When planning a new ground-floor addition, most homeowners naturally focus on the exciting aspects: layout options, bi-fold doors, and modern roof lantern designs. However, one of the most common—and potentially costly—surprises is hiding completely out of sight beneath your lawn.
If you are expanding a terraced, semi-detached, or closely situated detached home in the UK, there is a very high probability that a public sewer or shared drain runs directly through your property’s boundary.
Before a single spade hits the ground, you must address this underground hurdle. Here is what you need to know about securing a Water Authority Build-Over Agreement to protect your investment and keep your project fully compliant.
A Build-Over Agreement is a legal document issued by your regional sewerage undertaker (such as Thames Water, Severn Trent, or Anglian Water). It gives you formal permission to build a permanent structure over, or within 3 metres of, a public sewer.
This agreement ensures that:
The Shared Pipe Rule: Any drain that serves more than one property is legally classified as a public sewer. Even if a pipe starts within your boundary, the moment a neighbour’s waste pipe connects to it, it becomes the responsibility of your local water company.
Building without an agreement in place can have devastating consequences. Local authority building control officers cannot sign off on completed works if a required Build-Over Agreement is missing.
Furthermore, if the water company needs emergency access to a pipe beneath your floorboards and you do not have an agreement, they have the legal right to alter or remove your extension to access the sewer—and you will be liable for the repair costs.
To avoid these stressful scenarios, it is best to consult professionals who specialise in bespoke home extensions to assess your site’s drainage layout before submitting structural designs to local authorities.
Never rely on old property deeds to find your pipes. Hire a drainage specialist to conduct a CCTV drain survey. They will feed a camera down your access gully to map the exact route, depth, and structural condition of the pipes beneath your garden.
Once your structural engineer knows where the sewer sits, they must design foundations that bridge over it.
Submit your structural drawings, CCTV footage, and site plan to your local water authority. Application fees typically range from £300 to £800 depending on the size and complexity of the sewer.
Once the heavy building work is complete, you will often need to conduct a second CCTV survey. This proves to the water authority that no structural damage occurred to the public pipework during excavation or concrete pouring.
Navigating public utilities is one of the most complex parts of groundworks. By identifying sewer positions early, coordinating with structural engineers, and budgeting for the necessary agreements, you can proceed with your build knowing your structure is fully compliant and legally protected.
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