Grease trap rules for commercial kitchens in London

It is an offence to let fat, oil or grease from a commercial kitchen into a drain or sewer if it is likely to interfere with the flow. Building Regulations guidance also says the drains from a commercial hot food kitchen should have a grease separator or another effective way of removing grease. In London, Thames Water visits food businesses about fat, oil and grease.

The law on fat in the sewer

Section 111 of the Water Industry Act 1991 says nobody may put into a public sewer, or into any drain or sewer connected to one, anything likely to injure it or interfere with the flow of its contents. The section also covers allowing it to pass, so a kitchen that lets fat go down the sink is covered as well as one that pours it.

The penalties:

• in a magistrates’ court, a fine, plus up to £50 for each day the offence continues after conviction
• in the Crown Court, up to two years in prison, a fine, or both

The FAQ on fat and the sewer gives the short version.

When a grease separator is needed

Paragraph 2.21 of Approved Document H, the government’s guidance on drainage under the Building Regulations, says drainage serving kitchens in commercial hot food premises should be fitted with a grease separator, or another effective means of removing grease. The separator should comply with BS EN 1825 part 1 and be designed in line with part 2, which covers choosing its size, installing it and maintaining it.

Approved Document H is guidance for building work, so it applies when a kitchen’s drainage is built or altered. A kitchen without a separator is still bound by the law against letting fat into the sewer. The grease trap FAQ sets out the rule in short.

Thames Water’s visits

Thames Water looks after the public sewers in London. In September 2026 it said that:

• its network protection team made more than 19,000 visits to restaurants and food businesses in the past year
• more than 2,500 restaurants and food businesses have installed fat traps
• fat, oil and grease cause more than 20,000 sewer blockages a year, costing around £40m a year to clear
• fat, oil and grease contribute to more than 60% of sewer flooding incidents

Thames Water says its first approach is to educate and support businesses and to recommend suitable grease management equipment. In the past five years it has also prosecuted three businesses for offences relating to improper trade effluent disposal and sewer misuse, with fines and costs totalling over £50,000. The figures are in its September 2026 announcement.

Keeping fat out of the drain

• Let leftover cooking oil, fat and food cool, then put them in the bin, as Thames Water advises, rather than washing them down the sink.
• Used cooking oil is waste produced by the business, so the waste duty of care applies. It goes only to an authorised person, such as a registered waste carrier, and the business keeps its copy of each waste transfer note. The waste duty of care code of practice sets out the rules.
• A grease separator or fat trap only works while it is emptied and maintained, as its maker instructs and as BS EN 1825 part 2 covers.

Fat that does reach the pipe cools and sets on its walls, and a drain that blocks again and again usually has a cause that one clear does not remove. See why a drain keeps blocking.

When the drain blocks anyway

Neos Engineering clears blocked commercial kitchen drains across all of London, 24 hours a day, and traces blockages that keep coming back to their cause. At a business, a drainage call out is £150 + VAT an hour per engineer, 8am to 5pm, Monday to Friday, and £290 + VAT an hour at any other time. The first hour covers the call out and finding the fault, and after that the visit is charged by the hour, with any return visit or further work quoted first. Materials, parking, the congestion charge and waste removal are extra where they apply.

See commercial drainage, gas, plumbing and drainage for commercial kitchens, or contact the office.

If you need us

• Commercial drainage
• Commercial Catering
• Why a drain keeps blocking