Checking sewer records and the water undertaker position
An asset location search tells you whether a public sewer crosses your plot. What it does not tell you is exactly where, which is why verification on the ground matters.
A sewer records check development enquiry starts with one request: an asset location search from the sewerage undertaker covering the site and the land immediately around it. It comes back as a plan of public sewers, water mains and sometimes clean water assets, with manhole references and, on request, cover and invert levels. That plan decides whether your layout works, because a public sewer crossing the plot is a constraint you cannot design around without the undertaker's agreement.
Order it before the architect draws anything. It costs a small fee and a few working days, and it prevents the sequence where a footprint is designed, approved and then moved.
Requesting an asset location search
Go to the developer services or property searches part of your sewerage undertaker's website. The company is the one that bills you for wastewater, which in many areas is different from the company supplying your drinking water.
Ask for the sewer records and water main records together, and specify a search radius wider than the red line. Fifty metres is a reasonable default on a small site, because a sewer running along the road frontage matters as much as one crossing the garden.
Request levels at the same time. Manhole cover and invert levels are usually a separate, chargeable item, and you will need them for any gravity drainage design. Requesting them later means a second fee and a second wait.
Reading the plan and its accuracy limits
Public sewer records are indicative. Every undertaker says so on the plan, and it is worth taking seriously: the line shown can be several metres from the pipe on the ground, and the depth may be recorded from a survey decades old.
Read the plan in this order.
- Identify the public foul sewers, surface water sewers and combined sewers, which are shown in different line styles and colours by convention rather than by any national standard.
- Note the manhole references and, where given, cover and invert levels with pipe diameters and gradients.
- Check whether any line crosses the site, runs along a boundary or passes within 3 m of where you intend to build.
- Identify the likely point of connection for foul drainage and for surface water, if a surface water sewer exists at all.
- Note anything marked as a rising main or pumping station, which brings a different set of constraints and easements.
Then verify on the ground. Lift the chambers you can safely lift, record what you see, and where a pipe position matters to the design, commission a survey with tracing equipment or a trial hole. The records tell you a sewer is probably there. They do not tell you where.
Sewers that are not on the map
This is the part that catches self-builders.
On 1 October 2011 most private sewers and lateral drains serving more than one property in England and Wales transferred into the ownership of the sewerage undertakers. Many of those transferred assets have never been surveyed and do not appear on the public sewer map at all.
So a pipe in the garden that the map does not show may still be a public sewer. If it serves more than one property and it was in use before the transfer, treat it as public until the undertaker confirms otherwise. Building over an unrecorded public sewer produces exactly the same problem as building over a recorded one, discovered later and at worse cost.
Drains serving only your own property remain private and are your responsibility.
Build over and build near agreements
Where a proposal sits over a public sewer, or within 3 m of one, the undertaker's build over agreement is required. This is separate from planning permission and separate from building control approval, though Approved Document H deals with building over drains and sewers and the building control body will ask whether the agreement is in place.
The undertaker will look at pipe size, depth, condition, material and access. A shallow 150 mm clay sewer in poor condition under a proposed extension is a different conversation from a 225 mm concrete sewer 3.5 m deep. Typical outcomes are approval with conditions on foundation design, a requirement for a lintel or bridging detail over the pipe, a CCTV survey before and after construction, or a refusal that forces a diversion.
Diversion is the expensive option. It is carried out under the Water Industry Act 1991 at the developer's cost, and it takes months rather than weeks. Where a sewer bisects a plot, price the diversion early or design around it. Our separate note on building over or near a public sewer goes through the agreement process in more detail.
Foul and surface water connection enquiries
The right to connect foul drainage to a public sewer comes from section 106 of the Water Industry Act 1991, and it is a right rather than a favour. The undertaker can refuse only on specified grounds, broadly that the mode of construction or condition of the drain is unsatisfactory or that the connection would prejudice the sewerage system.
Apply for it properly all the same. A connection application identifies the point of connection, the method, the flow and the responsible contractor. On larger schemes the undertaker may carry out a capacity assessment and, where the network is constrained, require phasing or off-site reinforcement.
Surface water is a different story. There is no equivalent right to discharge surface water to a public sewer, and most undertakers will refuse where infiltration or a watercourse is available. The drainage hierarchy in national policy puts infiltration first, then a watercourse, then a sewer, and the undertaker's position should be obtained in writing before the Drainage Strategy commits to an outfall.
Where foul capacity is in real doubt, the council may ask for a Foul Sewage Assessment setting out the proposed disposal method and the undertaker's response.
Timescales for each
Work backwards from when the drainage design has to be fixed.
| Enquiry | What it gives you |
|---|---|
| Asset location search | Plan of public sewers and mains around the site |
| Levels and manhole schedule | Cover and invert levels for gravity design |
| Pre-development enquiry | Capacity position and indicative point of connection |
| Build over or build near agreement | Consent to construct over or close to a public sewer |
| Connection application | Formal approval of the connection itself |
| Diversion under the Water Industry Act | Relocation of a public sewer, at developer cost |
Published service standards vary between undertakers, so check the target timescales on your company's developer services pages rather than assuming. As a planning rule of thumb, the search and levels are quick, the pre-development enquiry takes weeks, and anything involving an agreement or a diversion should be started before the planning application rather than after permission.
Our companion piece on ordering sewer records and pre-development enquiries covers the forms and the fee structure.
Feeding the result into the layout
Three outputs go straight onto the drawings. The easement corridor along any public sewer, usually shown as a no-build zone either side of the pipe. The point of connection and the fall available to reach it, which sets the finished floor level as much as any flood consideration. And the surface water outfall, which determines whether a soakaway, an attenuation tank or a restricted connection is designed.
Where levels are marginal, a pumped foul connection is possible but unpopular with adopting authorities and with future owners. Try to keep gravity.
If the site is also in a flood zone, the sewer position and the modelled flood level need reading together, because a gravity outfall that surcharges during a flood event is a design that fails in the one condition it was meant to survive. The Flood Risk Assessment should state the assumed outfall and its level.
PlanScope can draft the drainage and foul sewage documents to the structure councils expect, with every level and capacity figure marked as still to be confirmed until the undertaker's records are supplied. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.
This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it.
Frequently asked questions
How accurate are public sewer records?
Indicative only. Every undertaker states this on the plan, and the pipe on the ground can sit several metres from the line shown, at a depth recorded from an old survey. Where a pipe position affects the design, verify it with tracing equipment or a trial hole rather than relying on the record.
Can a sewer be public if it is not on the map?
Yes. Most private sewers and lateral drains serving more than one property in England and Wales transferred to the sewerage undertakers on 1 October 2011, and many transferred assets have never been surveyed or mapped. Treat any shared pipe predating the transfer as public until the undertaker confirms otherwise.
When is a build over agreement needed?
Where the proposal is built over a public sewer or within 3 m of one. The agreement is separate from planning permission and from building control approval, and the undertaker will consider pipe size, depth, condition and access. Outcomes range from conditions on the foundation design to refusal, which forces a diversion.
Do I have a right to connect foul drainage?
Section 106 of the Water Industry Act 1991 gives a right to connect foul drainage to a public sewer, refusable only on specified grounds relating to the construction or condition of the drain or prejudice to the system. There is no equivalent right for surface water, which normally has to follow the drainage hierarchy.
When should these enquiries be made?
Before the layout is fixed. The asset search and levels are quick, a pre-development enquiry takes weeks, and any build over agreement or diversion takes considerably longer. Starting a diversion after planning permission has been granted is the sequence that costs the most time on small residential schemes.
Sources and further reading
This article is general guidance about the planning system in England and, where stated, the rest of the UK. It is not legal or professional advice. Check your local planning authority’s validation requirements and take professional advice for your specific site.