Drainage
PremiumFoul and surface water drainage for your extension: what connects where, gradient rules, build-over agreements, soakaway requirements, and what building control inspects before backfilling.
Get drainage wrong and you'll know about it for years. A foul drain with insufficient fall blocks up every few months. A surface water pipe connected to the foul system is a pollution offence that the water company can prosecute. And if you build over a public sewer without permission, the water company has the legal right to demolish your extension to access it.
Drainage isn't glamorous. Nobody puts it on Instagram. But it's one of the few things on your extension project where a mistake creates ongoing problems rather than a one-off fix. A cracked tile gets replaced. A badly graded drain gets dug up.
Your extension creates two drainage requirements. First, foul water: the kitchen sink and dishwasher need waste pipes that connect to the foul sewer. Second, surface water: your new roof collects rainwater that has to go somewhere. These are separate systems, and mixing them up is both illegal and surprisingly common.
Drainage runs are installed during or immediately after foundation work. Your groundworker needs the trenches open and the foundations poured before laying drainage pipes alongside them. Building control must be notified before drainage work begins, and the drainage inspection happens before any pipes are backfilled. If a public sewer runs under or near your extension, you'll need a build-over agreement from your water company before construction starts (allow 4-8 weeks for processing).
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