Drainage Strategy

Sustainable Drainage Systems (SuDS) are drainage techniques that mimic natural water management by slowing, storing, and cleaning surface water runoff before it is discharged. Examples include permeable paving, rain gardens, swales, green roofs, and detention basins. SuDS have been a requirement for major developments since April 2015 under national planning policy and associated planning guidance. They reduce flood risk downstream, improve water quality, and create biodiversity and amenity benefits.

For greenfield sites, the surface water discharge rate must not exceed the existing greenfield runoff rate for a range of storm events up to the 1-in-100-year event plus a climate change allowance. For brownfield (previously developed) sites, the discharge rate should be reduced as close to greenfield rates as reasonably practicable. Many LLFAs will accept a maximum discharge rate of 5 litres per second per hectare as a minimum, even if the calculated greenfield rate is lower. Always check your LLFA's specific requirements.

The Environment Agency publishes climate change allowances that must be applied to rainfall intensities. As of current guidance, the standard approach requires testing the drainage system against a range of climate change scenarios. For most residential developments, the drainage should be designed to function without flooding for the 1-in-100-year storm event plus a percentage uplift for climate change — typically 40% for the upper end estimate, though this varies by river basin district. Check the Environment Agency's climate change allowances guidance for the applicable figures.

If your Drainage Strategy proposes soakaways or any form of infiltration drainage, you will need to demonstrate that the ground conditions are suitable. This requires infiltration testing (soakaway testing) carried out in accordance with BRE Digest 365 or equivalent methodology. The tests determine the soil infiltration rate and confirm whether soakaways are feasible. If infiltration testing shows the ground is unsuitable (for example, heavy clay with very low permeability), alternative drainage solutions must be proposed.

An exceedance flow route is the pathway that surface water will take if a storm exceeds the design capacity of the drainage system. Even a well-designed system has a finite capacity, and storms more severe than the design event will produce flows that exceed it. The Drainage Strategy must demonstrate that these exceedance flows will be safely conveyed away from buildings and other sensitive areas without causing flooding to properties, either on or off site.

For major applications, the Lead Local Flood Authority (LLFA) — usually the county council or unitary authority — is the statutory consultee on surface water drainage matters. The LLFA reviews the Drainage Strategy and advises the local planning authority on its acceptability. For foul drainage, the water company (such as Thames Water, Anglian Water, etc.) should be consulted on capacity. The local planning authority makes the final decision taking all drainage advice into account.

Connection of surface water to the public combined or surface water sewer requires consent from the water company under Section 106 of the Water Industry Act 1991. However, the SuDS hierarchy requires that surface water discharge to a public sewer should be considered as a last resort. You must demonstrate that infiltration and discharge to a watercourse are not feasible before the LLFA will accept a sewer connection. Many water companies actively oppose new surface water connections to combined sewers due to capacity constraints.

A SuDS maintenance plan sets out the ongoing maintenance activities required to keep the drainage system functioning as designed throughout its lifetime. It covers routine tasks such as clearing debris from inlets, cutting vegetation in swales, inspecting and cleaning attenuation tanks, and monitoring infiltration performance. The planning authority will require a maintenance plan and details of the responsible management body as a condition of planning permission. If the SuDS are to be offered for adoption by the LLFA, they must meet adoption standards.

A Flood Risk Assessment (FRA) considers the risk of the site being flooded from external sources — rivers, the sea, groundwater, overland flow from adjacent land — and whether the development increases flood risk elsewhere. A Drainage Strategy focuses specifically on how the development manages its own surface water and foul drainage. In practice, both documents are often needed and should be consistent with each other. For sites in Flood Zones 2 or 3, the Drainage Strategy is typically incorporated into the FRA.

For a single new dwelling, most planning authorities require at least a drainage statement showing how surface water and foul water will be managed. This need not be a full Drainage Strategy with hydraulic modelling, but it should demonstrate a viable drainage approach — whether through soakaways, connection to existing drains, or other suitable means. If the site has particular drainage challenges (high water table, impermeable soils, flood risk), a more detailed assessment may be required.