A blocked drain in London is usually a toilet that will not clear, a basement gully that backs up in heavy rain, or a shared stack in a converted house where two flats go slow on the same day. Tell us the problem and the postcode. We pass that to local drainage companies that cover it.

Common signs of a blocked drain in London

• Toilet water sitting high after a flush, or not clearing at all
• Grey water coming up through the shower when the washing machine empties
• A yard or basement gully bubbling or spilling when it rains
• More than one sink in the flat going slow at the same time
• A sewage smell in a lower ground floor, a lightwell or a basement
• The neighbour in the flat below reporting a smell before you notice anything

If only your flat is slow and the rest of the building runs, the restriction is usually on your own pipework. If two or more flats in a converted house are slow at once, that is the shared stack or the drain under the garden, and it is a different job with a different bill payer.

What people typically pay in London (2026)

London prices sit above the rest of the country. Some firms advertise from around £75 or £99 for a simple daytime job. Treat those as starting points. Parking, the congestion charge, a basement, a drain cover under a patio, or a mansion block stack puts them up again. National ranges sit on what drain unblocking costs, and survey prices on CCTV drain survey cost.

Wipes and fat are why London blocks

This is not a guess or a general warning. Thames Water publishes its own figures, and they are the starkest of any water company in the country.

• 75,000 blockages cleared from its sewer network in a year
• 3.8 billion wet wipes removed each year, at a cost of around £18 million
• 28,899 rag blockages in 2025, mainly wipes, plus 14,810 caused by fat, oil and grease
• Wipes are the number one cause, by their own count

The Feltham fatberg in west London is what that looks like when nobody catches it early. It weighed 100 tonnes and ran 125 metres, which Thames Water compared to eight double decker buses. It took more than a month to clear and was finished on 6 October 2025.

A domestic blockage is the same material at a smaller scale. Wipes catch on a joint or a bend, fat cools and sticks to them, and the pipe closes over weeks rather than in one go. That is why the drain that backs up today has usually been running slow for a while.

The practical version

• Nothing but paper down the toilet. Wipes labelled flushable are the single biggest cause on the network
• Fat goes in the bin, not the sink. It pours warm and sets cold, on the inside of your own pipe first
• In a shared house or a converted flat this matters more, because several kitchens feed one stack

What the super sewer did not fix

The Thames Tideway Tunnel, the super sewer, became fully operational in February 2025 and was officially opened by King Charles III on 7 May 2025. It runs 25 kilometres, about 16 miles, under the river.

It intercepts flows from 34 of the most polluting combined sewer overflows and cuts discharges into the Thames from an average of around 60 a year to fewer than five. It cost roughly £5 billion, and Thames Water customers pay for it through their bills.

Here is the part that matters to a householder. Tideway was built to stop sewage going into the river. It was not built to stop your kitchen sink backing up, and it does not.

If your drain is blocked in 2026, the cause is almost certainly the same as it was in 2016: wipes, fat, roots or a broken pipe on private ground. A tunnel under the Thames does not reach the run between your kitchen and the boundary.

Where it does change things is heavy rain. Fewer overflows means the network is less likely to be completely full when a storm hits. That is the moment when basements and lower ground floors have always suffered most.

Victorian sewers under most of inner London

Inner London still runs largely on the system Joseph Bazalgette built in the second half of the nineteenth century. Most of it is combined: foul waste and rainwater share the same pipe.

That design is why a downstairs toilet in a Victorian terrace can back up during a downpour while everything is fine in dry weather. The pipe is not blocked in the ordinary sense. It is full.

Two things follow from that, and they point at different bills.

• If it only happens in heavy rain, and the neighbours have it too, that is the network under load. Report it to Thames Water
• If it happens in dry weather, or only in your property, the restriction is on private pipework and you need a contractor

Basement and lower ground floor flats

London has a great deal of below ground living space: original basement flats, lower ground conversions and newer basement digs.

Every one of them sits below the level of the sewer. When the network surcharges, gravity decides where the water goes, and it goes to the lowest opening in the building.

• The gully in the lightwell is usually the first thing to spill
• A basement shower or toilet can back up while the flat upstairs is unaffected
• Many basement conversions use a pump, because there is no fall to the sewer. A pumped system that stops is an emergency, not a slow drain

If waste is coming into the property, ring Thames Water as well as a contractor. Do not wait to see whether it goes down on its own.

Mansion blocks in Maida Vale, Kensington and Marylebone

Mansion blocks share stacks vertically. One flat puts the wrong thing down, and the flat two floors below finds out first.

The practical problem is rarely the pipe. It is working out who instructs and who pays.

• Inside your flat is normally yours, or your landlord’s on a tenancy
• The shared stack and the drains under the building are normally the freeholder or the managing agent, through the service charge
• Beyond the building boundary is normally Thames Water

Tell the managing agent first if more than one flat is affected. They should already know whose pipe it is, and calling a contractor yourself for a communal stack can leave you holding an invoice that belongs to the block.

Converted houses in Hackney, Camden and Lewisham

A three storey terrace split into three flats has one drain designed for one household now serving three. Three kitchens, three bathrooms, three sets of habits, one pipe under the front garden.

These are the addresses where a CCTV survey earns its money. It shows whether the run is genuinely blocked, or whether it was never upgraded when the house was divided.

Tree roots and London Clay in the outer boroughs

Bromley, Barnet, Enfield, Croydon and Richmond have older clay drain runs with jointed pipes, mature street trees and gardens.

Roots do not break into a sound pipe for the sake of it. They find a joint that has already moved, and London Clay moves. It shrinks in a dry summer and swells again when the rain returns, and the ground carrying your drain moves with it.

The pattern is recognisable. The drain runs slow for months, clears when it is rodded, then blocks again the following autumn once the leaves are down and the ground is wet.

Rodding a root mass buys you months. Jetting cuts it back properly, and a survey afterwards shows whether the pipe is cracked or displaced. If it is, repair or lining is the fix and another unblock is money spent twice.

Who looks after the drain in London

• You, or your landlord. Everything inside the property, and the pipe outside that serves only your property up to the boundary or up to the point it joins shared pipework
• The freeholder or managing agent. In a block or a converted house, the shared stack and the drains serving more than one flat, usually funded through the service charge
• Thames Water. The public sewer beyond the boundary, and most shared pipes that serve two or more properties. Their guide to private sewer ownership sets out where the line falls. Blockage on their network is cleared at no charge
• Your borough council. Road gullies and highway drainage. Standing water in the street is a council matter, not a Thames Water one and not yours

From 1 October 2011 most private sewers and lateral drains in England transferred to the water company. In practice that moved a lot of shared London pipework off householders and on to Thames Water, which is why a blockage in a rear garden serving several houses is usually not your bill.

The 33 borough problem

London is not one council. Highway drainage, gully cleaning and flooding reports are handled borough by borough, and the reporting routes and response times are not the same in Camden as they are in Croydon.

For a household drain this rarely matters, because it is Thames Water or a contractor either way. It matters when the water is in the road rather than the house. Report that to the borough, and be ready for the borough to say the gully is theirs but the sewer under it is not.

Student and shared houses

Around the university areas, and anywhere with a high concentration of shared houses, one drain serves several households who have never discussed what goes down it.

Report it to the landlord or agent in writing. A tenant can still use the quote form if the landlord has agreed to it. Say on the form that it is a shared house, because it changes what the company expects to find.

FAQs in London

The toilet only backs up when it rains. What is that?

Usually the combined sewer taking foul waste and rainwater together and running full. If the neighbours have it at the same time, report it to Thames Water. If it is only your property, a contractor still needs to check the private section.

Did the super sewer not fix this?

Tideway was built to stop sewage entering the Thames, and it does that well. It was not built to clear a blockage between your kitchen and the boundary, and it makes no difference to one.

Two flats in our converted house are slow. Who pays?

If the pipe serves more than one flat it is normally the freeholder or the managing agent, or Thames Water beyond the boundary. Tell the agent before you instruct anyone yourself.

The basement flat is flooding with waste. What do I do first?

Ring Thames Water and a contractor. If it is their sewer they should attend. Do not wait to see whether it drains away.

Is my drain blocked, or is it a fatberg?

A fatberg is the same problem on the network rather than in your pipe. The Feltham one weighed 100 tonnes and took more than a month to cut out of the sewer. Yours will be a handful of wipes caught on a joint, and an hour or two of work.

Are wet wipes really the problem?

By Thames Water’s own count they are the number one cause, at 3.8 billion removed a year costing around £18 million. Flushable on the packet does not mean it breaks down in a pipe.

Is a CCTV survey worth it before buying in London?

Yes on a Victorian terrace, a converted flat, anything with a basement, and anywhere with mature trees. A standard building survey does not film the drain.

Will parking and the congestion charge be on the bill?

Often, in central boroughs. Ask when you book so it is not a surprise on the invoice.

Do I need to be there?

Usually yes, so the drain covers can be lifted. In a block, be able to say which flats are affected and who the managing agent is.

How quickly can someone attend?

Same day and next day cover is normal across London postcodes. Emergency rates apply out of hours. If waste is entering the property, ring as well as filling in a form.

Getting a quote in London

Describe the problem and the postcode. We pass it to drainage companies covering that London borough. Say if it is a basement, a mansion block, or more than one flat is affected.

Not in London? See all areas.