Approved Document H (Drainage): What Homeowners Need to Know

Part H is divided into four sections:

• H1 — Foul water drainage: pipes, gradients, access points (rodding eyes) and connections to the public sewer.
• H2 — Wastewater treatment systems and cesspools: for properties not connected to a public sewer.
• H3 — Rainwater drainage: gutters, downpipes, soakaways and sustainable drainage systems (SuDS).
• H4 — Building over sewers: if you want to build over or close to a public sewer, you need consent from the water company.

For foul drainage to work by gravity, pipes must be laid to a sufficient gradient. Part H recommends:

• 100 mm diameter pipes: minimum gradient of 1:80 (1.25%), preferred gradient 1:40 (2.5%)
• 150 mm diameter pipes: minimum gradient of 1:150 (0.67%)

Access points (rodding eyes or inspection chambers) must be provided at changes of direction, junctions and the head of long runs so blockages can be cleared. For extensions adding a new WC or bathroom, the new drainage must connect to the existing soil stack or, if that is not practicable, via a new gulley and rodding eye to the nearest inspection chamber.

Rainwater from roofs and paved areas must be managed to prevent flooding and reduce surface water entering the public sewer. Part H3 sets a hierarchy of disposal options:

1. Infiltration to ground (soakaway or permeable paving) — preferred where soakage rates allow
2. Discharge to a watercourse
3. Discharge to a surface water sewer
4. Discharge to a combined sewer — only if none of the above are feasible

A soakaway must be at least 5 metres from the foundation of any building. A simple percolation test will establish whether the ground drains sufficiently to use a soakaway.

If your extension or new building would be built within 3 metres of a public sewer, or over it, you need a Build Over Agreement from your water company (e.g. Thames Water, Severn Trent, Yorkshire Water). This protects their right to access and maintain the sewer.

The water company will typically want to see a CCTV survey of the sewer before and after construction. Foundation design near sewers may need to be strengthened. Building control cannot issue a completion certificate without confirmation that the Build Over Agreement has been obtained.

Key inspection stages for drainage include:

• Drain run inspection: before backfilling, the BCO checks the pipe gradient, bed and surround, and access point positions.
• Water test or CCTV: building control may require a drain test (water pressure test) to confirm there are no leaks before backfilling.
• Connection to sewer: if you are making a new connection to the public sewer, this must be approved by the water company and building control.