout the fit-out: the number and type of sinks, whether a commercial dishwasher is installed and how it’s plumbed, deep fryer volume and how often oil is disposed of, and the realistic peak covers the kitchen will run at service. Locking in the trap size and location before those decisions are finalised, rather than after, is the difference between a trap installed once and a trap that has to be dug up and replaced because the kitchen it was sized for changed underneath it.
The practical problem with sizing late is that an undersized trap doesn’t fail quietly. It fills faster than expected, needs far more frequent servicing than a correctly sized unit, and in a worst case overflows during a busy service, precisely the moment a venue can least afford a compliance incident or a bad smell reaching the dining room.
What determines the right size for a specific kitchen?
- Fixture count and type. Pot wash sinks, prep sinks and dishwasher pre-rinse points all contribute flow, and a kitchen with more of them at once needs a larger trap than one with a single combined wash-up point.
- Commercial dishwasher capacity and cycle frequency. A dishwasher discharges a concentrated burst of hot, greasy water rather than a steady trickle, and that peak flow matters more to sizing than the kitchen’s average daily volume.
- Deep fryer volume and oil disposal habits. A kitchen running multiple fryers, or one that changes oil frequently, adds a materially heavier fats and oils load than a menu with no deep-frying at all.
- Realistic peak service volume, not the venue’s quietest expected trading pattern. A trap sized for a modest opening-week crowd and never revisited once the venue finds its real trading rhythm is a common and avoidable gap.
A licensed trade waste contractor or plumber calculates the required trap size and location from the kitchen’s actual plumbing plan, and that calculation is worth getting right before the trap goes in the ground, since retrofitting a larger unit later usually means breaking concrete or floor coverings that were only just finished.
What does council signoff actually require?
Two different regulatory pathways apply depending on how the venue disposes of wastewater, and it’s worth knowing which one applies to your site before you’re partway through the fit-out.
Sewer-connected premises need a Trade Waste Agreement with the water utility before the business opens. The agreement specifies grease trap size, type, location and minimum cleaning intervals, and PMHC’s trade waste team typically needs to sign off the plumbing plan showing the trap before final connection is approved. Applying for this agreement in parallel with the fit-out, rather than after the trap is already installed, avoids the situation where an installed trap doesn’t match what the agreement ultimately requires.
Premises on an onsite sewage management system need the grease trap included as a component of the property’s OSSM approval, sized appropriately for the commercial kitchen’s output rather than treated as an afterthought to a domestic-scale system already on site. This applies to rural venues, cellar doors, catering kitchens and any commercial food premises not connected to reticulated sewer, and it’s a materially different approval process to a straightforward trade waste agreement. Our septic services FAQ covers more of the general approval questions that come up around onsite systems if you’re unfamiliar with how PMHC’s OSSM framework works.
Common fit-out timeline mistakes
Ordering equipment before confirming the trap’s plumbing connection point. A kitchen layout finalised without the trade waste contractor’s input can end up with drainage points that don’t line up sensibly with where the trap needs to sit, forcing an expensive replumb.
Treating the trap as a plumber-only decision. The trap size and the trade waste agreement (or OSSM approval) need to match. A plumber can install a trap to a given specification, but confirming that specification against council or utility requirements is a separate step that needs to happen in parallel, not be assumed to follow automatically.
Sizing for opening week, not for growth. A café that plans to expand its menu or add a commercial dishwasher within its first year is better served sizing the trap for that near-term reality now, rather than opening with a marginal unit that needs upgrading again within twelve months.
Leaving the application until construction is nearly finished. Trade waste agreements and OSSM approval variations both take processing time, and a signoff requested at the last minute can delay an opening date that’s already been advertised.
What does grease trap servicing cost once the kitchen is operating?
| Job type | Indicative price | Notes |
|---|---|---|
| Small trap service (up to ~1,000 L) | $250-$450 | Typical for a café, takeaway or small kitchen |
| Mid-size trap service (1,500-3,000 L) | $400-$650 | Pubs, clubs and busier kitchens |
| Large trap service (4,000-5,000 L+) | $600-$950+ | Larger venues and function kitchens |
| Recurring service contract | Quoted per venue | Set schedule matching your trade waste or OSSM conditions |
These are indicative guide ranges for ongoing servicing, not a fit-out or installation quote. Trap installation itself is arranged through your plumber or trade waste contractor as part of the fit-out; once it’s operating, the ongoing servicing, cleaning and maintenance is what this site takes enquiries for, and the published tables give realistic servicing intervals while you’re still finalising the sizing decision.
Commercial kitchen grease trap FAQs
Who decides the size of our grease trap, us, our plumber, or council?
All three have a role. Your plumber or a licensed trade waste contractor calculates the required size from your kitchen’s actual fixtures and plumbing plan, but the final specification needs to satisfy whichever approval pathway applies to your site, a Trade Waste Agreement for sewer-connected premises or an OSSM approval for onsite systems. Confirming both in parallel avoids installing a trap that later needs to be resized.
We’re renovating an existing kitchen rather than starting from scratch. Does the old trap still count?
Not necessarily. Check whether the existing trap was sized for the previous kitchen’s menu and volume, and confirm whether your renovation changes fixture count, dishwasher capacity or fryer volume enough to require a larger unit. An undersized inherited trap is a common issue on renovated premises that change hands and change menus more than once.
How long does the council or trade waste approval process typically take?
Timeframes vary and depend on the specific council or utility process, so it’s worth confirming directly with PMHC’s trade waste team or the OSSM approvals area as early in your fit-out as possible, rather than assuming a fixed turnaround. Building the approval timeline into your overall fit-out schedule, rather than treating it as a formality to sort out later, is the safest approach.
Can you help us work out what size trap our new kitchen needs?
Sizing calculations and formal approval are handled by your plumber or trade waste contractor, since they need your specific plumbing plan. The published tables here give realistic servicing frequency and costs for the trap size you’re considering, and once the kitchen is operating, ongoing cleaning and service contract enquiries come through this site.
What happens if we open with a trap that turns out to be undersized?
An undersized trap needs far more frequent servicing to avoid overflowing, and in some cases needs upgrading once the true volume is clear. Catching a sizing gap early, ideally through more frequent servicing while a permanent fix is arranged, is far cheaper than an overflow that triggers a compliance issue with council or your trade waste agreement.
Get your commercial kitchen grease trap servicing sorted
Whether you’re partway through a fit-out and want to talk through realistic servicing intervals, or your kitchen is already open and needs a trap put on a proper schedule, get a free quote and tell us your premises type and trap size if known. For the full picture on food business grease trap obligations in the region, see our grease traps for food businesses guide.