No — not without the right approvals. Whether you can build over a manhole depends on who owns the drain underneath it. If it sits on a public sewer, you’ll usually need a water company build-over agreement, and in many cases the manhole will have to be relocated outside the extension’s footprint rather than built over and sealed. If it serves only your own property as a private drain, you have more flexibility, but Building Regulations Part H still requires the chamber to stay accessible for maintenance and rodding. Simply covering, burying or boxing in a manhole without agreement is not an acceptable fix — it can hold up building control sign-off, cause problems at sale, and leave you without emergency drainage access.
When Building Over a Manhole Is — and Isn’t — Allowed
The short answer is: not without checking first, and in many cases not at all without formal agreement. Whether an extension can go over a manhole comes down to one question — who owns the drain running beneath it?
If the manhole sits on a public sewer (one that serves more than one property), you’ll typically need a build-over agreement with the local water company, and many water companies won’t allow you to build directly over a chamber at all. Instead, they’ll ask for the manhole to be relocated outside the extension’s footprint, or for the sewer run to be diverted.
If the manhole covers a private drain serving only your own house, you have more scope to build over it — but you still need to meet Building Regulations, keep the drain accessible for maintenance, and get sign-off from building control.
Either way, “building over” doesn’t mean hiding the problem under a new floor slab. It means proving, with proper documentation, that the drain can still be inspected, maintained and repaired once your extension is up.
Why Drainage Access Is Protected by Law
Manholes — or inspection chambers, to use the more accurate term for the smaller ones — aren’t just covers in the lawn. They’re access points that let drainage engineers get a rod, camera or pump down into the pipe network to clear blockages, carry out repairs or investigate a collapse. Build over one without proper provision and you lose that access, which is exactly why it’s controlled.
Building Regulations and the duty to keep drains accessible
Part H of the Building Regulations (Drainage and Waste Disposal) sets out the requirement, generally referred to as H4, that any proposal to build over or near a drain must still allow reasonable access for maintenance. Building control will expect to see this addressed in your drainage plans and application, whether the drain in question is public or private.
Who owns the sewer under your garden?
Ownership matters because it decides whose permission you need. Most sewers running beneath a public road, or connecting more than one property, were transferred into water company ownership some years ago. That means Thames Water, Anglian Water, Severn Trent, Yorkshire Water or whichever company covers your area has a legal duty to maintain that pipe — and a legal right to get at it, even once it sits on your property.
Private Drain or Public Sewer? Why the Distinction Matters
This is the single most important thing to establish before you design your extension.
A private drain serves one property only, running from the house to the point where it joins a public sewer or another drain — usually at the first inspection chamber or manhole. Up to that point, it’s your responsibility to maintain, and you generally don’t need the water company’s permission to build over it, though you still need to satisfy building control that it remains accessible and protected.
A public sewer serves more than one property, or has been formally adopted by the water company. If your manhole sits on one of these, or your extension will be built within roughly three metres of one (the exact distance depends on depth and the individual water company’s criteria), you’ll usually need to apply for a build-over, or build-near, consent before work starts.
Some water companies offer a simplified self-certification route for smaller sewers — some set the limit around 150mm (6 inches) — provided no manhole needs to be relocated. Where a manhole falls within the footprint of the extension, though, most won’t allow self-certification; you’ll need a full build-over application, which typically involves drainage drawings, a CCTV survey of the pipe and sometimes structural details showing how the foundations will bridge or avoid the sewer.
If you’re not sure whether a manhole on your plot is public or private, don’t guess. Your local water company can check its records, or a drainage contractor can trace the run for you.
Your Options If a Manhole Is in the Way
Finding a manhole in the footprint of a planned extension isn’t the end of the project — it just means building a solution in from the design stage rather than discovering the problem on site.
Relocate the manhole. Where it’s a private drain, this is often the simplest fix: a contractor moves the chamber to a new position just outside the extension wall and reroutes the pipework. On a public sewer, relocation needs the water company’s agreement and, usually, their own or an approved contractor to carry out the diversion.
Create a new external access point. Rather than moving the whole chamber, it’s sometimes possible to install a new inspection chamber or rodding eye outside the building line, while the original run beneath the extension is covered by a build-over agreement.
Use an internal access cover, where permitted. In some cases, water companies will allow a manhole to remain within the extension if it’s fitted with a sealed, double-sealed access cover in a suitable room — though this is the exception rather than the rule, and many won’t permit it over a public sewer at all.
Redesign the extension. Sometimes the cleanest answer is adjusting the footprint, footing design or layout so the drain run sits outside the new structure altogether, avoiding the need for an agreement in the first place.
Which of these applies to you depends on the sewer’s depth, condition, diameter and ownership, so it’s worth getting advice before you commit to a design.
Why You Can’t Just Cover, Bury or Seal It
It’s tempting to think a manhole is just a cosmetic problem — lay a floor over it, box it in, or bury it under hardcore, and it disappears. It doesn’t, and treating it that way causes real problems.
A buried or inaccessible manhole can no longer be opened for a CCTV survey, a rod-and-clear, or an emergency repair. If a blockage or collapse happens under your new extension, engineers may have to break through the new floor to reach it — a far more expensive and disruptive job than if the chamber had been relocated or protected properly in the first place.
It also creates a legal and practical problem at sale. A buyer’s solicitor will typically ask for drainage searches and, if a public sewer runs under the building, evidence of a build-over agreement. Without one, you may struggle to sell, or the buyer’s mortgage lender may raise concerns.
And if building control or the water company find, after the fact, that a manhole has been built over without consent, you could be required to expose it again, carry out remedial work, or in the worst cases apply retrospectively and hope for approval — which isn’t guaranteed.
Steps to Take Before You Start Building
Sort the drainage question out before the extension design is finalised, not after.
Find your drainage plans. Ask your water company for the sewer records for your property, and dig out any CCTV survey or drainage plan from when the house was built or previously extended. This tells you exactly where pipes and chambers run, and whether they’re public or private.
Contact the water company early. If there’s any chance a public sewer passes near or under the extension, get in touch with the relevant water company’s developer services or building-over team before you apply for planning permission, not after. Early contact gives you time to redesign if needed, rather than discovering a problem once foundations are being dug.
Bring in a drainage professional. A reputable drainage contractor, such as Fast Drains, can run a CCTV survey of the pipework, confirm the depth, condition and ownership of the drain, and put together the plans and documentation most water companies ask for as part of a build-over application.
Loop in building control. Whether you’re using full plans or the building notice route, make sure your building control surveyor — council or an approved inspector — knows about the manhole and how you intend to deal with it. They’ll want it resolved before signing off the build.
Keep your paperwork. Save copies of your build-over agreement, drainage plans and any correspondence with the water company. You, or a future buyer, will likely need to produce this at some point.
Risks, Delays and Mistakes Worth Avoiding
Most of the problems homeowners run into with manholes and extensions are avoidable, and they tend to follow a pattern.
Leaving it until planning permission is granted. Planning permission doesn’t cover drainage approval — they’re separate processes. Waiting until after planning to think about the manhole often means a redesign at a stage when it’s expensive and disruptive to change.
Assuming a drain is private because it’s on your land. Ownership isn’t about whose garden the pipe runs through; it’s about whether the sewer serves other properties too. A manhole in your own garden can still sit on a public sewer.
Underestimating how long approval takes. Build-over applications can take anywhere from a few weeks to a few months, depending on the water company, the complexity of the site and how quickly you can supply the information they ask for. Build this into your programme rather than assuming it’ll be quick.
Trying to hide the manhole instead of resolving it. As covered above, covering or burying it doesn’t make the requirement go away — it just defers the problem to a worse moment.
Not keeping access clear during construction. Even once an agreement is in place, contractors sometimes stack materials, pour concrete or route scaffolding over a manhole during the build itself, which can breach the terms of the agreement.
Build in time for the drainage side of the project from the outset, and most of these issues simply don’t arise.
Frequently Asked Questions
Can you build an extension over a manhole without permission?
No. If the manhole sits on a public sewer, you’ll need the water company’s agreement — usually a formal build-over agreement — before building work starts, and many won’t allow the chamber to be built over at all. If it’s on a private drain, you have more freedom, but building control still needs to see that access is maintained.
What’s the difference between a private drain and a public sewer?
A private drain serves a single property, running from the house to the point it joins another drain or the public sewer. A public sewer serves more than one property and is owned and maintained by the local water company, which has a legal right of access to it — even where it runs through your garden.
Can a manhole be relocated to make way for an extension?
Often, yes. On a private drain, this is usually straightforward: a contractor repositions the chamber and reroutes the pipework. On a public sewer, relocation needs the water company’s agreement and is typically carried out by them or an approved contractor, as part of the build-over process.
What happens if you build over a public sewer without an agreement?
You risk enforcement action, including being asked to expose and remediate the work, and difficulties at sale, since conveyancing searches usually ask whether a build-over agreement is in place for any sewer beneath the property. Building control can also refuse to sign off the work.
Do you still need permission to build over a private drain?
You don’t normally need the water company’s agreement for a private drain, since it isn’t their asset. You do still need to satisfy building control, under Building Regulations Part H, that the drain remains accessible, protected and able to be maintained.