The short answer
Yes, you can usually divert a drain that runs under your planned extension, but whether you may do it freely depends on who owns it. A private drain serving only your property can generally be re-routed as part of the groundworks, provided the new line meets Part H (correct fall, access, bedding). A public sewer, or a drain shared with neighbours, involves the water and sewerage company: you normally need their agreement to divert it, or you keep it where it is and apply for a build-over agreement instead. Diversion is real groundworks — excavating, laying a new run to the right gradient, connecting it and reinstating — so it typically costs from around £1,500 to £5,000+ depending on length, depth and whether the water company is involved. The first step is always to establish what the drain is and who owns it.
Finding a drain crossing your extension footprint is common and not a dead end. The question is whether you move it or build over it, and that turns on ownership.
Diverting a drain
- Private drainUsually divertible, meet Part H
- Public sewerWater company consent / build-over
- Typical cost~£1,500–£5,000+
- AlternativeBuild-over agreement
- First stepIdentify owner via survey/records
Divert or build over?
When a drain crosses your footprint you have two main options:
- Divert it — re-route the drain around the extension. Cleaner long-term, keeps the pipe outside the building, but means new groundworks and (for a public sewer) water company agreement.
- Build over it — leave the drain where it is and build the extension over it, under a build-over agreement with design measures to protect the pipe and keep access. Often cheaper than diverting, but ties a drain under your structure.
Which is better depends on the drain's depth, position, ownership and the water company's stance. Sometimes diversion is the only acceptable option; sometimes building over is.
Who owns the drain decides the rules
Establish ownership before anything else.
| Drain type | Can you divert it? | Process |
|---|---|---|
| Private drain (your property only) | Generally yes | Re-route to Part H, Building Control |
| Shared / public sewer | Only with consent | Agree diversion with water company |
| Keep public sewer in place | — | Build-over agreement instead |
Indicative; confirm ownership with sewer records and a survey. Source: GOV.UK build-over / diversion guidance.
What diversion involves and costs
Diverting a drain is a proper groundworks job: locating and exposing the existing run, excavating the new route, laying pipe at the correct gradient with bedding and surround, providing access (a chamber or rodding point where needed), making the connections, testing, and reinstating. Cost is driven by the length and depth of the new run, ground conditions, and whether a public sewer means water company involvement and inspection. Expect roughly £1,500–£5,000+, with deep or long diversions, or those crossing other obstacles, costing more. The new drainage must satisfy Part H, and Building Control will inspect and test it before it is covered. If a public sewer is diverted, the water company's own approval and inspection apply on top.
The gradient trap and getting it right first time
The hidden difficulty in diverting a drain is levels. A foul drain has to keep falling towards its destination at a steady gradient, so when you re-route it around an extension you are not just moving it sideways — you have to keep the new, longer run within an acceptable fall while still arriving at the connection point at the right depth. Make the diversion too long or too flat and solids settle and block; too steep and the liquid outruns the solids. On a tight plot, or where the connection point is shallow, there may be very little room to play with, which is why a diversion that looks simple on a plan can be awkward on site.
- Survey before you commit: establish the invert levels (the pipe bottom) at the start and end of the existing run, so the diversion can be designed to a workable fall before any digging.
- Keep access: changes of direction need a chamber or rodding access under Part H, so a diversion with bends usually adds at least one access point.
- Mind other foundations and services: the new route must avoid clashing with the extension's own foundations and any water, gas or electricity runs, which can dictate the line and depth.
Because of the gradient constraint, diversion is sometimes ruled out and building over the drain under a build-over agreement becomes the sensible route instead — particularly where there is no room to re-route at an acceptable fall. The decision is best made with the existing levels known and, for any public sewer, in discussion with the water company. Get the survey and the strategy done before the groundworks are priced, and a drain crossing your extension becomes a planned task rather than a mid-build problem dug up by surprise.
Manholes in the footprint and protecting a drain you keep
Even when you decide not to divert, a drain crossing your extension brings its own requirements, and one situation in particular catches people out: a manhole or inspection chamber sitting inside the proposed footprint. You cannot simply build over an open chamber and lose access to it.
- Relocate or rebuild the chamber: the access point may need to be moved out of the footprint, or rebuilt within it with a sealed, accessible double-sealed cover so the drain can still be rodded and maintained.
- Keep access overall: Part H and any build-over agreement require the drain to remain accessible for clearing blockages, so the design has to preserve a way in even where the pipe is built over.
Where a drain you are keeping passes under or close to the new foundations, it must be protected from the building's movement and load. Common measures are a lintel bridging the pipe so foundation loads do not bear on it, a small gap or compressible filler around the pipe where it passes through a wall, and flexible joints either side so minor settlement does not crack a rigid pipe. On a public sewer, the build-over agreement usually specifies these protections explicitly, and the foundations may have to be designed to bridge the sewer entirely.
The decision between diverting and building over therefore comes down to the same early groundwork: identify the drain, confirm its ownership and levels, and find any chambers in the footprint before the design is fixed. With that known, you can weigh a clean diversion against building over with protection, and bring the water company in where a public sewer is involved — rather than uncovering a chamber mid-dig and having to redesign around it under time pressure.
Frequently asked questions
Do I need permission to divert a drain?
For a private drain serving only your property, you generally don't need the water company's permission, but the new run must meet Part H and be inspected by Building Control. For a public or shared sewer, you need the water company's agreement.
Is it cheaper to divert a drain or build over it?
Building over with a build-over agreement is often cheaper than diverting, since you avoid re-laying the run. But diversion keeps the pipe outside your structure. The drain's depth, ownership and the water company's view decide which is sensible.
How do I find out who owns the drain?
Check the title deeds, look for shared manholes, and ask the water company for sewer records. A CCTV survey traces the line. A drain serving more than one property is commonly a public sewer.
Sources & further reading
- GOV.UK — building over or near a public sewer
- Planning Portal — Approved Document H (drainage)
- Checkatrade — drain diversion and drainage costs
Figures on this page are typical UK ranges drawn from published sources and depend on your specific site. They are guidance, not a quotation.