What is the difference between foul and surface water drainage?

Foul water is waste from sinks, dishwashers, and WCs, which goes to the foul sewer for treatment. Surface water is rainwater from the roof, which goes to a soakaway, a surface water sewer, or a watercourse. The two systems must stay separate, and cross-connecting them is a pollution offence.

What gradient should an extension drainage pipe be laid to?

For a standard 110mm domestic foul pipe, Approved Document H sets a self-cleansing target of 1:40, a drop of 25mm per metre. Too steep and the water outruns the solids, too shallow and the flow is too weak to carry them, and both leave deposits that block the pipe.

What is a soakaway and when does an extension need one?

A soakaway is an underground chamber that collects roof rainwater and lets it soak slowly into the surrounding soil. Most extensions use one for surface water because water butts cannot handle the volume and a surface water sewer may not exist, but the soil has to drain well enough, which a percolation test confirms.

Do I need to tell the water company before connecting to the public sewer?

Yes. Section 106 of the Water Industry Act 1991 gives you the right to connect, but you must give the water company at least 21 days' notice first. They can set conditions on how the connection is made but cannot refuse it, and connecting without notice is a criminal offence.

Should I get a CCTV drain survey before building an extension?

It is worth doing. A camera survey of your existing drains shows their condition, depth, and connection points before you start, which tells your groundworker the level to work the gradient back from and flags any unmapped pipes before the digger finds them.

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Phase 3 · Groundwork · Task 05 of 10

Drainage

Foul and surface water drainage for your extension: what connects where, gradient rules, build-over agreements, soakaway requirements, and what building control inspects before backfilling.

15 min readUpdated 2026Premium

Process infographic showing the drainage system for a UK house extension, with foul and surface water paths from extension to public sewer and soakaway

Prerequisites

Complete this first

Foundations and Footings

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.

Do this first

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 6-8 weeks for processing).

What this guide covers

01Foul Water vs Surface Water: Two Systems, No Mixing
02What Your Extension Actually Needs
03Connecting to Existing Drains
04Build-Over Agreements
05Section 106: Your Right to Connect
06Gradient and Fall
07Surface Water and Soakaways
08Building Control: The Drainage Inspection
09What It Costs
10What Goes Wrong
11The CCTV Survey: Don't Skip It
12What Happens Next

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