Building Over a Sewer: Build-Over Agreements

Written by the Farnborough Contracting Services team.
over 20 years on the drains and ground of north Hampshire and west Surrey.

Of all the ways a domestic extension goes wrong, this is the one that costs the most and is the easiest to avoid. There is a pipe under the footprint. Nobody checked. The footings are dug, or worse, concreted, and then somebody notices the chamber that used to be in the middle of the lawn.

The rule of thumb

If you are building over, or within roughly three metres of, a public sewer or a lateral drain, you normally need a build-over or build-near agreement from your water company before you start. And since the 2011 transfer, a great many pipes that everyone assumes are private are now public.

Why this catches so many people out

Two reasons, and they compound each other.

The first is the 2011 transfer. Private sewers and lateral drains connected to the public system became the water company's property. That means the run crossing your garden, which your neighbour also uses, and which everyone has always treated as a private matter between the two of you, may well be a public sewer now. Building over a private drain that serves only you is a building control matter. Building over a public sewer is a water company matter, with a formal agreement and a fee.

The second is that the maps are incomplete. Your water company must make sewer records available on request, and it will. But the pipes that transferred in 2011 were transferred without plans, because no comprehensive mapping of private sewers and lateral drains existed. Owners received notices with no plans attached. So a clean sewer map is not proof that there is nothing under your extension. It only proves that nothing is recorded there.

Finding out what is actually there

In the right order, cheapest first.

Ask for the sewer map. Free or nearly so, and it tells you about recorded public sewers. Treat an empty map as inconclusive rather than as an all clear.
Lift every cover on the property and work out the direction of flow. Two chambers and a direction gets you most of the way to knowing where the run goes.
Camera and trace it. A camera with a sonde in the head, and a locator above ground, gives you the line and the depth from the surface, and we can mark it out on the ground in paint. This is the only method that tells you what is under the footprint rather than what is recorded as being under it. On any extension where a chamber is anywhere near the footprint, it is worth doing before the drawings are finalised, not after.

What Part H actually requires

Building over existing sewers is dealt with in Approved Document H, the drainage and waste disposal part of the Building Regulations, in the 2015 edition which came into force on 1 October 2015. H4 covers building over existing sewers specifically, and its concerns are practical rather than bureaucratic:

Risk from failure of the sewer. If the pipe fails under your new floor, what happens to the building, and can it be got at?
Maintaining access. Somebody has to be able to rod, jet and inspect the run in future. Burying an inspection chamber under a solid floor with no access is not acceptable.
Protection during construction. A pipe survives being built over. It often does not survive a machine bucket or a load of hardcore tipped on it.
Preventing settlement damage. The new structure must not transfer its load onto the pipe, which is what lintels and bridging details are for.

The other parts of H are worth knowing exist if you are doing anything with drainage: H1 foul drainage, H2 wastewater treatment and cesspools, H3 rainwater drainage, H5 separate systems and H6 solid waste storage.

The engineering options, and roughly what each involves

Option	When it works	What it involves
Divert the run	Where there is space outside the footprint and the levels allow	New pipework laid around the extension, new chamber, old run capped. Often the cleanest answer, and it removes the problem permanently.
Bridge it with a lintel	Where the pipe crosses the line of a wall	The footing is bridged over the pipe so the load bears either side rather than on the pipe. Usually needs the engineer's detail.
Deepen the footing	Where the pipe is shallow and the footing can go below it	Footing taken down past the invert so the load is transferred below the pipe. Cost rises quickly with depth.
Rebuild the chamber with access	Where a chamber ends up inside the footprint and cannot be moved	Chamber rebuilt, often with a double sealed cover inside the building, so access is retained. Acceptable, but always the last choice.
Encase or replace the pipe	Where the existing pipe is old and would be hard to reach later	Replace the length under the footprint in modern pipe, sometimes concrete encased, before building over it.

What happens if you build over without an agreement

Nothing, for a while. Then one of three things.

The water company finds out and requires the work to be altered, or in the worst case removed. Or your building control sign-off runs into trouble because the drainage element cannot be certified. Or, most commonly, it surfaces years later when you sell, a solicitor raises it, and your buyer's lender wants an indemnity or a retrospective agreement before completion, at the worst possible moment in a chain.

The cost of applying before you start is a fee and a few weeks in the programme. The cost of sorting it out afterwards is measured in cancelled trades and, occasionally, in concrete coming back out.

How we handle it

On any groundworks job where there is a chamber anywhere near the footprint, we camera and trace the run before we set out, and we mark the line and depth on the ground. If it turns out to be a public sewer or a lateral drain, we tell you what consent is needed, we design the drainage around it, and we programme the work so the pipe is protected while we are on site. It is an hour or two at the start of the job that regularly saves weeks.

If you have drawings and there is a chamber on the site plan, send them over. We will tell you whether you have a problem before anybody digs.

Questions people ask

Do I need permission to build over a drain?

If the pipe is a public sewer or a lateral drain, which since 2011 includes many runs that used to be private, you normally need a build-over or build-near agreement from your water company before you start. Part H of the Building Regulations covers building over existing sewers, including maintaining access and protecting the pipe from settlement.

How do I find out if there is a sewer under my garden?

Ask your water company for a sewer map, which they are required to make available. Maps are incomplete for pipes that transferred in 2011, because owners were never given plans, so the reliable method is a CCTV survey with a sonde traced from the surface. That gives you the line and the depth.

What happens if I build over a sewer without an agreement?

The water company can require the work to be altered or removed, it can create problems on sale because a solicitor will raise it, and it can invalidate parts of a building control sign-off. It is far cheaper to locate the pipe and apply before the footings are set out.

Can an extension be built over a drain at all?

Usually yes, with the right design. Options include diverting the run outside the footprint, bridging the pipe with a lintel so the load does not bear on it, or rebuilding a chamber with access maintained. What is not acceptable is burying an inspection chamber under a floor with no access to it.

Where this comes from

The rules, fees and figures on this page are taken from the sources below. Fees change, so check the current figure before you send anyone money.

This is general information about how the rules work, not advice on your particular property. If something here matters to a purchase or a dispute, get it checked on your own facts.

The drain under your extension: what a build-over agreement is and when you need one