Build Over Agreement: Can You Extend Over a Sewer?

Most rear extensions go into the garden, and in many houses that's exactly where the drains run. If one of those pipes is a public sewer, you need the water company's say-so before you build over it or near it. It's one of the most common surprises at the start of an extension, and it's much cheaper to find early than halfway through digging foundations.

The short answer

You need approval from the water company if your extension, or its foundations, will be:

• built over a public sewer, or
• within 3 metres of a public sewer.

In England, this comes from Part H of the Building Regulations. Building control has to consult the water company, and the water company decides whether to agree and on what terms. People call this a build over agreement. Some companies call it build over approval, or a building near a sewer application. They all mean the same thing.

It's not a planning matter. Permitted development doesn't cover it, and planning permission doesn't replace it.

Public sewer or private drain?

This is the part that catches people out. Since October 2011, in both England and Wales, most shared sewers were transferred to the water companies. That means a pipe running under your garden can be public even though it's on your land.

Type of pipe	Who owns it	Need a build over agreement?
A drain that serves only your house and stays inside your boundary	You	No, but building control will still check it
A pipe that serves your house and a neighbour's	Usually the water company	Yes, if you build over or within 3m
A bigger sewer running along the back of a terrace or under the road	The water company	Yes, and some can't be built over at all

Terraced and semi-detached houses are the ones to watch. A shared run of pipe along the back of a row of houses is very often public, and it's often right where a rear extension would go.

How to find out what's under your garden

1. Look at your sewer map. Your water company holds records of public sewers. You can usually ask for a plan of your address, sometimes for a small fee.
2. Check your house purchase papers. The drainage and water search your solicitor ordered when you bought the house may already show it.
3. Lift the manhole covers. If you can see pipes coming in from next door, that run is probably shared.
4. Don't rely on the map alone. Sewer records aren't always accurate, especially for older pipes. A drainage contractor can do a CCTV survey to show the exact route, depth and condition.

Do this before you have drawings done. The position of a sewer can change the shape of the extension.

What the water company looks at

Each water company has its own process, fees and timescales, so check with yours. In general they want to know:

• How big and how deep the pipe is. Small, shallow pipes in good condition are usually straightforward. Some companies let you self-certify for these. Larger or deeper sewers need a full application, and the biggest ones usually can't be built over at all.
• The condition of the pipe. Many will ask for a CCTV survey before work starts, and sometimes again after.
• How the foundations protect it. Foundations may need to go deeper, bridge over the pipe, or keep the weight of the wall off it. A structural engineer usually designs this.
• Whether there's a manhole in the way. Water companies generally don't allow a public manhole inside a building. If there's one where your kitchen will go, expect to move it.

If the answer is no, the extension has to be redesigned to keep clear, or the sewer has to be moved. Moving a sewer is a formal process with the water company, and it's slow and expensive. Sometimes the honest answer is that a smaller extension, or a different spot, is the better choice.

When to apply

Apply as soon as you have drawings that show the extension and the sewer, and before any digging starts. It runs alongside building regulations, so it fits in after planning (or after you've confirmed the work is permitted development) and before the builder starts. Allow several weeks, and don't book a builder's start date until you have it.

Your building regulations drawings should show the sewer, the 3m zone and the foundation design. That way the water company and building control are looking at the same thing.

England and Wales

The basic rule is the same in both: build over or within 3m of a public sewer and you need the water company's approval. The 2011 transfer of shared sewers applied to both.

What's different in Wales:

• Wales has its own Building Regulations. They're similar on drainage, but check the Welsh version and talk to your council's building control.
• The water company is different. Most of Wales is served by Dŵr Cymru Welsh Water. Parts of north-east and mid Wales are served by Hafren Dyfrdwy. Your water bill tells you which.
• Sustainable drainage rules. Wales has separate approval rules for sustainable drainage on larger schemes. Most house extensions are too small to be caught, but check with your council if yours is big.

Our guide to planning permission in Wales covers the other differences.

What it means for cost and timing

A sewer in the way doesn't usually stop an extension, but it can add cost: a survey, an application fee, deeper or special foundations, and possibly moving a manhole. None of that is unusual, but it's exactly the kind of thing that blows a budget if it turns up late. Our guide to avoiding unexpected costs covers the others worth checking.

It also matters at the end. Building control won't issue a completion certificate if work near a public sewer hasn't been approved, and that certificate is what a buyer's solicitor will ask for. See how long it takes to sign off an extension.

Check your home first

Use the planning checker to see what rules apply to your plot, and the local authority finder to find your building control team. Then ask your water company for the sewer map. Those three things together tell you most of what you need before anyone draws a line.

If you're still deciding whether an extension is worth it at all, the Build, Wait or Move framework is a good place to start.

Talk it through

If you've found a sewer where you wanted to build and you're not sure what it means, that's a good thing to bring to a free Build, Wait or Move Call. We'll look at where the pipe runs, what it's likely to mean for the design, and whether extending is still the right move. Sometimes it is. Sometimes it's better to wait or think again, and we'll tell you that too.