Building Regs Part H (Drainage): A Trade's Guide to Doing Drainage Right (2026)

What Part H Actually Covers

Approved Document H (Drainage and Waste Disposal) is split into several sub-sections, and it helps to know which one applies to the work in front of you. The headline areas are:

• H1 — Foul water drainage: carrying soil and waste water from WCs, basins, baths, sinks and appliances away to a sewer or treatment system.
• H2 — Wastewater treatment systems and cesspools: septic tanks, package treatment plants and cesspools where there's no mains sewer to connect to.
• H3 — Rainwater drainage: roof gutters, rainwater pipes and surface water drainage from paved areas.
• H4 — Building over sewers: the rules for building over or near a public sewer or drain.
• H5 — Separate systems of drainage: keeping foul and surface water apart where the network requires it.
• H6 — Solid waste storage: adequate provision for storing household and trade waste (bin storage).

Most domestic jobs touch H1 and H3. Rural and off-mains work brings in H2. Extensions and new builds frequently run into H4. Knowing which section governs the work tells you what evidence and what permissions you'll need.

Foul Drainage: Getting the Falls Right

The single most common mistake on foul drainage is the gradient. Pipe needs enough fall to keep solids moving with the flow of water, but too steep and the water runs away faster than the solids, leaving them stranded — a self-clearing run becomes a blockage waiting to happen. Part H, via the relevant design guidance, gives recommended minimum falls based on pipe diameter and the number of fittings discharging into the run.

For a 100mm foul drain serving a typical domestic property, a fall of around 1:40 to 1:80 is the usual working range — roughly 12.5mm to 25mm of drop per metre of pipe. The 1:40 figure is often quoted as the standard for smaller domestic runs with at least one WC; flatter runs down towards 1:80 or 1:100 can be acceptable on larger-diameter pipe or where the flow rate supports self-cleansing. For 150mm pipe, gradients as flat as 1:150 can work because the larger bore carries more flow. The principle to remember on site: lay it consistently, avoid back-falls and dips, and don't over-steepen short runs.

Quick reference: pipe size and recommended gradient

Treat these as a starting point, not gospel — the correct fall depends on flow rate, the number of dwellings served and the design method used. But if you're laying 100mm domestic foul drain and you aim for 1:40, you'll be in safe territory for most jobs.

Access, Rodding Points and Inspection Chambers

A drain you can't clear is a drain that will eventually flood someone's garden. Part H requires adequate access so the system can be rodded and cleared. In practice that means providing rodding eyes, access fittings, inspection chambers or manholes at the right points:

• At or near the head (start) of a drain run.
• At a change of direction or gradient where you can't rod through.
• At a junction, unless every run can be cleared from another access point.
• At regular intervals on long straight runs so the whole length is reachable with rods.

Modern shallow inspection chambers and rodding-point systems make this straightforward on domestic work — but you still have to install them. Skipping an access point to save a fitting is a false economy that leaves a buried run no-one can clear without excavation.

Ventilation: Don't Forget the Air

A drainage system has to breathe. Without ventilation, the movement of water through the pipework can siphon the water out of trap seals, letting drain smells into the building. Part H requires the system to be ventilated, normally by carrying a vent pipe up to open air at high level — the familiar soil vent pipe (SVP) terminating above the eaves and clear of any opening window. On smaller installations, air admittance valves (AAVs) can be used internally to admit air without a roof penetration, provided at least one vent open to atmosphere remains on the system to let foul air escape. Get the ventilation wrong and the customer ends up with gurgling traps and a smell they'll call you back about.

Surface Water: The Disposal Hierarchy

This is where a lot of trades trip up. You cannot simply connect rainwater and surface water into the foul sewer because it's the nearest pipe. Overloading foul sewers with rainwater causes them to surcharge and flood in heavy weather, which is exactly what the regulations are trying to prevent. Part H sets out a hierarchy: you must dispose of surface water as high up this list as is reasonably practicable before dropping to the next option.

On most domestic jobs the right answer is a soakaway, sized for the area of hardstanding or roof draining into it and located the required distance from buildings and boundaries. A percolation test tells you whether the ground will actually take the water — clay-heavy ground often won't, which is when you move down the hierarchy. Don't assume; test the ground.

Building Over or Near a Public Sewer

If you're building an extension, a garage or any structure over or close to a public sewer, H4 applies and you almost certainly need a build-over agreement with the relevant water and sewerage company (the water authority for the area). Public sewers — including many that used to be private and were transferred to the water companies in 2011 — have to remain accessible for the water company to maintain and repair. Build a wall or footing across one without permission and you can be ordered to take it down.

A build-over agreement (or, for work near but not over a sewer, a build-near consent) sets conditions: minimum distances, foundation design that bridges the sewer rather than loading it, retaining access to manholes, and sometimes a CCTV survey before and after. The trigger distance is usually building within around 3 metres of a public sewer, but check the local water company's requirements. The key point for trades: this is separate from Building Regulations approval and from planning — you need the water company's sign-off in addition, and it's the homeowner's (or your) job to apply for it early, not after the footings are poured.

Septic Tanks and Package Treatment Plants

Off the mains sewer, H2 governs how foul water is treated on site. The two common options are a septic tank (which settles solids and discharges partially treated effluent to a drainage field) and a package sewage treatment plant (which treats the effluent to a higher standard, usually with an aeration process). Cesspools — sealed tanks that store everything and must be emptied — are a last resort where neither treatment option is feasible.

The big regulatory point trades must know is the General Binding Rules, enforced by the Environment Agency. Under these rules, a septic tank must not discharge directly to a watercourse — its effluent has to go to a drainage field (infiltration into the ground). If a property's septic tank currently discharges to a ditch or stream, it has to be replaced with a treatment plant or connected to a drainage field. A package treatment plant produces cleaner effluent and can discharge to a watercourse, but only if it meets the discharge standards and the conditions of the binding rules — and larger discharges need a bespoke environmental permit. Site the tank and drainage field the required distances from buildings, boundaries, watercourses and any well or borehole, and make sure there's tanker access for emptying.

When Is Drainage Work Notifiable to Building Control?

New drainage runs, new connections to sewers or treatment systems, the installation of a septic tank or treatment plant, and most underground drainage work are notifiable to Building Control. That means you either submit a building notice / full plans application to your local authority, or use an Approved Inspector, before the work is covered up. Building Control will typically want to witness or be satisfied about the drain test before backfilling.

Like-for-like repairs and minor maintenance generally are not notifiable, but anything that creates or materially alters the drainage system usually is. If in doubt, a quick call to Building Control before you start saves you exposing a covered-up run later. Don't backfill notifiable work without the inspection — re-digging it is on you.

Testing the Drains: Air and Water Tests

Before a new drain is signed off, it has to be tested for leaks and that test is part of demonstrating compliance. There are two standard methods:

• Air test: the run is plugged at both ends and air is pumped in to a small set pressure, read on a manometer (a U-gauge of water). The pressure must hold without dropping below the allowed limit over the test period. Quick and clean, and the usual first choice on site.
• Water test: the run is plugged and filled with water to give a head of pressure (typically around 1.5m at the top), left to stand, then topped up and the loss over time measured against the permitted limit. More demanding than an air test and useful where a section fails on air.

Test before you backfill, and ideally test again after backfilling to make sure nothing was disturbed. Document the result — a photo of the manometer holding pressure and a note of the time and date is cheap insurance and exactly the kind of evidence Building Control and the customer want to see.

Common Part H Failures on Site

Most Part H problems aren't exotic — they're the same handful of mistakes repeated. Watch for these:

• Wrong fall: too flat (standing water, solids stranded) or too steep (water races away, solids left behind). Use a laser or boning rods, don't eyeball it.
• Back-falls and dips: a sagging section between supports creates a low spot that silts up. Bed and support the pipe properly.
• Surface water into the foul sewer: the classic cross-connection that overloads the network — keep rainwater off the foul system.
• No rodding access: a buried junction or bend with no way to clear it.
• Inadequate ventilation: trap seals lost, smells in the property.
• Building over a sewer with no agreement: footings across a public sewer with no water-company consent.
• Septic tank discharging to a watercourse: a breach of the General Binding Rules.
• Covering up before testing: backfilling a notifiable run before it's been tested and inspected.
• Poor bedding and surround: pipe laid on hardcore or unsupported, leading to cracks and joint failures under load.

Frequently Asked Questions

What fall should a 100mm foul drain have?

A working range of roughly 1:40 to 1:80 suits most domestic 100mm foul runs, with 1:40 commonly used as the standard for shorter runs serving at least one WC. The exact figure depends on flow rate and run length — but aiming for around 1:40 on domestic 100mm pipe keeps you safe.

Can I connect rainwater to the foul sewer?

Not as a default. You must work down the surface water hierarchy — soakaway or infiltration first, then a watercourse, then a surface water sewer — and only connect to the foul (combined) sewer where nothing higher up is reasonably practicable. Overloading foul sewers with rainwater is a key thing Part H is designed to prevent.

Do I need a build-over agreement for an extension?

If the extension is over or close to (usually within about 3 metres of) a public sewer, yes — you need a build-over or build-near agreement from the water company, in addition to Building Regulations and planning approval. Apply early, before the foundations go in.

Can a septic tank discharge to a stream?

No. Under the Environment Agency's General Binding Rules a septic tank must discharge to a drainage field, not directly to a watercourse. If you need to discharge to a stream you'll need a package treatment plant meeting the discharge standards (and a permit for larger discharges).

Is drainage work notifiable to Building Control?

New drains, new connections and installing treatment systems are generally notifiable — submit a building notice or full plans application, or use an Approved Inspector, and have the drain tested and inspected before you backfill. Like-for-like repairs usually aren't notifiable, but check if you're unsure.