A drain survey for a new build or extension is no longer something to leave until after drawings are approved. In 2026, the real risk is not just a blocked pipe. It is discovering too late that a public sewer crosses the footprint, a manhole cannot stay where it is, surface water cannot discharge the way you assumed, or building control needs a different approval route.
Need to confirm the drainage layout before you commit to drawings? Start with CCTV drain surveys and, where the layout conflicts with the proposal, review options for moving drains.
When a CCTV drain survey is needed for an extension or new build
A CCTV drain survey is most useful when the drainage route is unclear, the site includes older pipework, the extension sits near a manhole, or the proposal could pass over a public sewer or lateral drain. The survey should do more than film the pipe. It should map layout, ownership, depth, flow direction, diameter and condition, because those details shape both the drawing package and the approval route.
For London and Kent sites, this matters even more. Kent has many older Victorian buildings with traditional drainage systems, which increases the chance of clay pipes, displacement and root-related issues appearing before work starts.
That is exactly why we favour early investigation rather than redesign after foundations have already been priced. We also carry out surveys to WinCan standards and deliver reports with footage and findings through our app, which makes them easier to share with designers and contractors.
How a build over agreement can change your design
A build over agreement is usually the key issue when an extension approaches public drainage. An application is likely if you are building within 3 metres of a public sewer or within 1 metre of a public lateral drain. Also, if you are building a new property, the sewer must be diverted rather than handled through a standard build over agreement. The sewerage undertaker should be consulted where a building is proposed over or near a mapped drain or sewer.
That can change a scheme in several ways. A manhole serving more than one property cannot simply disappear into the footprint. A poor-condition sewer, a brick sewer, a deep sewer, or a larger diameter pipe can trigger extra protection measures or permission requirements. In short, the survey findings affects whether the design stays as drawn, needs protection details, or needs the drainage rerouted before the build proceeds.
| Site condition | Likely consequence |
| Public sewer within 3m of extension | Build over approval route may apply |
| Public lateral drain within 1m | Water company approval may apply |
| Manhole serving more than one property | Build-over layout often needs redesign |
| New build over public sewer | Diversion is often required, not a simple build-over route |
| Sewer in poor condition | Repair or redesign may be needed before approval |
The cheapest time to solve a drainage problem is before the drawings are fixed and before the foundations are priced.
What Building Regulations Part H and building control expect
The planning conversation is only part of the picture. Building regulations Part H drainage guidance covers building over sewers, surface water drainage, separate systems and access for maintenance. It states that surface water should discharge to a soakaway or other infiltration system where practicable. Buildings or extensions should not be constructed over a manhole or inspection chamber on a sewer serving more than one property.
One detail many homeowners miss is the building control application route. The current England Building Notice form asks whether there is a public sewer within 3 metres of the proposed extension, and says that if the answer is yes, you must apply using the Full Plans form instead. That is an important practical point because it affects how early the drainage information needs to be ready. Ongoing asset care also matters, which is why projects with ageing or vulnerable systems should also factor in drain maintenance once the build is complete.
When moving drains is the better option
There are projects where protecting the existing sewer is not the best answer. If the extension covers the only sensible access point, crosses a poor-condition pipe, or leaves no practical route for maintenance, moving the drain can be the better option than forcing the design around it. A satisfactory diversionary route should be available so the drain or sewer can be reconstructed without affecting the building.
Why surface water and soakaway design matter on new builds
On new builds especially, foul drainage is only half the issue. Surface water design is often what decides whether the layout is efficient and compliant. Surface water should discharge to a soakaway or other infiltration system where practicable, before discharge to a sewer is considered. It also supports separate foul and rainwater drainage systems where required. With rising surface water flood risk, that drainage hierarchy is becoming increasingly important.
Where infiltration is suitable, soakaway installation should be planned early, not added at the end as a workaround. This is especially important where site levels, paved areas or hard landscaping could push water back towards the property. A drainage plan that deals properly with both foul and rainwater is more robust than one that only solves the sewer line under the extension.
What a usable drainage report should include
The report needs to be usable by more than one reader. It should show the drainage layout, pipe sizes, ownership, condition, access points and recommendations in a format that your architect, builder, building control officer and water company can all understand.
Get drainage certainty before work starts
Drainage problems delay projects because they are discovered too late. The better route is simple: survey first, confirm ownership and condition, choose the right approval path, and only then lock in the design. That applies whether you are planning a rear extension in London, a self-build plot in Kent, or a remodel that changes the drainage line entirely. The rules are manageable when the evidence is clear.
If you need a survey that can support design decisions and regulatory checks, speak to us. Call us on 0800 654 6065 or email info@sedrainage.co.uk.
Frequently Asked Questions
Do I need a drain survey for a house extension in the UK?
Usually, yes when the extension is near a drain, manhole, sewer or unclear drainage route. Pre-construction survey gives architects critical information before design is completed.
Do I need a build over agreement if my extension is near a public sewer?
Often, yes. Approval is likely if you are building within 3 metres of a public sewer or within 1 metre of a public lateral drain.
Can I build over a manhole?
Buildings and extensions should not usually be constructed over a manhole or inspection chamber on a sewer serving more than one property.
Is planning permission the only approval I need?
No. Drainage can also affect building control and water company approvals. If the work is within 3 metres of a public sewer, the Full Plans route may be required instead.
Does a new build handle public sewers differently from an extension?
Often, yes. For a new property, the sewer must be diverted rather than handled through a standard build over agreement.
