Construction & Property

Construction drainage design and compliance

How to design and install foul and surface water drainage that meets Approved Document H requirements. Covers pipe sizing, gradients, access points, testing, connection to public sewers under Section 106, and build-over agreements.

UK-wide
Guide summary

Follow drainage rules when building or altering properties in England and Wales. Separate foul and rainwater systems, use correct pipe sizes and gradients, and get approval for building near sewers. Check local requirements for soakaways and sustainable drainage.

  • Separate foul water and rainwater drainage systems
  • Use minimum pipe sizes: 32mm (basin), 40mm (shower), 100mm (WC)
  • Maintain gradients: 1:80 for 100mm pipes, 1:150 for 150mm pipes
  • Provide access points every 45m or at junctions
  • Get build-over agreement if within 3m of a public sewer
  • Use soakaways 5m from buildings if ground conditions allow
  • Sustainable drainage (SuDS) required for new Welsh developments
  • Septic tanks need 2,700L capacity minimum, cesspools 18,000L
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Related guides in Building Regulations

If you are installing or altering drainage on a construction project in England or Wales, the work must comply with Building Regulations 2010, Approved Document H. This applies to new buildings, extensions, and any material alteration that involves drainage work.

Approved Document H covers six areas:

  • H1 – foul water drainage (WCs, basins, sinks, baths)
  • H2 – wastewater treatment systems and cesspools (where no public sewer is available)
  • H3 – rainwater drainage (gutters, downpipes, soakaways)
  • H4 – building over sewers
  • H5 – separate drainage systems (keeping foul and surface water apart)
  • H6 – solid waste storage (not covered in this guide)

Getting drainage wrong can cause blockages, flooding, pollution, and failed building control inspections. This guide explains what you need to do at each stage of a drainage installation.

Design the drainage layout

Before starting any installation, you need a drainage design that meets Approved Document H and satisfies building control. Your design must address pipe sizing, gradient, and access points for each part of the system.

Pipe sizing and gradients

Pipe sizes and minimum gradients depend on the number of appliances connected and the expected flow rate. The key requirements are:

  • Branch discharge pipes: minimum 32mm (basin), 40mm (bath, shower, sink), 100mm (WC)
  • Main drain: minimum 100mm diameter for domestic buildings; 150mm or larger may be needed for commercial or multi-unit developments based on BS EN 752 hydraulic design
  • Gradients for 100mm pipes: minimum 1 in 40; a flatter gradient down to 1 in 80 is only acceptable where at least 1 WC is connected to the drain
  • Gradients for 150mm pipes: minimum 1 in 150, and only where at least 5 WCs are connected

If the site topography makes minimum gradients difficult to achieve, the gradient may be reduced where the design meets BS EN 12056-2 and BS EN 752 flow requirements. Discuss this with building control before installation.

Access points

Rodding eyes, access fittings, inspection chambers, and manholes must be provided:

  • At every change of direction
  • At every change of gradient
  • At junctions between drains
  • At intervals not exceeding 45 metres on straight runs

All access points must be accessible for maintenance and clearing blockages. Position manholes where they will not be obstructed by the finished building or landscaping.

  1. 1

    1. Check the sewer type in your area

    Contact the local water and sewerage company to confirm whether the area is served by separate or combined sewers. Foul water must connect to the foul sewer and surface water to the surface water sewer. Connecting to the wrong sewer is a criminal offence under the Water Industry Act 1991.

  2. 2

    2. Prepare a drainage layout drawing

    Produce a scale drawing showing pipe routes, diameters, gradients, access point locations, connection point to the public sewer, and any SuDS features for surface water. This drawing forms part of your Building Regulations application.

  3. 3

    3. Submit to building control

    Submit a full plans application or building notice to the local authority building control or a registered building control approver (RBCA). Building control will review your design against Approved Document H before construction starts. Full plans applications receive a formal approval decision.

  4. 4

    4. Install drainage to the approved design

    Lay pipes to the specified gradients using appropriate bedding material. Ensure joints are correctly made and access points are installed at the positions shown on the approved drawing. Building control will inspect the installation before trenches are covered.

  5. 5

    5. Test the drainage before covering

    Foul drains must pass an air or water pressure test before being backfilled. Air test: maintain 100mm water gauge for 5 minutes with a maximum permitted drop of 25mm. Water test: apply 1.5 metres head of water and check for leaks. Record the results for building control inspection.

  6. 6

    6. Arrange building control sign-off

    Building control will inspect at key stages: before covering trenches, on completion of drainage runs, and at final testing. A completion certificate is issued when the work satisfies Building Regulations requirements. Do not backfill trenches before inspection.

Connect to the public sewer

You have a statutory right to connect drains and private sewers to the public sewer system under Section 106 of the Water Industry Act 1991. However, you must follow the correct procedure and the water company can impose conditions on the connection.

Before connecting

You must give the water and sewerage company a minimum of 21 days' written notice before making any connection to a public sewer. The notice must specify the mode and point of connection. The water company will confirm whether your proposed connection is acceptable or specify alternative arrangements.

If the water company unreasonably refuses your connection, you can refer the matter to Ofwat for determination.

Trade effluent

If your premises will discharge trade effluent (commercial or industrial wastewater), you need separate consent under Section 118 of the Water Industry Act 1991 in addition to the Section 106 connection. Apply to the water company before connecting.

Building over or near existing sewers

If your proposed building or extension is over or within 3 metres of a public sewer, you must obtain a build-over agreement from the water company before starting work. The water company will assess the risk to the sewer and may require:

  • A CCTV survey of the sewer before and after construction
  • Specific foundation designs such as bridging foundations to protect the sewer
  • Adequate access for future sewer maintenance

Apply to the water company before submitting your Building Regulations application. Provide site plans showing the sewer location, proposed building footprint, and foundation details. Building control approval is also required separately.

Failure to obtain a build-over agreement can result in enforcement action from the water company and difficulties selling the property in future.

Surface water and SuDS

Rainwater drainage must be kept separate from foul drainage where the area has separate sewer systems. Surface water should discharge to a soakaway, watercourse, or surface water sewer where possible.

For major developments (10 or more dwellings, or sites of 1 hectare or more), sustainable drainage systems (SuDS) are expected as a condition of planning permission. The Lead Local Flood Authority (LLFA) is a statutory consultee on surface water drainage for major planning applications.

In Wales, SuDS approval is mandatory for most new developments. You must obtain approval from the local SuDS Approving Body (SAB) under Schedule 3 of the Flood and Water Management Act 2010 before construction begins.

In England, Schedule 3 has not been commenced. SuDS are delivered through planning conditions rather than statutory approval. However, LLFA objection to your surface water drainage strategy will normally result in refusal of planning permission.

Competent person schemes

Registered installers under competent person schemes (such as APHC, SNIPEF, or NAPIT Plumbing) can self-certify certain drainage work without a separate Building Regulations application. The installer issues a certificate of compliance and notifies the local authority. The work must still meet all Approved Document H requirements.

Using a competent person scheme can save time and cost on smaller projects, but is not suitable where the drainage design is complex or involves connection to the public sewer.

What to do next

Official guidance and legislation