Building Regulations Part H: Foul Water Explained

Building Regulations Part H is the part of the building rules that governs foul water drainage: how the used water from a building's toilets, sinks, baths and appliances is carried away safely. If you are building, extending or converting anything in England, the foul drainage has to satisfy Part H before building control will sign the work off. This guide explains what Part H requires for foul water in plain terms. It covers what counts as foul water, where it is allowed to go, the pipe sizes and gradients the guidance sets, and how the building control test differs from the separate planning one.

The detail is not difficult once the structure is clear. It is easy to get wrong, though, on a flat site, a plot with no nearby sewer, or a scheme where planning and building control pull in different directions. On a development, the same decisions also have to be evidenced for the planning system in a foul drainage strategy for planning, a separate exercise from Part H sign-off that we return to below.

Part H sets no single "pass mark". A foul drainage system is compliant when it carries the flow to a suitable outfall without blocking, leaking, letting foul air in or raising the flood risk to the building.

What is Building Regulations Part H?

Part H of the Building Regulations covers drainage and waste disposal. The legal requirement itself is short. The practical detail sits in the government's Approved Document H, the statutory guidance that shows one accepted way to meet it. An Approved Document is guidance, not the law itself. You can meet Part H another way if you can demonstrate it works, but following Approved Document H is the route building control expects, and departing from it puts the burden of proof on you.

Approved Document H is divided into six sections:

• H1, foul water drainage: carrying used water from appliances to a suitable outfall.
• H2, wastewater treatment systems and cesspools: septic tanks, package treatment plants and cesspools where there is no sewer.
• H3, rainwater drainage: roof and surface water.
• H4, building over sewers: building on or near an existing public sewer.
• H5, separate systems of drainage: keeping foul and surface water apart.
• H6, solid waste storage: refuse storage.

Foul water sits mainly under H1, with H2 and H4 close behind. The current version is the 2015 edition, which took effect on 1 October 2015 and is still in force in 2026. The requirements in Part H of Schedule 1 to the Building Regulations 2010 have not changed since.

Approved Document H is guidance on how to meet the law, not the law itself. Building control still assesses against it, so a design that ignores it has to carry its own evidence.

What counts as foul water?

Foul water is the contaminated water a building produces. It is the used water from toilets, sinks, basins, baths, showers, bidets, kitchen appliances and any fitting used to wash dirty containers. Part H treats it as waste water containing either soil contamination, from a WC or similar, or waste from food preparation, cooking or washing.

The distinction that matters is foul water against surface water. Rainwater running off roofs and paving is surface water, and it is dealt with separately under H3, not sent into the foul system. Mixing the two is one of the most common drainage errors, and it causes problems in both directions. Rainwater overloads the foul sewer and the sewage treatment works, while foul flows have no place in a surface water system that discharges to a watercourse. If you are unsure which is which, our guide to foul, surface and combined sewers sets out the difference in full.

What does Requirement H1 actually require?

The legal requirement is deliberately brief. It sets an outcome, an adequate drainage system discharging in a set order of preference, and leaves the "how" to the guidance.

Approved Document H then spells out what "adequate" means. A foul drainage system meets Part H when it does all of the following:

• Carries the flow of foul water to a suitable outfall, whether a sewer, a treatment system or a cesspool.
• Minimises blockage and leakage, so waste water does not escape or back up.
• Keeps foul air out of the building in normal use, through traps and ventilation.
• Is ventilated, so pressure changes do not empty the traps.
• Stays accessible, so blockages can be cleared without excavation.
• Does not raise the flood risk to the building, for example from a surcharging sewer.

Where can foul water discharge to? The Part H hierarchy

This is the question that shapes most foul drainage decisions, and Part H answers it with a clear order of preference. You work down the list and use the highest option that is reasonably practicable for the site. You do not skip to a septic tank because the sewer is a little further away, nor default to a cesspool because it is simplest to install.

A mains connection is the default the whole system is built around. It is treated off-site at a sewage works, it needs no maintenance from you, and building control will expect it wherever a public sewer is within reach. As a rule of thumb, a connection is looked for where a public foul sewer lies within about 30 metres of the building. Distance is only part of the test, though: levels, third-party land and the practicality of the run all count. Only where mains connection is genuinely impractical does the design move down to a private sewer, then to a treatment system, and finally to a cesspool as the option of last resort.

Pipe sizes, gradients and flows: the Part H numbers

A foul drain has to satisfy two demands that pull against each other. It must be large enough to carry the peak flow, but small and steep enough that the flow keeps solids moving rather than letting them settle. Approved Document H sets minimum sizes and gradients to strike that balance. The headline figures for below-ground foul drains are these.

The aim behind the gradients is a self-cleansing flow. Approved Document H looks for a velocity of at least 0.7 metres per second when the pipe is running at around a third full, fast enough to scour solids along. Laying a 100mm drain too shallow is a common cause of standing waste and recurring blockages, so on flat sites the fall is designed carefully rather than set at a nominal minimum. A 100mm foul drain at the standard 1 in 40 fall drops 25mm for every metre of run.

The drain also has to be sized for the peak flow it will carry, which rises with the number of dwellings served rather than in a simple multiple. Approved Document H gives design flow rates for domestic foul drainage.

A single dwelling generates a peak foul flow of about 2.5 litres per second; thirty dwellings together generate roughly 5.8 litres per second. Flows pool, so drains are sized on the combined peak, not the sum of the parts.

Above ground, the same logic sizes the soil and vent stacks. Approved Document H links stack diameter to flow capacity, from 50mm at 1.2 l/s up to 100mm at 7.2 l/s. Ventilation is part of the requirement, not an optional extra. An open vent pipe must finish at least 900mm above any opening, such as a window, within 3 metres, so foul air disperses safely.

Keeping foul and surface water separate

Section H5 requires foul and surface water to be drained by separate systems wherever the development connects to a sewer that is itself separate. In practice this means a new scheme should keep its foul and rainwater apart and connect each to the right sewer: foul to the foul or combined sewer, surface water to the surface water sewer or a sustainable drainage system.

You cannot send surface water into a foul-only sewer, and you cannot connect foul water to a surface water sewer that discharges, untreated, to a watercourse. Where only a combined sewer is available, foul and surface water may share it, but the design still has to account for the combined peak. For the surface water side of a development, the more demanding requirements now come from planning rather than building control, through a sustainable drainage strategy built to the 2025 National Standards for SuDS.

Building over or near a public sewer

Section H4 deals with building on or close to an existing public sewer. As a general rule, building work within 3 metres of a public sewer, or directly over one, needs the agreement of the water company through a build-over agreement. The concern is straightforward: the building must not damage or overload the sewer, must not obstruct access for maintenance, and must not sit where a future repair would mean underpinning the structure.

A build-over agreement is a separate consent from the water and sewerage undertaker, obtained alongside building control approval rather than instead of it, and it can take several weeks to arrange. Because most private drains and lateral drains transferred into public ownership in 2011, sewers that a developer might once have treated as private are now often public. That brings more everyday extensions within H4 than people expect.

Off-mains foul drainage: septic tanks, treatment plants and cesspools

Where no public sewer is within reach, foul water is treated on site. This is the H2 territory of septic tanks, package treatment plants and cesspools. They are not interchangeable, and choosing the wrong one is an expensive mistake.

An off-mains system also has to satisfy the Environment Agency's rules as well as Part H. Small domestic discharges are usually covered by the general binding rules for small sewage discharges, which cap discharge to ground at 2 cubic metres a day and to surface water at 5 cubic metres a day; a larger or more sensitive site needs a permit. Since January 2020 it has been an offence for a septic tank to discharge directly to a watercourse, so many older systems have had to be upgraded or redirected. A drainage field is sized from a percolation test to BS 6297. That is a different test from the BRE 365 soakaway test used for surface water, and the two are frequently confused in submissions.

Connecting to the public sewer: the right to connect

Where a mains connection is the answer, you have a statutory right to make it. Section 106 of the Water Industry Act 1991 gives an owner or occupier the right to connect their drains to the public sewer, and a water company can refuse only on narrow grounds: the construction or condition of the connection, or genuine prejudice to the sewerage system. A busy network is not one of them. The Supreme Court confirmed how strong that right is in Barratt Homes Ltd v Dŵr Cymru (Welsh Water) [2009] UKSC 13, holding that the developer chooses the point of connection and that capacity alone cannot justify a refusal.

Connecting is not the same as having new sewers adopted. Adoption brings a new sewer into public ownership so the water company maintains it, and it runs through a separate Section 104 agreement arranged before construction. Both sit apart from Part H: building control checks that the drainage is built to an adequate standard, while the connection and adoption are matters for the water company.

Why Part H sign-off is not the same as planning approval

This is the point that catches developers out, so it is worth stating plainly. Part H is a building control regime. It asks whether the drainage is built to an adequate technical standard. It does not ask whether the scheme should have been permitted, and it does not discharge a planning drainage condition.

A Building Regulations completion certificate for Part H does not discharge a planning drainage condition. The two regimes run in parallel, and neither checks the other's work.

The planning test is usually the more demanding of the two. A drainage condition on a planning permission can require greenfield run-off rates, attenuation storage sized to a 1-in-100-year storm with a climate change allowance, infiltration testing and a maintenance plan. Part H asks for none of that. Worse, drainage and flood-risk conditions are excluded from the deemed-discharge fast track, so silence from the council is not consent. A developer who follows building control's simpler route can end up holding a valid completion certificate alongside a live, enforceable planning breach that surfaces on a legal search. Our guide to how building control and planning differ on drainage works through the trap in detail.

When to bring in a drainage specialist

Minor repairs and like-for-like replacements are usually straightforward. Foul drainage warrants proper design, and often a planning drainage strategy alongside the Part H work, when a project involves any of the following:

• A new building, extension or change of use that adds or alters foul drainage.
• No sewer within reach, so an off-mains treatment system and drainage field are needed.
• A low-lying site, basement or history of flooding, where surcharge and pumping have to be designed for.
• A public sewer over or near the site, triggering a build-over agreement under H4.
• A planning condition requiring a drainage strategy to be approved before work starts.

In these cases the answer is rarely a bigger pipe. It depends on the flows, the levels, the ground and the receiving system, and on a development, on satisfying planning as well as building control. Unda prepares foul drainage strategies for planning and handles the water company enquiries and drainage condition discharge that sit alongside Part H, so the drainage stands up to both regimes. For a fixed-fee quote, talk to our drainage team.

Frequently asked questions