SuDS Drainage Systems: Part H Hierarchy & Design Guide UK

Summary

Conventional drainage moves rainwater off a site as fast as possible through pipes to a sewer or watercourse. That approach has driven flooding, pollution and overloaded combined sewers across the UK. Sustainable Drainage Systems (SuDS) flip the logic: deal with water at source, mimic the natural water cycle, and release it slowly and clean. For a contractor, SuDS is not a single product — it is a toolkit of techniques (permeable paving, soakaways, swales, basins, rain gardens, rainwater harvesting) combined to meet four objectives: quantity (flood control), quality (pollution control), amenity and biodiversity.

The legal anchor in England is Building Regulations Part H, which requires surface water to be disposed of in a defined order of preference: first to the ground via infiltration (soakaways, permeable paving), then to a watercourse, and only where neither is reasonably practicable, to a public sewer. This "discharge hierarchy" is the single most important rule to internalise — design always starts at the top and only moves down with justification. Where a sewer connection is unavoidable, the flow rate is usually restricted to greenfield run-off rates so the development does not make downstream flooding worse.

The wider framework comes from the Flood and Water Management Act 2010. Schedule 3 of that Act, which would have made a SuDS Approval Body (SAB) mandatory in England, has not been fully commenced in England, so SuDS in England is largely delivered through the planning system and Part H rather than a standalone approval. (Wales has implemented mandatory SuDS via a SAB; Scotland has its own SuDS requirements.) Design typically follows the non-statutory technical standards for SuDS and the CIRIA SuDS Manual (C753), with infiltration tested to BRE Digest 365 and pipe/sewer design referencing BS EN 752.

Key Facts

• Discharge hierarchy (Part H) — surface water to ground first, then watercourse, then sewer as a last resort.
• Four SuDS objectives — quantity, quality, amenity, biodiversity.
• Source control — manage rainfall where it lands (permeable paving, water butts, green roofs) before it concentrates.
• Attenuation — temporarily store run-off (basins, tanks, oversized pipes) and release it slowly.
• Infiltration — let water soak into the ground (soakaways, permeable paving, infiltration trenches) where soils permit.
• Greenfield run-off rate — discharge from a developed site is commonly limited to the rate before development, often via flow controls.
• BRE Digest 365 — standard method for soakaway percolation testing and sizing.
• BS EN 752 — design of drain and sewer systems outside buildings.
• Flood and Water Management Act 2010 — framework for surface-water management; Schedule 3 SAB not commenced in England.
• CIRIA C753 (The SuDS Manual) — the principal industry design guidance for SuDS.
• Non-statutory technical standards — government technical standards for SuDS used by planners in England.
• Treatment train — chain multiple SuDS components so water is progressively cleaned and slowed.
• Clay soils — heavy/impermeable ground often fails infiltration; design moves down the hierarchy or uses attenuation.
• Adoption — who maintains SuDS long-term (highway authority, water company, management company) must be agreed; non-standard features can attract commuted sums.

Quick Reference Table

Detailed Guidance

The discharge hierarchy in practice

Where can the surface water go? (Part H order)
        |
   1. INFILTRATE to ground?  -- Yes --> Soakaway / permeable paving (test soils, BRE 365)
        | No (clay / high water table / contamination)
   2. Discharge to WATERCOURSE? -- Yes --> Restrict to greenfield rate, flow control
        | No (no nearby watercourse / not permitted)
   3. Discharge to SEWER (last resort) --> Surface-water sewer preferred over combined;
                                           restrict flow; agree with water company

You must justify each step down. Reaching for a sewer connection without first proving infiltration and watercourse options are impractical will fail planning and Part H scrutiny. Even where a sewer is the final route, the flow is normally throttled to greenfield rates using a flow-control device and attenuation upstream.

Testing the ground: BRE Digest 365

Before designing any infiltration component you must know whether the ground actually drains. BRE Digest 365 sets out the percolation test: dig a trial pit, fill with water, and measure the time for the water level to fall between defined levels to derive a soil infiltration rate. That rate sizes the soakaway. Heavy clay typically gives very slow rates and may rule infiltration out — at which point the design moves to attenuation with a controlled outflow. Never size a soakaway from assumption; see soakaway sizing and soakaways.

Source control and the treatment train

The strongest SuDS schemes deal with water early and chain components together — the "treatment train". For a domestic plot that might be: permeable driveway (source control + infiltration) → rain garden for roof run-off → water butt for reuse → swale to a soakaway for overflow. Each stage removes a little pollution and reduces volume, so the final discharge is small and clean. For larger sites the train scales up to swales, basins and ponds.

Permeable paving as SuDS

Permeable paving is the workhorse of small-site SuDS. Special blocks (or porous asphalt/resin-bound) sit over an open-graded, no-fines sub-base that stores rainfall in the voids and either infiltrates it to the ground or releases it slowly through a controlled outlet. It satisfies the front-garden planning rules (see driveway planning permission) and doubles as attenuation. Construction depth and sub-base design matter — get the base build-up right per block paving installation and block paving base preparation.

Attenuation when infiltration fails

On impermeable ground or constrained urban sites, store-and-release attenuation is the answer: oversized pipes, geocellular crate tanks, or above-ground basins, fitted with a flow-control device (orifice plate, vortex flow control) that limits discharge to the agreed rate. Size the storage for the design storm (commonly the 1-in-100-year event plus a climate-change allowance). See foul drainage design for how foul and surface water are kept separate.

Maintenance and adoption

SuDS only works if it is maintained — silt removal from soakaways and permeable paving, vegetation management on swales and basins. Decide early who adopts and maintains each component: the water company (for some piped systems), the highway authority (roadside SuDS, often with a commuted sum — see section 278 highway agreements), or a private management company. Unmaintained permeable paving silts up and stops working within a few years.

Frequently Asked Questions

Is SuDS a legal requirement in England?

SuDS is required in substance through the planning system and Building Regulations Part H rather than a single standalone approval. The mandatory SuDS Approval Body under Schedule 3 of the Flood and Water Management Act 2010 has not been fully commenced in England, so major developments deliver SuDS via planning conditions and the discharge hierarchy. Wales operates a mandatory SAB; Scotland has its own SuDS rules.

Why can't I just connect everything to the sewer?

Because Part H puts the sewer last in the hierarchy, and water companies will normally refuse or restrict surface-water connections to combined sewers to prevent flooding and pollution. You must show infiltration and watercourse routes are impractical first, and even then discharge is throttled to greenfield rates.

How do I size a soakaway?

Run a BRE Digest 365 percolation test to get the soil infiltration rate, then size the soakaway storage volume for the contributing impermeable area and design rainfall so it empties within the required time. Our soakaway sizing tool walks through the inputs.

What if the ground is clay?

Clay usually fails infiltration. Move down the hierarchy: attenuate the run-off in tanks, crates or basins and release it slowly to a watercourse or, last resort, a surface-water sewer at greenfield rate. Don't force a soakaway into ground that won't take water — it will simply back up and flood.

Does a single domestic driveway need full SuDS design?

A standard domestic permeable driveway draining within its own plot satisfies the principles without a formal engineered scheme, provided the ground drains and run-off stays on site. Larger paved areas, poor ground, or sites in flood-risk zones need proper percolation testing and sizing, and possibly attenuation.

Regulations & Standards

• Building Regulations Part H — drainage and waste disposal, including the surface-water discharge hierarchy.

• Flood and Water Management Act 2010 — surface-water management framework; Schedule 3 (SAB) not fully commenced in England.

• BS EN 752 — drain and sewer systems outside buildings.

• BRE Digest 365 — soakaway design and percolation testing.

• CIRIA C753 — The SuDS Manual — principal industry SuDS design guidance.

• Non-statutory technical standards for SuDS (England) — government standards used by planning authorities.

• Building Regulations Approved Document H — surface-water disposal hierarchy.

• GOV.UK: Sustainable drainage systems non-statutory technical standards — design standards.

• Flood and Water Management Act 2010 — statutory framework.

• GOV.UK: Surface water management guidance — planning and SuDS.

• soakaways — soakaway construction and design detail.

• driveway planning permission — the 5m² front-garden SuDS rule.

• rainwater harvesting — capturing and reusing roof run-off.

• soakaway sizing — sizing infiltration from percolation tests.