If your extension, garage conversion, or loft build happens to sit above — or close to — a public sewer, you cannot simply lay foundations and hope for the best. You will almost certainly need a build-over agreement from your sewerage undertaker before work begins. Getting this wrong can mean building regs sign-off is refused, or worse, the water company demands you dig everything up to access a blocked drain.
What Is a Build-Over Agreement?
A build-over agreement (sometimes called a “build near” agreement) is a formal consent issued by your regional sewerage undertaker — Thames Water, Severn Trent, United Utilities, and so on — confirming that your proposed structure will not damage or obstruct a public sewer and that maintenance access can be preserved.
Under the Water Industry Act 1991, sewerage undertakers have the right to access public sewers for inspection and repair at any time. If your extension straddles one, the company can demand removal of the obstruction at your cost. A build-over agreement provides legal clarity for both sides.
When Do You Need One?
The trigger is proximity, not just whether your build physically sits on the sewer:
- Within 3 m of a public sewer — most undertakers require a formal agreement or at least a build-near notification.
- Directly over a sewer — a full build-over agreement is almost always mandatory.
- Over an adopted lateral drain (the section between your property boundary and the public sewer, adopted under the 2011 transfer rules) — also triggers the requirement.
Private drains serving only your property are not covered by this regime, but it is worth clarifying ownership with a drainage search before assuming.
Finding Out If a Sewer Is Affected
Before spending money on designs, instruct your solicitor (or do it yourself via your undertaker’s mapping portal) to obtain a CON29DW drainage search. This shows the location of public sewers on or near the site, typically to within 0.5 m accuracy, and costs around £75–£150. Large undertakers such as Thames Water also offer a free online sewer map for indicative checks.
If a sewer is shown within 3 m of your proposed works, treat a build-over application as a project dependency — not an afterthought.
Build-Over Agreement: The Application Process
1. Pre-application check
Most undertakers will review outline proposals informally. Send a location plan (1:1250), a floor plan, and a site section through the proposed building. You will get a steer on whether a full agreement, a build-near notification, or no action is required.
2. Structural design
Your structural engineer must design foundations that span the sewer and transfer loads to either side. Typical solutions include:
- Reinforced concrete ring beam or raft — spreads load away from the sewer.
- Pad and beam foundations — pads positioned outside the exclusion zone, with beams spanning over.
- Piled foundations — used where ground conditions or proximity require it; piles must not be driven within 1.5 m of the sewer barrel.
The undertaker’s own structural requirements, usually published in their technical guidance, govern minimum cover depths (typically 600 mm over a sewer in a domestic situation), inspection chamber access, and pipe bedding.
3. Submit the formal application
Submit to your undertaker’s build-over team with:
- Completed application form
- Site location plan
- Foundation drawings (plan and section) — usually drawn to 1:50 or 1:100
- CCTV survey of the sewer in its existing condition (most undertakers require this pre-work)
- Application fee
4. Assessment and approval
The undertaker reviews the structural approach, may ask for design amendments, and issues a conditional agreement. Conditions typically include a post-construction CCTV survey and a requirement to keep an inspection chamber accessible within or adjacent to the building.
5. LABC notification
Build-over agreements sit alongside — not instead of — Building Regulations approval. Your local authority building control (LABC) or an approved inspector must also sign off on the foundations. Many building inspectors will ask to see the undertaker’s agreement before approving the relevant inspection stage.
Costs and Timescales
| Item | Typical cost (2026) |
|---|---|
| CON29DW drainage search | £75–£150 |
| Pre-application CCTV survey | £350–£700 |
| Build-over application fee (varies by undertaker) | £300–£700 |
| Structural engineer fee (foundation redesign) | £800–£2,000 |
| Post-construction CCTV survey | £300–£600 |
| Typical total add-on to project cost | £1,800–£4,200 |
Timescales vary by undertaker. Thames Water currently targets 30 working days for straightforward applications; Severn Trent quotes 20–25 working days. Factor 6–10 weeks into your programme to be safe, as complex cases or heavily loaded drainage systems can trigger additional consultation.
What Happens If You Don’t Apply?
Building without an agreement where one is required is not technically a criminal offence, but the consequences are serious:
- Building regs sign-off refused — inspectors can and do check drainage compliance.
- Retrospective agreement — many undertakers will grant these, but they cost more (often 50% premium) and require a CCTV survey that may reveal damage already caused.
- Indemnity insurance — increasingly hard to obtain for undisclosed build-over situations, and mortgage lenders commonly ask specifically about sewer proximity.
- Demolition risk — if the sewer is damaged or obstructed, the undertaker can require removal of the structure at the homeowner’s expense.
Build-Near vs Build-Over: Key Difference
If your proposed structure is within 3 m but not directly over the sewer, your undertaker may require only a build-near notification rather than a full agreement. The notification process is lighter-touch — you submit a plan and a covering letter confirming the sewer will not be affected — and the fee is usually lower (£100–£250). Check with your specific undertaker, as thresholds and processes differ across the nine water and sewerage companies in England and Wales.
Practical Tips
Commission the drainage search early. Sewer positions discovered mid-design can force expensive foundation redesigns or even a change to the building footprint.
Use a structural engineer experienced in drainage proximity work. The undertaker’s technical guidance documents (all published online) contain specific requirements for concrete mixes, pipe surround materials, and minimum cover that a generalist may overlook.
Keep access open. One of the most common reasons an agreement is refused is that the proposed layout eliminates all practical access to an inspection chamber. If your design covers an existing chamber, you will need to build a new chamber immediately adjacent, accessible from outside the building envelope.
Check whether a lateral drain is involved. Since the 2011 transfer of private sewers to water company ownership, many drains that homeowners assumed were private are now public. The CON29DW search will confirm this, but it catches many applicants off guard.
[!tip] Check the sewer map before you design Most sewerage undertakers publish interactive sewer maps on their websites. A five-minute check before commissioning drawings can save thousands in redesign fees.
Build-over agreements add cost and time to a project, but they are straightforward when planned early. Treat the drainage search as step one of any ground-floor extension project and the rest of the process follows logically.
