Building Regulations Part H: Foul Water Drainage Explained

Foul drainage tends to be noticed only when problems arise. While it works, waste water leaves the building quietly and goes unnoticed. When it stops working as it should, the result is smells, blockages, backed up appliances, and in the worst cases waste water flooding either externally or into the building itself.

Building Regulations Part H sets out to prevent those problems. This guide explains what Part H requires for foul drainage, in plain terms, for anyone planning a new build, an extension, a change of use, or work on an older system.

What is Building Regulations Part H?

Part H of the Building Regulations covers drainage and waste disposal. The detailed guidance sits in a document called Approved Document H. This article refers to the 2015 edition, which applies to building work in England. Wales, Scotland and Northern Ireland have their own equivalent rules, so it is important to check the version that applies to the site in question.

It helps to be clear about what an Approved Document is. It is guidance on how to meet the legal requirement, not the only permitted method. The document states this directly: there are often other ways to comply, and they may be used provided they meet the requirement. In practice, following the Approved Document is the most straightforward route to satisfying Building Control. Where a project takes a different approach, that approach has to be justified.

Approved Document H is divided into several sections:

• H1 Foul water drainage
• H2 Wastewater treatment systems and cesspools
• H3 Rainwater drainage
• H4 Building over sewers
• H5 Separate systems of drainage
• H6 Solid waste storage

This guide concentrates on foul drainage, which sits mainly under H1, with H2 and H4 close behind. Rainwater and surface water are covered by H3 and form a separate subject, although the two systems often share a site.

What counts as foul water?

Foul water is the contaminated water a building produces. Part H defines it as waste water that includes either of the following:

• waste from a toilet, bidet, or an appliance used for washing dirty waste containers
• water used for food preparation, cooking, or washing

In practical terms, this is the water from toilets, sinks, basins, baths, showers, kitchen appliances, and washing machines.

Foul water is distinct from rainwater. Rainwater that runs off roofs and paving is surface water, and it is handled differently. Confusing the two causes real problems, which is why it is worth keeping foul and surface water clearly separated from the outset.

What does Part H require for foul drainage?

The legal requirement itself is brief. It states that an adequate system of drainage must be provided to carry foul water from the appliances in a building to a suitable outfall.

Approved Document H then sets out what “adequate” means in practice. A foul drainage system is considered to meet the requirement if it does all of the following:

• carries the flow of foul water to a suitable outfall, such as a sewer, a treatment system, or a cesspool
• minimises the risk of blockage and leakage
• prevents foul air from entering the building during normal use
• is ventilated
• is accessible so that blockages can be cleared
• does not increase the building’s vulnerability to flooding

Together, these six points describe what the system has to achieve. The technical detail in the guidance, the pipe sizes, gradients, materials and access points, exists to deliver them.

Where is foul water allowed to discharge to?

This question shapes most foul drainage decisions, and Part H sets out a clear order of preference. The waste water has to go somewhere, and the guidance ranks the options from most preferred to least preferred:

1. A public sewer
2. A private sewer that connects to a public sewer
3. A septic tank with secondary treatment, or another wastewater treatment system
4. A cesspool (or cesspit), as a last resort

The approach is to work down the list and use the highest option that is reasonably practicable for the site.

The principle is easier to picture with an analogy. Connecting to a public sewer is comparable to having waste collected at the kerb: the network takes it away and treats it. A private sewer to a public sewer is a shared arrangement that still reaches that network. A treatment system is a managed plant that cleans the water on site. A cesspool simply stores everything until a tanker empties it, which is why it sits at the bottom of the list.

Which option applies depends on what is available, the ground conditions, the levels, and the distance to a sewer.

How large do foul drains need to be, and how steep?

A foul drain has two requirements that work against each other. It has to be large enough to carry the peak flow, and it needs enough fall to keep solids moving without either silting up or allowing the liquid to run ahead and leave the solids behind.

Part H provides minimum pipe sizes and minimum gradients to balance this. As a starting point, a drain carrying foul water needs an internal diameter of at least 75mm, and a drain carrying the discharge from a toilet needs at least 100mm.

Setting the size and gradient correctly is where careful design matters most, particularly on flat sites or where the existing levels are constrained.

What happens on low lying sites and basements?

Sewers that carry rainwater are designed to fill and back up during heavy rain, a condition known as surcharge. When a sewer surcharges, water can only travel back the way it came, which presents a risk to any building lower than the sewer, and a particular risk to basements.

Part H addresses this through measures such as anti-flooding valves and pumped drainage, and it requires the sewerage undertaker to be consulted about how often surcharge is likely. Where a site is low lying, or a basement is being drained, this needs attention early in the design. Our backflow protection and flood risk assessment services can help assist here.

What if there is a sewer under or near the site?

Building over or close to an existing public sewer is governed by Part H4. As a general rule, this applies to work over a sewer, or within 3 metres of the centre line of a sewer shown on the sewage undertaker’s records.

The concern is straightforward. The building must not damage or overload the sewer, must not obstruct access for maintenance, and must not make future repairs impractical. This requirement often emerges partway through a project, so checking the sewer records at an early stage is worthwhile.

What about rainwater and surface water?

Foul drainage is only half of the picture. The rainwater that runs off roofs and paved areas is surface water, and it is managed separately. Within Building Regulations, surface water sits under Part H3 rather than H1.

For most development, the more demanding requirements now come from the planning system rather than the Building Regulations. National planning policy expects surface water to be managed using sustainable drainage systems, usually shortened to SuDS. The idea behind SuDS is to manage rainwater close to where it falls, slowing it down and cleaning it, rather than sending it straight to a sewer.

Surface water follows a discharge hierarchy that mirrors the foul water order of preference. It should drain, in order of priority, into the ground by infiltration, then to a watercourse, then to a surface water sewer, and only as a last resort to a combined sewer. The aim is to keep clean rainwater out of the foul system and to limit the flood risk it creates downstream.

In England, SuDS design is guided by the National Standards for Sustainable Drainage Systems, updated by Defra in 2025, together with national planning policy and the CIRIA SuDS Manual.

The practical result is that most development of any scale now needs a surface water drainage strategy at the planning stage, setting out how rainwater will be managed before the layout is fixed. We cover this through our surface water drainage strategy and SuDS design services.

When is an engineered drainage design needed?

Minor repairs and like for like replacements are usually straightforward. An engineered design is more likely to be needed where any of the following apply:

• a new building, an extension, or a change in how a site is used
• a connection to a public sewer, or the adoption of new sewers
• no sewer within reach, requiring an off grid solution
• a low lying site, a basement, or a history of flooding
• a sewer over or near the site
• an existing system that is underperforming, undersized, or flagged by a survey

In these cases, the answer is rarely a larger pipe on its own. It depends on the flows, the levels, the ground conditions, the available connection, and what Building Control and the water company will accept. This is the purpose of our detailed drainage design service: producing the construction ready drawings and calculations that turn a planning approval into a system a contractor can build and Building Control can approve.

Who this matters to

Foul drainage under Part H affects different parties at different stages:

• Architects and design teams need to allow for drainage routes, levels and sewer constraints at the design stage, before the layout is fixed.
• Developers and contractors need designs that discharge planning conditions and can be priced and built correctly.
• Asset owners and commercial property managers need to understand drainage constraints when extending, refurbishing, or changing the use of a building.

Getting foul drainage right under Part H

Part H exists to ensure foul water leaves a building reliably, without smells, without backing up, and without increasing flood risk. The guidance is clear about the outcomes it requires, and it provides sensible default figures for achieving them. The judgement lies in applying it to a specific site, with its own levels, ground conditions and connection options.

If you are unsure whether a project meets Part H, or you need a design that will satisfy Building Control and the water company, our engineers can help. Our detailed drainage design service is a sensible starting point.

This article is general guidance based on the 2015 edition of Approved Document H for England. It is not a substitute for site specific advice. Always confirm the current version of the regulations and have drainage designed by a competent professional.