What pipe size do I need for extension drainage?

For foul drainage with a WC, you need 110mm diameter pipe minimum. Without a WC and with fewer than 12 fixture units, 50mm or 82mm may suffice for waste pipes. Below-ground foul drains are almost always 110mm. Surface water drains are typically 110mm. Combined drains use 110mm or larger depending on total flow.

What gradient should drainage pipes be laid at?

The standard gradient for 110mm foul drains is 1:40 (25mm fall per metre). For a single WC, 1:80 is acceptable. Surface water drains can be 1:100 or flatter. Steeper gradients are fine. The gradient must be consistent — drains must not have low points where water can pond.

What is a build-over agreement?

If your extension is built over or within 3 metres of a public sewer, you need a build-over agreement from the water company. This requires a CCTV survey of the existing drain, an application (typically £200–£400) and protective measures such as concrete lintels or surrounds. Allow 8–12 weeks for approval before starting work.

Where do I need inspection chambers?

Inspection chambers (manholes) are required at every change of direction or gradient, at every junction of drains, and at intervals not exceeding 45m on straight runs. In practice, domestic extensions typically need one chamber at the connection to the existing drain and one at the point where the extension drain changes direction.

Do I need a new SVP for an extension WC?

Not always. If the new WC is within 6m of the existing SVP and the pipe can be routed with adequate fall, you can connect to the existing SVP. If the extension is further away or the route is complex, a new SVP is needed. The SVP must extend to 900mm above the highest opening window or use an air admittance valve internally.

TradeCalculator

Extension Drainage Calculator — Pipe Sizing & Inspection Chambers

Calculate below-ground drainage for UK extensions including pipe diameter, gradient, SVP requirements, inspection chambers and build-over agreements.

Connection type

Drain type

Drain run length

WCs

Basins

Baths

How We Calculate This

This calculator determines drainage requirements based on fixture unit count per BS EN 12056 and UK Building Regulations Part H.

Fixture units

Each sanitary fitting contributes a fixture unit value: WC = 7, basin = 1, bath = 7, shower = 2, kitchen sink = 6. The total determines pipe diameter and flow capacity.

Pipe sizing and gradient

110mm pipe at 1:40 gradient handles up to 32 fixture units. Inspection chambers are placed at direction changes and connections to existing drains.

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.