People use the words drain, lateral drain and sewer as if they mean the same thing. They don't, and the difference decides who pays when something goes wrong. Thames Water and the Water Industry Act 1991 define these terms. The 2011 private sewer transfer regulations changed who owns what. Here is what each term means for a London homeowner.
A Drain
A drain is a pipe that serves one property. It carries wastewater from the toilets, sinks, showers, baths and kitchen of a single house or flat. It runs to the point where it meets a shared pipe or the public sewer.
A drain is always private, so you look after it and you pay for repairs. Your duty starts at the first pipe fitting inside the property. It ends where the pipe leaves your boundary or joins a shared pipe.
Since a detached house has its own link to the road sewer, the whole run is a private drain. That includes the part under the front garden.
A Lateral Drain
A lateral drain also serves one property. The difference is where it runs. It crosses the property boundary, usually under the pavement or road, to reach a public sewer.
Before 2011, lateral drains were private. Homeowners paid for repairs all the way to the sewer, even under a public road. That was unfair. A cracked pipe under the pavement became your bill, although you had no real way to reach it.
The 2011 rules changed this. Since October 2011, sewerage undertakers (Thames Water in London) have adopted lateral drains. They now count as public sewers for repair purposes. This means:
- Thames Water maintains the drain under the pavement and road, up to the main sewer.
- If that section blocks or collapses, Thames Water repairs it at no cost to you.
- The private drain inside your boundary stays your responsibility.
This rule still catches people out. Some owners got a repair quote before 2011 and assume nothing has changed. Our post on drain ownership for flats and terraces covers the 2011 changes in more detail. It includes advice for leasehold and shared properties.
A Public Sewer
A public sewer carries wastewater from more than one property. Thames Water owns and maintains it. Public sewers usually run under roads and pavements. They collect flow from lateral drains until it reaches a treatment works.
Thames Water handles all repairs to public sewers, because it owns them. If one blocks and sewage backs up into your home, their emergency team responds. If one collapses under your road, they repair it.
Although the pipe may sit in your garden, you cannot build over or near a public sewer without a build-over agreement from Thames Water. The pipe is Thames Water's asset wherever it sits.
Shared Private Sewers: The In-Between Case
Before 2011, a pipe that served several homes but had no public owner was a shared private sewer. The connected owners shared the duty to maintain it. This caused many disputes, because nobody agreed who should pay.
The 2011 rules moved most of these pipes into the public network. Some remain private, however. These are typically pipes built after 1937 under specific arrangements, or pipes inside private developments. If you are unsure, a CON29DW drainage search will tell you.
Why This Matters When Something Blocks
The table shows who pays in each case:
| Location of blockage | Responsibility | Who pays |
|---|---|---|
| Internal drain (within property) | Homeowner | You |
| Private drain under garden | Homeowner | You |
| Lateral drain under pavement | Thames Water | Thames Water |
| Public sewer | Thames Water | Thames Water |
| Shared private sewer | Joint homeowners | Split between connected properties |
The hard part is finding where the blockage sits, because the pipes are buried. A camera survey from an accessible inspection chamber can locate it exactly. If it lies in your private drain, that is our work. If it lies in the lateral drain or public sewer, Thames Water takes over.
Our drain repair service covers private drain checks and blockage location. We also work with Thames Water when the fault lies in the public network.
How to Check Which Pipes Are Public
There are three simple routes.
CON29DW search. Your solicitor can order a drainage search during conveyancing. It shows the public sewers and adopted lateral drains near your property.
Thames Water sewer maps. Thames Water publishes maps of its sewers. They are not always exact for older pipes, but they give a good start.
CCTV survey. For an active problem, a survey from your inspection chamber shows the layout and finds the blockage. Combined with the search, it gives a full picture.
Worked Examples: Three Common London Property Types
The rules are easier to follow with real layouts, so here is how one blockage plays out in three typical homes.
A Victorian mid-terrace in Walthamstow (E17). The drain runs under the rear yard and joins a shared party drain. That drain serves three or four houses. It then crosses under the front pavement to the road sewer. A blockage in the kitchen branch is your bill. A blockage in the shared party drain is now Thames Water's job, because the 2011 rules adopted it. A blockage under the front pavement is the lateral drain, so it is also Thames Water's. Many owners are surprised that most of their drainage is not their responsibility.
A 1930s semi in Chiswick (W4). The drain runs from the rear bathroom, round the side return and under the front garden. It meets the public sewer near the kerb. Since nothing is shared, the whole run to the boundary is private. Anything beyond that is the lateral drain, which belongs to Thames Water. Many Chiswick semis have one inspection chamber halfway down the side return. It often shows where private ends and public begins.
A two-bed flat in a converted Hackney house (E8). The flat shares a soil stack with three other flats. The stack feeds one drain for the whole conversion. Your lease decides who pays, not the 2011 rules. The freeholder or management company usually maintains shared drainage through the service charge. Your own duty usually ends at the wall of your flat. A CCTV drain survey before you buy, plus a careful read of the lease, is the only sure way to confirm this.
FAQ
Has the 2011 Act been fully implemented — are all lateral drains now adopted?
Most lateral drains were adopted when the rules came into force in October 2011. A small number of cases were disputed or delayed. A drainage search will confirm the status of the pipe at your property.
If Thames Water is responsible for the lateral drain, why do I need my own drain survey?
Your private drain still belongs to you, yet the survey will not cover the rest. You need your own survey for the private part. Thames Water holds records for the adopted pipes.
What if a public sewer blockage causes damage to my property?
If a public sewer blocks and sewage backs up into your home, Thames Water may be liable. Keep records of all the damage and contact their claims team. You should also tell your home insurer.
Is a soakaway drain a public or private drain?
A soakaway sends surface water into the ground while the sewer stays unused. It is always private. Thames Water does not adopt it, so the homeowner is responsible.
How do I find out who is responsible for the drain under my garden?
A CON29DW search plus a CCTV survey is the most reliable route. Request a quote online via our contact form to arrange a survey. Your solicitor can order the drainage search.
Because every property is different, start with our guides to blocked drains and bathroom drain blockages advice. We also cover blocked drains in Bromley and blocked drains in Camden, so you can see how the work runs near you.
