17/07/2026
Before you dig your extension foundations, read this!
When you start planning a home transformation, you naturally focus on the exciting details—modern floor layouts, vaulted ceilings, and bringing in natural light.
But there is an invisible, underground hurdle that catches out thousands of UK homeowners every year: public sewers.
If you live in a terraced, semi-detached, or closely situated detached home, there is a very high chance a shared drain runs right through your garden.
Here is what you need to know about navigating a Water Authority Build-Over Agreement:
🧱 What is it? A legal approval from your water utility company (like Thames Water or Severn Trent) allowing you to build within 3 metres of their sewer pipe.
🏗️ Why do you need it? To prove your new foundations won't crack or collapse the pipe, and to protect your home if the water company ever needs emergency access.
🚪 The risk of ignoring it: If you build over a sewer without an agreement, the water company can legally demolish or alter your extension to get to their pipes. Plus, you won't be able to sell your house without Building Control sign-off!
How to build safely:
1️⃣ CCTV Survey: Run a camera down the drains to map their exact depth and route.
2️⃣ Bridge the Gap: Have your structural engineer design foundations (like pile and beam) that safely bridge over the pipe.
3️⃣ Secure Sign-Off: Submit plans to the water authority and run a post-build CCTV check to prove no damage was done.
We make expanding your home stress-free. Our expert team coordinates everything from drainage mapping to structural engineering, ensuring your home extensions are completely compliant, beautifully built, and legally secure.
Thinking of extending? Drop us a message or click the link below to talk to our team!
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https://singlestoreyextensions.com/extensions/