Basement flooding and sewage backup

A flooded basement is one of the most frightening calls we take, and in this part of London one of the most common. Kensington and Chelsea has one of the highest densities of basement homes in the country, Westminster and Camden are full of lower ground floors and converted cellars, and the sewers beneath them were built in Victorian times. When water comes in, the first question is always the same: where is it coming from?

The first few minutes

1. Switch off the electricity to the basement or lower ground floor at the fuse board, if you can reach it without standing in water. If you cannot, stay out.
2. Call 999 if anyone is trapped or at risk. Fire crews deal with serious flooding and rescues.
3. Stop using water in the home if the water is coming up through drains, toilets or showers. Every flush and tap adds to it.
4. Move people, pets and valuables upstairs, and keep away from water that has come up from the drains.
5. Call 07861 933265 or WhatsApp us a photo so we can tell you what to do next.

Four kinds of basement flood

Storm water and sewage backing up. Most of central London’s sewers carry rainwater and waste together. In a cloudburst they fill up, and water rises back through the lowest outlets it can find, which are basement toilets, showers and floor drains. That is what happened across Notting Hill, Holland Park and Kensington on 12 July 2021, when Kensington and Chelsea estimated rainfall as intense as a 1 in 185 year event in places. Camden and Westminster saw the same that day.

A burst main in the street. When a large water main fails, water pours into the nearest low point, often a basement. On 25 September 2025 a main burst near Enford Street in Marylebone and filled basement flats with up to two metres of water. On 21 January 2026, a 30-inch main burst near the Holland Park roundabout, affecting more than 50 homes.

Your own pipes. A failed supply pipe, a burst hot water cylinder or a leaking pump in a basement extension can flood a lower floor from the inside, often out of sight.

Groundwater. In some streets, water seeps through walls and floors after long, heavy rain. It is slower and steadier than the other three.

Worth knowing: Clean water that keeps coming in dry weather is almost always a pipe. Water that appears during or just after heavy rain, especially through drains, is the sewer. Muddy water pouring in from the street is usually a burst main.

What we do

We make the basement safe, find out which of the four it is, and deal with what is ours to deal with: stopping a failed pipe, restoring your supply, clearing and checking your drains and making your plumbing safe to use again. If the cause is the public sewer or a main in the street, we tell you exactly who to contact and what to say, and give you a written account for your insurer.

Protecting a basement for next time

• A pumped device on the drains. Kensington and Chelsea’s planning policy asks new basements to be protected from sewer flooding by a pumped device, and says a non-return valve alone is not enough. Since 2007, Thames Water has fitted more than 1,000 FLIP pumping devices to basements in the Counters Creek area alone.
• A non-return valve on older drains where a pump is not practical, checked and maintained.
• Electrics and valuables off the floor, and a fuse board you can reach without going into the basement.
• Know where your stopcock is, and keep the route to it clear.
• Check your insurance covers flood and sewage damage, not just escape of water.

We deal with flooded basements across Chelsea, Notting Hill, Kensington, Marylebone, Swiss Cottage, West Hampstead and every other area we cover. If the water is coming up through a toilet or drain, see blocked toilet or drain, and read our flood timeline for north-west London.