If a sewer runs under the garden or close to an extension, establish who is responsible for it, what the drainage records show and which approvals relate to any building work. A map, a physical survey and a build-over agreement answer different questions. This guide covers England and Wales, where the relevant sewerage provider’s requirements matter.
The practical issue is often discovered late: an extension looks complete, but a search raises a pipe or access question. Start by collecting the evidence rather than treating the presence of a sewer as an automatic verdict on the purchase. A clear account of its location, condition and legal paperwork will help your advisers explain the actual implications.
A pipe in your garden may be a public sewer
Ofwat explains that sewerage companies maintain public sewers, which may run through private gardens, and have access rights for maintenance. Householders normally maintain private drains up to the public connection. Do not decide ownership from which side of a fence a pipe happens to occupy.
Ask the conveyancer and relevant provider to establish the arrangement for this property. The terms private drain, lateral drain and public sewer are useful, but a label in a conversation is not enough: connect it to the particular pipe, route and responsible body.
| Evidence | What it can help establish | Important limit |
|---|---|---|
| Drainage and water search | Recorded connections, assets and relevant search answers | Read its coverage and mapping qualifications |
| Public sewer map | The provider’s recorded asset locations | Some sewers or lateral drains may be unrecorded |
| Drain survey | Observed route and condition within the agreed survey scope | It does not establish all legal approvals |
| Build-over paperwork | The provider’s consent and any specifications or conditions | Match it to the actual work and pipe |
| Planning and building-control records | Those separate approval processes | Do not assume they replace provider consent |
Do not treat a blank map as a clear site
Thames Water’s property-search information notes that public sewers or lateral drains may be absent from the public sewer map and still affect development. A search saying no pipe is shown should be read with its qualifications, particularly if the surveyor notices inspection covers or a route inconsistent with the map.
Ask what a proposed drainage survey would cover: which runs can be inspected, whether the route can be traced and how any inaccessible section will be reported. Keep the findings with the search and building plans. The purpose is to reconcile the records, not to assume one drawing must be a complete physical survey.
Keep search findings connected to the property
Use the wider address research to organise the questions, then add the drainage search, survey and consent documents as they arrive.
Find a propertyBuild-over rules depend on the provider and pipe
A build-over agreement concerns construction over or close to the sewerage provider’s assets. Thames Water explicitly distinguishes this from planning permission. Its published proximity criteria also show why a single national distance is a poor shortcut.
| Provider | Published proximity guidance | What the buyer should do |
|---|---|---|
| Thames Water | Likely application within 3 metres of a public sewer or 1 metre of a public lateral drain | Check the particular pipe, proposed work and application criteria |
| Dŵr Cymru Welsh Water | Notify the provider about extension work over or within 3 metres of a public sewer or lateral drain | Check the pipe details and the provider’s consent criteria |
Dŵr Cymru’s guidance also describes circumstances in which a proposal may not be acceptable. Distance alone is not an approval: the pipe, proposed structure, foundations and access requirements can all need consideration. Ask the relevant provider about the actual proposal before treating an extension as feasible.
Worked example: an extension and no agreement in the pack
Imagine a drainage search shows a public sewer crossing the rear of a house. The seller supplies planning drawings and a building-control document for the kitchen extension, but no build-over agreement. First, ask whether the indicated route corresponds to the pipe’s actual location and whether the documents cover the extension now standing.
Record when the extension was built and give that information to the conveyancer. Ask them to establish which provider requirements applied and what further evidence is necessary. Do not infer a universal exemption from the age of the work or assume that a missing document proves the extension must be removed. The question needs the actual history, provider position and legal review.
If the seller proposes insurance or another solution, ask the conveyancer what it covers and what remains unresolved. Keep legal risk, pipe condition and future maintenance access as separate questions. A financial protection document should not be treated as a physical inspection of the drain.
Your pre-purchase drainage checklist
- Identify the sewerage provider and obtain the complete drainage search, including its maps and qualifications.
- Ask the seller for drainage plans, dates of alterations and any build-over correspondence.
- Compare provider approvals with the work actually carried out and ask about relevant conditions.
- Discuss survey scope with the surveyor or drainage specialist where the location or condition is uncertain.
- Ask how maintenance access works and whether any proposed extension requires a different layout.
- Give future building plans to the conveyancer and designer early; permission for existing work does not answer every new proposal.
- Keep unresolved questions visible until the responsible adviser explains the evidence and remaining implications.
Read conveyancing searches explained for the wider search pack and how to find planning history for the alteration record. Where access crosses other land, the rights-of-way guide helps you prepare that separate title enquiry.
Build the evidence before deciding on the issue
Start an address research file and keep each drainage question tied to the document or professional answer needed to resolve it.
Find a propertyFrequently asked questions
Am I responsible for a sewer because it is inside my garden?
Not necessarily. Public sewers can cross private gardens. Establish the particular pipe’s status and the responsible body using the search and provider’s information.
Does planning permission replace a build-over agreement?
Do not assume so. Provider consent concerns its sewer assets and access. Review the relevant provider requirements separately from planning and building-control documents.
Is the build-over distance always three metres?
No universal shortcut should be used. For example, Thames Water publishes different distances for public sewers and lateral drains, while Dŵr Cymru’s published guidance uses three metres for both. Check the current provider criteria.
Does no sewer shown on a map mean there is no sewer?
No. Provider records may not show every public sewer or lateral drain. Read the search qualifications and investigate any conflicting physical evidence.