Who is responsible for a blocked drain outside my house?

It depends where the blockage is. Pipes inside your property boundary that serve only your house are yours. Pipes that serve more than one property — a shared drain — transferred to the water and sewerage company on 1 October 2011, along with lateral drains, which are the sections of your own pipe running beyond your boundary to the public sewer. So a blockage in your garden serving only your house is your bill; a blockage in a shared pipe, or in your pipe once it has left your boundary, is generally the water company's. In Cornwall that means South West Water.

Should I call a drainage company or the water company first?

Call the water company first if the problem looks like it is outside your boundary, affects more than one property, or shows up in a shared manhole — it costs nothing and they attend blockages on pipes they own. Call a private drainage company if the problem is clearly inside your boundary and serves only your house, or if you are on a septic tank or treatment plant, because private off-mains systems never transferred and are always yours. If you are not sure, calling the water company first is the cheaper mistake.

My neighbours have the same problem — what does that tell me?

It is a strong signal that the blockage is in a shared pipe or the public sewer rather than in your own drainage, which usually makes it the water company's responsibility rather than yours. If several properties on the same run are backing up at once, report it to South West Water and say explicitly that more than one property is affected. Do not let a private contractor start work on a shared system before that call is made — you may be paying for something you are not liable for.

Does the water company pay if sewage floods my house?

If the flooding was caused by a problem in a public sewer or a pipe the water company is responsible for, they have obligations to homeowners affected by sewer flooding, and there are compensation schemes. Report it to them immediately, and photograph everything before you clean up. If the cause was your own private drainage — a blocked pipe serving only your house, a failed pumping station within your boundary, or a septic tank problem — it is a matter for you and your insurer instead.

What if I'm on a septic tank rather than mains drainage?

Then none of the transfer rules help you: private off-mains systems — septic tanks, treatment plants, cesspits and the drainage fields serving them — were never transferred and are entirely the property owner's responsibility. The one exception worth checking is a sewage pumping station that discharges into the public sewer, which may have transferred in 2016 even though the rest of your drainage did not.

Drains backing up: whose responsibility is it?

Something is backing up. Before you call anyone, it is worth spending five minutes working out whose pipe it is, because the answer decides whether you get a bill or a free visit.

The rules changed in 2011 and again for pumping stations in 2016, and a lot of what people believe about drain ownership is out of date.

The three cases

1. Your own drainage, inside your boundary, serving only your house. Yours. You pay. This covers the pipes under your garden that nothing else connects to.

2. A shared drain, or your pipe beyond your boundary. Generally the water company’s. On 1 October 2011, private sewers serving more than one property transferred to the water and sewerage companies, along with lateral drains — the length of your own pipe that runs beyond your property boundary to the public sewer. In Cornwall that is South West Water.

3. A sewage pumping station discharging to the public sewer. Possibly the water company’s, under a separate transfer that took effect on 1 October 2016. Two tests: does it serve more than one property, or does it sit outside the boundary of the property it serves? If either is true, it likely transferred. Full detail in is my pumping station mine or the water company’s?.

And the exception that swallows all of it: if you are on a septic tank or treatment plant, none of the above applies. Private off-mains systems never transferred and are always yours.

How to work out which one you’re in

Is more than one property affected? If neighbours on the same run have problems at the same time, it is almost certainly a shared pipe or the public sewer. That is the single most useful signal there is.

Where is the water coming up? A gully or manhole at the bottom of your garden that only serves your house points at your own pipe. A shared manhole between properties, or one in a lane or verge, points the other way.

Follow the flow. Drainage runs downhill to the sewer. The blockage is downstream of the last thing working normally and upstream of the first thing backing up. If everything in your house is affected but the shared manhole beyond your gate is full too, the problem is past your boundary.

Do you actually have a public sewer? Plenty of Cornish properties do not. If your drainage ends at a tank in the garden, you are off-mains and it is all yours.

What to do, in order

Stop using water. Every flush, shower and washing machine cycle adds to what has nowhere to go.
Look, don’t lift. Check gullies and manhole covers you can see safely. Do not enter a chamber — sewage produces gases that have killed people, and confined spaces are genuinely dangerous.
Call South West Water first if there is any chance it is theirs. It costs nothing. They attend blockages on pipes they are responsible for, and if it turns out not to be theirs you have lost a phone call rather than a callout fee.
Then call a private drainage engineer if it is confirmed as your own pipe, or if you are off-mains.
Photograph everything before cleaning up, especially if sewage has entered the building.

If sewage gets inside the house

Report it to the water company immediately if there is any prospect it originated in a public sewer — there are duties toward homeowners affected by sewer flooding, and schemes that may apply. Photograph everything first. Then contact your insurer.

If the cause was your own drainage, it is you and your insurer, and the photographs matter just as much.

Why this is coming up more in Cornwall

Cornwall’s sewerage has been under considerable strain, and South West Water has been the subject of significant regulatory and legal action over discharges from its network. That is a live picture and it has practical consequences for homeowners — including for the price and availability of tanker services. We have set out what it means for off-mains and part-mains properties in South West Water and your drainage.

None of that changes the ownership rules above. But it does mean that if you are reporting a problem you believe is theirs, it is worth being clear, keeping a record of what you reported and when, and following it up.

Sources: South West Water — sewer responsibility · Water Industry (Schemes for Adoption of Private Sewers) Regulations 2011