Extension Drainage Calculator — Pipe Sizing & Inspection Chambers

A below-ground foul drain carrying a WC must be at least 100mm internal diameter — 110mm OD in UK practice (AD H 2015 para 2.33). A foul drain that does not serve a WC must be at least 75mm; the smaller 50mm pipe is only for above-ground waste branches, not the buried drain. Surface-water drains are typically 110mm, and combined drains are 110mm or larger depending on total flow.

AD H 2015 Table 6 gives a 100mm foul drain a recommended self-cleansing gradient of 1:40 (25mm fall per metre, capacity 9.2 l/s) for flows under 1 l/s. Because a single 6 L WC flush produces about 2 l/s, a WC-bearing drain meets the over-1 l/s row, where the absolute minimum gradient is 1:80 (capacity 6.3 l/s) — but 1:40 is recommended for reliable self-cleansing. Surface-water drains can be 1:100. Steeper is fine; the gradient must be consistent with no low points where water can pond.

If your extension is built over or within 3 metres of a public sewer, you need a build-over agreement from the water company. Routes and fees vary by company and sewer size: a build-over declaration for a small, shallow sewer can be roughly £70–£150 and approved in about 2–3 weeks, while a full build-over application typically runs around £400–£800 or more, with pre- and post-build CCTV surveys and 6–8 weeks or longer for larger or critical sewers. Confirm the current fee and route with your water company before starting work.

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.

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. An open-air ventilating pipe must finish at least 900mm above any opening into the building within 3m (AD H para 1.31) — that is any window, door, rooflight or vent, not just windows — or you can use an air admittance valve internally where the pipe cannot reach open air.