AVA.AVA Home Design
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Build Over Agreements

A build over agreement is written consent from your sewerage undertaker to build over, or within three metres of, a public sewer or lateral drain. AVA Home Design establishes early whether your project needs one, prepares the location plan, drainage layout and foundation sections the application requires, and submits it alongside your building regulations application.

This is the requirement that most often surprises people halfway through a project. Approved Document H of the Building Regulations 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 records — and it applies whether that sewer is public or not. Building control will not sign the work off without the undertaker’s consent, so it is far cheaper to find out at design stage than after the foundations are dug.

A common misunderstanding is that the drain running down the side of your garden is yours. Since the 2011 private sewer adoption scheme, a great many drains that homeowners still think of as private are lateral drains or public sewers, maintained by the undertaker. In our patch that is usually Thames Water. The only reliable way to know is to check the sewer records, which is where we start.

We handle the drawing work and the application: the location plan, the existing and proposed drainage layout, and the sectional drawing showing your proposed foundations against the sewer. Where the design needs to change to win consent — moving a foundation, deepening it, bridging the pipe — we would rather redesign it at the drawing stage than have you find out on site.

Thames Water application classes

Class 1
Sewers with an internal diameter of 175mm or less. Thames Water’s published fee is £130, and the agreement is usually issued within 21 days of the technical review.
Class 2
Sewers from 176mm to 350mm. The published fee is £490. Usually issued within 21 days, though longer if the survey shows the sewer needs repair first.
Class 3
Sewers over 350mm. The published fee is £585 plus legal costs starting at £1,000, and Thames Water advise these can take more than three to six months — worth knowing before you set a start date.

Who this is for

  • Rear, side or wraparound extensions where the drainage runs behind or beside the house.
  • Any project where the foundations will go deeper than the invert of a nearby drain.
  • Garage conversions and outbuildings being built up to a boundary with a shared drain.
  • New build plots where the existing site drainage crosses the footprint.
  • Homeowners who have been told by building control that consent is outstanding.

Frequently asked questions

Do I need a build over agreement for my extension?
If any part of the work is over, or within 3m of the centreline of, a drain or sewer on the undertaker’s sewer records, then Approved Document H applies and you need the owner’s consent. We check the sewer records at the start of the project so this is settled before the drawings are finished, not after.
How much does a build over agreement cost?
Thames Water publish their fees by sewer size: £130 for Class 1 (175mm or less), £490 for Class 2 (176mm to 350mm), and £585 plus legal costs from £1,000 for Class 3 (over 350mm). A reduction of £130 is available for applicants on qualifying disability benefits. Other undertakers set their own fees, and a CCTV survey of the drain is usually an additional cost.
How long does a build over agreement take?
Thames Water carry out their technical review within 21 days of the application, and Class 1 and Class 2 agreements are usually issued in that time. Class 3 applications, on the largest sewers, can take more than three to six months. Build the timescale into your programme rather than treating it as a formality at the end.
Can I build over a manhole or inspection chamber?
No. Approved Document H is clear that buildings and extensions should not be constructed over a manhole, inspection chamber or other access fitting on a sewer serving more than one property, unless an alternative access point can be provided on the line of the sewer at a location the undertaker accepts. Where we find a chamber in the footprint, we look at diverting the run or relocating the access.
Is there a limit on how much sewer I can build over?
Yes. The length of drain or sewer running under a building should not exceed 6m without the owner’s permission. Approved Document H also says buildings should not be built over or within 3m of any drain or sewer more than 3m deep, or greater than 225mm in diameter, except with the owner’s permission.
What happens to the foundations where a drain runs underneath?
It depends on depth. Where a drain under a building is less than 2m deep, the foundation is extended locally so the drain passes through the wall. Where it is more than 2m deep to invert and passes beneath the foundations, the foundation is designed as a lintel spanning the line of the pipe, extending at least 1.5m either side, so no load is transmitted onto it. At least 100mm of granular or other flexible filling goes around a pipe running under a building.
Does permitted development mean I can skip it?
No. Build over consent is a drainage matter under the Building Regulations and the undertaker’s own rules, entirely separate from planning permission. A project can be permitted development, needing no planning application at all, and still require a build over agreement.
What if the sewer is in poor condition?
That changes the engineering. Approved Document H flags rising mains, brick or masonry sewers, and sewers in poor condition — cracked, fractured, deformed more than 5% or misaligned — as high risk in the event of failure, and requires special measures. It is one of the reasons the undertaker may ask for a CCTV survey before agreeing, and occasionally why a repair has to happen before your build can.

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