What does Building Regulations Part H cover for foul water drainage, soakaways and septic tanks?

Quick Answer: Building Regulations Part H governs the design and installation of foul water and surface water drainage systems in England. It covers everything from the sizing of underground foul drains (minimum 100mm internal diameter for domestic use) to the design of soakaways, septic tanks, and cesspools. All new drainage installations and material alterations require building control approval, and drainage serving new dwellings must be designed to prevent surcharge or nuisance under the design storm event.

Summary

Part H of the Building Regulations is the primary technical reference for drainage work in England. It is divided into six parts: H1 (foul water drainage), H2 (wastewater treatment systems and cesspools), H3 (rainwater drainage), H4 (building over sewers), H5 (separate systems of drainage), and H6 (solid waste storage). For most tradespeople — plumbers, groundworkers, and drainage contractors — H1 and H2 are the sections that matter on a daily basis.

The hierarchy of drainage disposal is established in Part H: foul water must be directed to the public sewer where one is available and connection is reasonably practicable. Only where no public sewer connection is feasible may a private treatment system (septic tank, packaged treatment plant) or cesspool be used. Surface water should be managed through sustainable drainage (soakaway, watercourse) rather than the foul sewer wherever possible. Mixing foul and surface water in a combined sewer is only permitted where no separate system exists.

For tradespeople working on extensions, new builds, or drainage repairs, the practical implications are: minimum pipe sizes must be respected; falls must be within specified ranges to prevent blockage and self-cleansing failure; access points must be positioned at intervals that allow rodding; and any work within 3 metres of a public sewer requires notification to the sewer authority (and possibly a build-over agreement if within 3 metres of the centreline). Getting drainage wrong is expensive to fix — building control inspections at drain-laying stage are standard practice.

Key Facts

• Minimum drain diameter: 100mm internal diameter for foul water drains serving domestic buildings
• Minimum gradient — 100mm pipe: 1 in 40 recommended; 1 in 80 acceptable only for straight runs with high flow; 1 in 150 minimum for 150mm pipes
• Self-cleansing velocity: Drainage must achieve 0.75 m/s mean velocity at part-full flow to remain self-cleansing
• Inspection chambers: Required at every change of direction, gradient, or pipe size, and at maximum 45m intervals on straight runs
• Rodding eye spacing: Maximum 22m from a rodding point to the next accessible fitting on a straight run
• Building over sewers: Any structure within 3m of the centreline of a public sewer requires consent from the sewerage undertaker (under s.185 Water Industry Act 1991)
• Soakaway location: Must be sited at least 5m from any building and from the boundary with an adjacent property; must not be sited in a groundwater source protection zone
• Percolation test: Required to establish soakaway feasibility — Vp (percolation value) must be between 12 and 100 seconds/mm for a soakaway to work reliably
• Septic tank effluent: Must pass to a drainage field (infiltration system) — direct discharge to surface water or groundwater is illegal without an Environment Agency permit
• Cesspool capacity: Minimum 18,000 litres (18m³) for two users; must be watertight and have no overflow
• Sewer adoption: Drains serving a single property are private; sewers serving two or more properties are typically adopted under Schedule 3 of the Flood and Water Management Act 2010
• Part H3 rainwater: Gutters and downpipes must be sized to handle a rainfall intensity of 75mm/hour (standard design event for England)
• Ventilated traps: All sanitary appliances must have a water seal of at least 25mm depth; all drains must be ventilated

Quick Reference Table

Detailed Guidance

Foul Water Drain Design (H1)

The design of below-ground drainage must ensure:

1. Self-cleansing: Gradient and diameter must produce the minimum 0.75 m/s flow velocity. Drains that are too flat silt up; drains that are too steep run dry and cause sedimentation.
2. Watertightness: All joints must be airtight and watertight. A hydraulic pressure test (water test) or pneumatic test is standard before backfilling.
3. Structural stability: Pipes must be bedded appropriately for the depth and ground conditions. Class D bedding (selected granular fill, minimum 100mm surround) is standard for domestic work at typical depths.
4. Access: Inspection chambers or manholes at every junction, change of direction, and at intervals not exceeding 45m.

Drain runs must be laid in straight lines between access points. Junctions should be angled in the direction of flow (oblique or curved branches, not square tees).

Inspection Chambers and Manholes

• Rodding eyes: Minimum internal diameter 100mm, used for straight runs only
• Inspection chambers: Minimum internal width 450mm for depths up to 1m; 600mm for depths up to 1.2m
• Manholes: Required for depths exceeding 1.2m to centreline of channel; step irons or ladders for depths over 1m
• Covers: Must be appropriate for the load — D400 (40 tonne) for road positions; B125 for light vehicle areas; A15 for pedestrian/light use

Wastewater Treatment Systems (H2)

Where a public sewer connection is not reasonably practicable, the options in order of preference are:

1. Packaged sewage treatment plant: Produces effluent to BS EN 12566 standard; typically requires an Environment Agency (EA) registration or permit for discharge
2. Septic tank with drainage field: Septic tanks only provide primary settlement — the effluent must then pass through a drainage field (infiltration trenches) for secondary treatment in the soil
3. Cesspool: A sealed holding tank with no treatment — contents must be pumped out regularly; expensive to operate and only appropriate where other options are impossible

Environment Agency requirements: From January 2020, any new septic tank discharge to surface water (a ditch, stream, or river) requires an individual EA permit. The general binding rules only permit discharge to ground via a drainage field. Installers should ensure clients understand this before any system is specified.

Drainage field design: Based on percolation test results (Vp value). Total pipe length calculated from design formula in the BRE Digest 365 / Approved Document H guidance. Pipes laid at consistent depth (typically 300–700mm), in parallel runs minimum 2m apart.

Soakaway Design (H3 / Surface Water)

For surface water disposal where mains drainage is unavailable:

• Percolation test: Dig a trial pit 300mm square to the proposed soakaway depth. Fill with water and measure the time for water to drop 75mm from a set datum. Repeat three times. Vp = average time (seconds) ÷ 75mm. Suitable if Vp is 12–100 seconds per mm.
• Soakaway sizing: Use BRE Digest 365 design method, or for small domestic installations, a minimum 1m³ volume per 50m² of drained area is commonly applied.
• Modern soakaways: Plastic crate systems (e.g., attenuation crates) are now common for new builds. They must still be sited correctly and sized to the design storm.
• Not suitable: Soakaways are not suitable in clay soils (Vp greater than 100 s/mm), in groundwater source protection zones (Zone 1), or where groundwater is close to the surface.

Building Over and Near Sewers (H4)

A public sewer is an adopted sewer maintained by the sewerage undertaker (e.g., Thames Water, Severn Trent). No structure may be built over a public sewer without consent.

• Within 3m of sewer centreline: A build-over agreement is required from the sewerage undertaker
• Within 1m of sewer centreline: Construction is generally not permitted; the sewer may need diversion (at the applicant's cost)
• CCTV survey: Sewerage undertakers typically require a pre-construction CCTV survey and a post-construction survey as a condition of the build-over agreement
• Private drains: A drain serving only your client's property is private — no consent required, but it must still comply with Part H

Frequently Asked Questions

What is the minimum cover over a drain under a garden?

Under a garden or landscaped area, drains should have a minimum cover of 600mm to the top of the pipe (900mm in agricultural ground subject to vehicle loading). Under driveways or areas with vehicle access, minimum cover is 900mm, and concrete surround or higher-strength pipes may be required.

Can a WC be connected to a 100mm drain running at 1 in 100?

In general, no. A 100mm drain requires at minimum a gradient of 1 in 80 for acceptable self-cleansing velocity, and 1 in 40 is the preferred gradient. At 1 in 100, a 100mm drain is likely to silt and will not achieve the minimum velocity. If the site constraints force a shallower gradient, a 150mm pipe at 1 in 100 may be acceptable with high design flow.

Does a rainwater pipe need to be separate from the foul drain?

Yes — in almost all new work, surface water must be kept separate from foul drainage. Surface water should drain to soakaway, attenuation, or surface water sewer. Connecting surface water to the foul sewer is only permitted where there is no separate system available and the sewerage undertaker consents. Mixed drainage in existing combined systems may be retained, but new connections to foul sewers for surface water are not permitted without consent.

How far must a septic tank be from a house?

Approved Document H recommends a minimum 7m from any habitable building. The Environment Agency's position is similar. The drainage field must be a minimum 10m from any watercourse, and the entire system should be sited to avoid waterlogged ground. In practice, the percolation test result determines feasibility more than distance.

Who is responsible for a shared private drain?

A drain serving only one property up to the point where it joins a shared run (or the public sewer) is the responsibility of the property owner. A shared drain serving two or more properties — a "private sewer" — is the joint responsibility of all connected owners. Since October 2011, private sewers that were formerly jointly owned have been transferred to the ownership of sewerage undertakers under the Water Industry Act 1991 reforms.

Regulations & Standards

• Building Regulations 2010, Schedule 1, Part H (SI 2010/2214) — Statutory requirements for drainage and waste disposal

• Approved Document H (2002 edition, as amended) — Statutory guidance for compliance with Part H

• BS EN 752:2017 — Drain and sewer systems outside buildings — sewer system management

• BS EN 1610:2015 — Construction and testing of drains and sewers

• BS EN 12566 — Wastewater treatment systems for 50 PT or less — covers packaged sewage treatment plants

• BRE Digest 365 — Soakaway design — used for sizing domestic soakaways

• Water Industry Act 1991 — Legal framework for sewer ownership and build-over consents

• Environment Agency General Binding Rules (2020) — Rules governing discharge from septic tanks and small sewage treatment plants

• Approved Document H (GOV.UK) — Official statutory guidance

• Environment Agency — Small sewage discharges — Discharge permits and general binding rules

• Water UK — Build over agreements — Overview of sewer consent processes

• BRE Digest 365 — Soakaway design — Technical soakaway sizing methodology

• part g sanitation — Sanitary pipework above ground, trap requirements and branch sizing

• part c contamination — Resistance to moisture and ground contaminants relevant to drainage design

• building regs overview — Overview of all Building Regulations parts and when they apply