Build Over Agreement: Building an Extension Over a Drain or Sewer

A build over agreement is written consent from the water company to build over, or within 3 metres of, a public sewer. If your extension, conservatory or garage will sit near a pipe shared with neighbours, you will probably need one, and the water company will usually want to see a CCTV drain survey of the pipe before it agrees. Building Regulations Requirement H4 applies too, whoever owns the pipe.

This guide explains when you need an agreement, how Severn Trent and Anglian Water handle applications in our area, what the fees have been, and how to avoid a build going wrong at the foundations stage.

When do you need a build over agreement?

You need one when you plan to build over or within 3 metres of a public sewer. The 3 metre distance is measured from the centreline of the pipe, and it applies to extensions, conservatories, garages and outbuildings with foundations.

The pipe does not have to be under the road. Since the 2011 transfer of private sewers, most pipes serving more than one property became public sewers, even where they run through back gardens. Citizens Advice explains that a lateral drain might run under your property if you share a sewer with your neighbour, and that the water company is responsible for it.

A drain that serves only your house, inside your boundary, is usually a private drain. The water company's build over process may not apply to it, but Building Regulations still do.

What does Building Regulations Part H4 say about building over sewers?

Requirement H4 covers building over or near drains and sewers. Approved Document H says its guidance applies to the construction, extension or underpinning of a building "over or within 3m of the centreline of an existing drain, sewer or disposal main shown on the sewerage undertaker's sewer records whether that sewer is a public sewer or not".

The condition point is why a camera survey matters. You cannot show a pipe is not "cracked, fractured, deformed more than 5% or misaligned" without looking inside it.

How does Severn Trent's build over process work?

Severn Trent covers most of Nottinghamshire, Derbyshire and Leicestershire. Its building over or close to sewer pipes guidance offers two routes.

Self-certification is for simpler domestic jobs. Its self-certification form says it is for an extension to a single existing domestic property, over pipes of 150 mm diameter or less, that does not need connections or a manhole moved outside the extension. Its guidance document also expects no change in pipe diameter, direction, material or gradient under the building, no manholes within the footprint, and suitable foundations with clearance around the pipe.

A formal build over agreement is needed for larger sewers, work involving manholes and inspection chambers, and projects that don't meet the self-certification criteria. Severn Trent's guidance says the application needs a CCTV survey unless the pipe is being replaced, and a CCTV survey report after construction to show the sewer is still structurally acceptable. Severn Trent says it aims to respond within 10 working days of receiving the application and payment.

How does Anglian Water's build over process work?

Anglian Water covers most of Lincolnshire. Its building over or near our assets guidance uses standard criteria for any work over or within 3 metres of a public sewer. Its published criteria include that the sewer is less than 3 m deep and 150 mm diameter or less, no more than 6 m of it passes under the building, there are no manholes inside the building, the sewer is not pumped, and the pipe is not pitch fibre or asbestos cement.

• If your plans meet all the criteria, Anglian Water says you don't need to consult it.
• If they don't, you apply online, and an agreement is issued once the drawings are acceptable.
• Building over manholes "will not be permitted"; a manhole may sometimes be relocated at the applicant's cost.
• Where piles are within 1 metre of the sewer, before and after piling CCTV surveys are required, at your cost.

Our guide to pitch fibre pipes explains why that pipe material is treated differently.

How much does a build over agreement cost?

Fees are set by each water company and change from year to year, so always check the current fee on the company's own site before you apply.

On top of the water company fee, budget for the before survey, any after survey, and any drainage work the survey shows is needed. Our CCTV drain surveys start from £149 + VAT. See our guide to CCTV drain survey costs for what affects the price.

Why do you need a CCTV survey before and after building?

A before survey records the pipe's route, depth, diameter, material and condition. It tells you whether the pipe is a private drain or a shared sewer, whether it meets the self-certification or standard criteria, and whether any defects need repairing before it disappears under a floor slab.

An after survey proves the build did not damage the pipe. Without one, a crack found years later is hard to pin on anyone, and repairing a pipe under an extension costs far more than repairing it in open ground. Our guide to reading a CCTV drain survey report explains the defect codes water companies look for.

Build over checklist for extensions

Work through this before your architect finalises the drawings:

1. Find the pipes. Check the sewer records and look for inspection chambers near the footprint.
2. Work out who owns them. Private drain, shared sewer or public sewer.
3. Measure the distance. Anything within 3 m of the pipe's centreline is in scope.
4. Survey the pipe. Record route, depth, diameter, material and condition, coded to MSCC5.
5. Check the criteria. Severn Trent self-certification, or Anglian Water standard criteria.
6. Deal with chambers in the footprint. Relocation needs the water company's agreement first.
7. Design the foundations. Approved Document H (para 1.12) says that where the pipe is more than 2 m deep and passes beneath the foundations, they should be designed as a lintel spanning at least 1.5 m either side of it.
8. Apply and wait for consent before breaking ground.
9. Protect the pipe during the build. No heavy plant or stored materials over it.
10. Resurvey afterwards where the water company asks for it, and keep the report for when you sell.

Our guide to a drain survey when buying a house covers what to ask if the home you're buying already has an extension over a sewer.

What if the pipe is in poor condition or in the way?

If a survey shows the pipe is cracked or displaced, it usually has to be repaired or replaced before building over it. For a public sewer that decision sits with the water company. If a manhole sits in the footprint, the options are usually to relocate it, with agreement, or redesign the extension. Our guide to drain relining vs excavation compares repair methods.

When to call a professional

Get the drains surveyed as soon as you know roughly where the extension will go. It is far cheaper to adjust a drawing than to stop work with the foundations open. Our CCTV drain surveys start from £149 + VAT and include MSCC5-coded condition grading, a written report, footage and drain mapping, which is the evidence water companies and building control ask for. If a chamber needs moving or a drain diverting, our groundworks team (from £500 + VAT) can quote for the work. The final price depends on the job, and you get a fixed quote first.

We cover Nottingham, Nottinghamshire, Derbyshire, Leicestershire and Lincolnshire, including CCTV drain surveys in Nottingham. Call 07300 600 950 or ask for a quote.

Sources

• Approved Document H: Drainage and waste disposal (gov.uk)
• Severn Trent: building over or close to our sewer pipes
• Severn Trent: building over or close to a public sewer guidance (PDF)
• Severn Trent: self-certification application form (PDF)
• Anglian Water: building over or near our assets
• Citizens Advice: who is responsible for repairing drains and sewers
• The Water Industry (Schemes for Adoption of Private Sewers) Regulations 2011 (legislation.gov.uk)