Do I Need a Build Over Agreement?

Yes, if any part of the extension will sit over a public sewer or a lateral drain, or within three metres of one. The agreement is with the water and sewerage company, which across Colchester and north Essex is Anglian Water, and it is nothing to do with the council, your neighbours or the planning system.

It is not planning permission and it is not Building Regulations approval either. It is a third consent, on its own timescale, and it is the one people find out about last.

The pipe under your lawn may not be yours any more

This is the fact the whole question turns on, and it changed in 2011.

Four words get used loosely and mean different things. A drain serves one building. A private sewer serves more than one and used to be the shared responsibility of everybody connected to it. A lateral drain is the length of your own drain beyond your property boundary. A public sewer is vested in the water company, which maintains it and controls what happens over it.

On 1 October 2011 most existing private sewers and lateral drains connected to the public network transferred to the sewerage companies. Private pumping stations followed by 2016. The effect is that an enormous number of ordinary back gardens now have a public sewer running under them, put there in the 1930s to serve a row of houses, with an owner who has never been near the place and a say in what you build on top of it.

Nothing visible changed. The covers are the same covers. What changed is who has to be asked.

Finding out what is under there

Ask the sewerage undertaker for a sewer map or an asset location search for the address. That shows what is recorded as public. Then lift the covers, because the map is a record of the network rather than a survey of your garden, and private runs do not appear on it at all.

The giveaway on a suburban plot is a run that takes flow from more than your own house, or one that heads off across the boundary rather than towards the front. On a terrace or an interwar semi, a shared run down the back of several gardens is close to standard, and it is very often public now.

Where the requirement actually comes from

Two separate regimes touch this, and neither can do the other’s job.

The Building Regulations deal with building over drains and sewers under Part H, and the requirement applies to work over or near a drain or sewer shown on the map of public sewers. Where it applies, the building control body consults the sewerage undertaker, and on the local authority route plans cannot be passed and a completion certificate cannot be issued until fifteen days have run from that consultation, unless the undertaker replies sooner.

So the two are connected, and they are still not the same thing. Building control cannot grant you a build over agreement, and the undertaker cannot sign off your extension. You need both, from two organisations, and the one nobody applies for early is the water company’s. Planning, for its part, takes no view whatever: an extension can be permitted development and still be sitting on a public sewer.

When you do not need one

Where the only pipe affected is a drain serving your house alone, staying inside your boundary, and no public sewer runs within three metres of the new work. That covers a good number of detached plots and most front porches.

It does not let you ignore the drain. Part H still applies to anything built over or beside it, an inspection chamber inside the new footprint still cannot simply be concreted over, and building control will still want to see how the pipe has been protected. What you are avoiding is the separate application, not the engineering.

The route that avoids a formal agreement

Companies publish standard criteria, and work that meets all of them can often proceed without a full application. Read Anglian Water’s current published set rather than a summary of it, because the company’s own thresholds are what it will actually apply. Several of the limits people quote as the water company’s are in fact Approved Document H: no more than six metres of pipe running under the building, and nothing built over an access fitting on a sewer serving more than one property, each except with the permission of the owner. The Approved Document also sets the point at which permission is needed at all, which is a drain or sewer more than three metres deep or greater than 225mm in diameter, or any work within three metres of one. A pumped rising main is treated differently again and wants three metres clear either side of it.

Fail any single criterion and you apply. These criteria get revised, so read the current published version rather than a summary of it, including this one.

What an application wants from you

• A site plan showing the run of the pipe, the footprint of the extension and the distance between them.
• A foundation detail showing how the load is kept off the pipe, which usually means bridging it or founding below it.
• A survey of the pipe’s condition, commonly a camera survey, because nobody wants a defective sewer sealed under a kitchen.
• An answer on access: how the pipe gets repaired in twenty years’ time without coming through your floor.

The engineering answers behind those, and the cases where the answer is a diversion instead, are covered separately in can I build an extension over a sewer.

Where it belongs in the programme

Early, and before the drawings are finished rather than after. The application takes weeks rather than days, and it can come back asking for a deeper foundation, a lintel over the pipe or a diversion, all of which change the design and the price. Applying once the slab is designed means doing that part twice.

The fee is the water company’s own and sits outside the building control charge entirely, which is worth remembering when you are reconciling what building control costs against your budget.

What skipping it costs

The undertaker keeps its statutory powers over its own apparatus whether you asked or not, and those include requiring alteration or removal of what has been built over it, at your expense. Because building control consults them on work caught by Part H, the omission tends to surface during the build rather than quietly.

The slower version is worse. It appears years later on a sale, in the enquiries, with a buyer’s solicitor wanting the agreement you never obtained. And if the sewer blocks under your extension in the meantime, the cost of getting to it is a conversation you will be having on your own.

The practical answer

Get the sewer map and lift the covers before the design is settled. If a public sewer or lateral drain is under the footprint or within three metres of it, an agreement or a self assessment against the published criteria is required, and it is a separate application to the water company. Everything else about the drainage still has to satisfy Part H, and there is more on both in our guide to build over agreements and extension drainage.

We do the drainage check at survey stage, apply for the agreement where the runs require it, and get the answer back before the foundation is designed rather than after it is dug. The survey and the quotation cost nothing.

Published criteria and fees are revised periodically. Treat the structure described here as reliable and confirm the detail against the undertaker’s current requirements for your address.

Last reviewed August 2026.