Published: 9 April 2026
What Is a Build-Over Agreement?
If a property has been extended over or near a public sewer, the work should have been approved by the local water authority. This approval is known as a build-over agreement.
In practice, the rules depend on when the work was carried out and which authority is responsible. Older alterations, in particular, are often less clearly documented, which is why this isn’t something to treat as routine paperwork.
Why This Matters When Buying
For a buyer, the question is straightforward: was the work properly approved, and has the drainage system been affected?
If that isn’t clear, it can slow down the legal process and introduce uncertainty at the point where you need confidence. In some cases, it can also affect how comfortable a lender or solicitor is with the property.
Where Problems Typically Arise
Issues are most common with older extensions, conservatories, or garden alterations where records are incomplete or missing altogether.
In these situations, drainage routes are often assumed rather than confirmed. That can create uncertainty around access, maintenance responsibility, and whether the pipework beneath the structure has been affected over time.
What to Check Before Exchange
Start by confirming whether any structural work has been carried out near likely drainage runs. Your solicitor should be asking specifically for build-over documentation where this applies.
If the layout of the drainage system is unclear, it’s worth resolving that before you proceed. A CCTV drain survey can provide direct visual evidence of pipe condition and routing, which helps remove guesswork from the decision.
When to Raise the Question
Build-over and drainage questions are best addressed early, while the transaction is still flexible.
Clear evidence at this stage makes it easier to proceed with confidence, agree on next steps if issues are found, or adjust your position before you are committed.
Getting Clear Evidence Before You Commit
If you are unsure whether drainage has been properly accounted for, it is usually better to clarify that before exchange rather than after completion.
A quick check now can prevent a situation where responsibility for an unclear or restricted drainage system becomes yours once the purchase is complete.