What Is a Drainage Strategy? Everything You Need to Know

A drainage strategy is one of the most critical technical documents in any planning application, yet it is frequently misunderstood, underestimated, or left to the last minute. The consequence is delay — sometimes measured in months — as the Lead Local Flood Authority, the water company, or the planning officer requests additional information, amendments, or a complete redesign.

This article explains what a drainage strategy is, when you need one, what it should contain, and how to navigate the technical and regulatory requirements to get it right first time.

Definition

A drainage strategy is a document that sets out how surface water (rainwater) and foul water (sewage) from a proposed development will be collected, managed, and disposed of. It is a planning document — its purpose is to demonstrate to the local planning authority and statutory consultees that the drainage arrangements for the development are technically sound, comply with policy, and will not increase flood risk or create a nuisance.

A drainage strategy is not the same as a detailed drainage design. The strategy sets out the principles and approach; the detailed design specifies exact pipe sizes, levels, materials, and construction details. In most cases, the strategy is submitted with the planning application, and the detailed design is prepared later, often as a condition of planning approval.

When Do You Need One?

Major Development

For major development — defined as 10 or more dwellings, or a site area of 0.5 hectares or more, or for non-residential development, a floor area of 1,000 sq m or more, or a site area of 1 hectare or more — a drainage strategy is almost always required as part of the planning application.

The Lead Local Flood Authority is a statutory consultee on major development applications for surface water drainage, and most LLFAs have published their own requirements for drainage strategies.

Minor Development

For minor development (below the major thresholds), the requirements vary by LPA. Some LPAs require a drainage strategy for all applications; others only require one where there is an identified flood risk or drainage constraint. Check the LPA’s validation checklist.

Householder Applications

Householder applications (extensions and alterations) do not usually require a full drainage strategy, although the LPA may request information about how surface water from the extension will be managed, particularly in flood-risk areas or Critical Drainage Areas.

Change of Use

Change of use applications may require a drainage strategy where the change of use involves external works that alter the impermeable area, or where the new use generates different foul drainage demands.

What Should a Drainage Strategy Contain?

A comprehensive drainage strategy should address both surface water and foul water drainage, and should contain the following elements:

1. Site Assessment

Existing drainage. The strategy should describe the existing drainage arrangements on the site. For brownfield sites, this may include existing sewers, soakaways, and connections to public sewers. For greenfield sites, the existing drainage is typically overland flow to watercourses or natural infiltration.

Topography. The site’s topography determines how surface water flows across the site and where it accumulates. A topographic survey is essential.

Ground conditions. The infiltration potential of the soil is a critical factor in the drainage design. This is typically determined through infiltration testing (BRE 365 soakaway tests or falling head tests) and/or geotechnical investigation.

Groundwater levels. High groundwater levels can limit the use of infiltration-based drainage and soakaways. Groundwater monitoring may be required.

Flood risk. The drainage strategy should be consistent with the Flood Risk Assessment (if one is required) and should address any flood risk identified in the FRA.

Receiving watercourses and sewers. The strategy should identify the receiving watercourse or sewer for surface water and foul water discharge, and confirm that there is capacity to accept the proposed discharge.

2. Surface Water Drainage

Surface water drainage is the management of rainwater that falls on the development. The key principles are:

The SuDS hierarchy. The drainage strategy must follow the SuDS (Sustainable Drainage Systems) hierarchy, which prioritises:

1. Infiltration — disposing of surface water by soaking it into the ground (soakaways, permeable paving, infiltration basins)
2. Attenuation and discharge to a watercourse — storing surface water temporarily and releasing it slowly to a nearby river, stream, or ditch
3. Attenuation and discharge to a surface water sewer — as above, but discharging to the public surface water sewer network
4. Attenuation and discharge to a combined sewer — the least preferred option, discharging to a sewer that carries both surface water and foul water

The strategy must demonstrate why each higher-priority option has been discounted before resorting to a lower-priority option. For example, if the strategy proposes discharge to a watercourse rather than infiltration, it must explain why infiltration is not feasible (e.g., because the ground conditions do not support it, or because groundwater levels are too high).

Greenfield runoff rates. For greenfield sites, the surface water discharge rate from the development must not exceed the existing greenfield runoff rate. This is calculated using standard methods (typically the IH124 method or the FEH statistical method) and is expressed as a rate in litres per second.

For brownfield sites, the discharge rate should be reduced to as close to greenfield as practicable. Many LLFAs require a minimum reduction of 50% from the existing brownfield rate, and some require discharge at greenfield rates regardless of the existing situation.

Attenuation storage. Where the development cannot discharge surface water at the rate it falls, attenuation storage is required. This is typically provided by:

• Attenuation tanks (underground geocellular storage)
• Attenuation ponds or basins
• Swales (vegetated channels)
• Oversized pipes or channels
• Green roofs (which retain a proportion of rainfall)

The volume of attenuation storage required is calculated using a computer model that simulates the development’s drainage network under a range of rainfall events, up to and including the 1% annual probability event (1 in 100 year storm) with a climate change allowance.

Climate change. The drainage design must account for climate change by applying the Environment Agency’s peak rainfall intensity allowances. The current allowances range from +5% to +40% depending on the time horizon and allowance category. Most LLFAs require the use of the “upper end” allowance for the lifetime of the development.

Exceedance. The strategy must address what happens when the drainage system capacity is exceeded — for example, in a storm event more severe than the design event. The exceedance strategy should demonstrate that:

• Excess surface water is directed away from buildings
• Excess surface water is contained within the site or directed to safe flood routes
• Excess surface water does not increase flood risk to neighbouring properties

Water quality. SuDS should provide appropriate treatment of surface water before it is discharged. The level of treatment required depends on the pollution risk of the surface — for example, car parks and roads generate more polluted runoff than roofs and gardens. The CIRIA SuDS Manual (C753) provides guidance on treatment requirements and the Simple Index Approach for assessing water quality.

3. Foul Water Drainage

Foul water drainage deals with sewage from the development. The strategy should address:

Connection to the public sewer. In most cases, foul water from the development will be discharged to the public foul sewer network. The strategy should identify the proposed point of connection and confirm that the sewer has capacity. A pre-development enquiry to the water company (Section 98/Section 106 of the Water Industry Act 1991) is recommended.

On-site treatment. Where a public sewer connection is not available (typically in rural areas), the strategy should propose an on-site treatment system — either a package treatment plant or, in some circumstances, a septic tank. The Environment Agency’s General Binding Rules or an environmental permit may apply.

Foul drainage assessment. For sites where a public sewer connection is not available, a Foul Drainage Assessment (FDA1 form) is required as part of the planning application.

4. SuDS Design

The drainage strategy should identify the SuDS features that will be used and explain how they integrate with the site layout:

Source control. SuDS features that manage surface water as close to where it falls as possible. Examples include permeable paving, green roofs, rain gardens, and water butts.

Site control. SuDS features that manage surface water from a wider area within the development. Examples include swales, attenuation basins, and detention ponds.

Regional control. SuDS features that manage surface water from the entire development or a large sub-catchment. Examples include large attenuation ponds, wetlands, and regional detention basins.

The SuDS design should follow the “management train” concept, using a sequence of SuDS features to progressively manage and treat surface water as it moves through the development.

5. Adoption and Maintenance

A drainage strategy that does not address long-term maintenance and adoption is incomplete. The strategy should explain:

Who will adopt the drainage infrastructure? Options include:

• The water company (through a Section 104 agreement for sewers)
• The highway authority (for highway drainage)
• A management company (for private drainage and SuDS)
• The local authority (in some circumstances)

How will SuDS be maintained? SuDS features require ongoing maintenance to function correctly. The strategy should include a maintenance plan that covers:

• Routine maintenance tasks (mowing, clearing inlets/outlets, removing sediment)
• Frequency of maintenance
• Responsibility for maintenance
• Estimated annual maintenance costs
• Arrangements for inspection and monitoring

SAB approval (Wales). In Wales, Schedule 3 of the Flood and Water Management Act 2010 has been commenced, creating SuDS Approving Bodies (SABs) that must approve drainage systems before construction can begin. The drainage strategy for Welsh sites must comply with SAB requirements.

Who Reviews the Drainage Strategy?

The Lead Local Flood Authority (LLFA)

The LLFA is a statutory consultee on major development applications for surface water drainage. The LLFA reviews the drainage strategy to ensure it complies with national and local policy, and provides comments or objections to the LPA. Common LLFA objections include:

• Failure to follow the SuDS hierarchy
• Discharge rates higher than greenfield
• Insufficient attenuation storage
• Inadequate climate change allowance
• No exceedance strategy
• No maintenance plan

The Water Company

The water company reviews proposed connections to the public sewer network. Where the drainage strategy proposes discharge to a public sewer, the water company will need to confirm that there is capacity and agree the point of connection and maximum discharge rate.

The Environment Agency

The Environment Agency may review the drainage strategy where the development is in a flood zone or where there is a proposed discharge to a main river or to groundwater.

The LPA

The LPA coordinates the responses from the LLFA, water company, and EA, and makes the final decision on whether the drainage strategy is acceptable.

Common Mistakes

Not Testing Infiltration

Many drainage strategies assume infiltration is not feasible without testing it. The LLFA will typically require evidence that infiltration has been properly investigated before accepting a lower-priority drainage solution. BRE 365 soakaway testing is the standard method and should be carried out early in the development process.

Underestimating Attenuation

Underestimating the volume of attenuation storage required is a common error that can lead to drainage systems that are inadequate for the design storm event. Use appropriate computer modelling software (such as MicroDrainage or InfoDrainage) and check the results against the LLFA’s requirements.

Ignoring Exceedance

A drainage strategy that does not include an exceedance strategy is incomplete. The LLFA will want to see evidence that the development is safe even when the drainage system capacity is exceeded.

Leaving Drainage to the Last Minute

Drainage should be considered from the earliest stages of the site layout design. Retrofitting SuDS into a layout that has already been designed without drainage in mind often results in suboptimal solutions, increased costs, and planning delays.

Not Engaging with the LLFA

Each LLFA has its own specific requirements and preferences. Some require greenfield runoff rates for all development; others accept a betterment approach for brownfield sites. Some have specific SuDS preferences; others are more flexible. Pre-application engagement with the LLFA can save significant time and cost.

Cost and Timescale

The cost of a drainage strategy depends on the scale and complexity of the development:

Development Type	Typical Cost	Typical Timescale
Single dwelling / minor	GBP 500 - GBP 1,500	2-3 weeks
Minor residential (2-9 units)	GBP 1,500 - GBP 3,000	3-4 weeks
Major residential (10+ units)	GBP 3,000 - GBP 8,000	4-8 weeks
Large commercial / industrial	GBP 5,000 - GBP 15,000+	6-12 weeks

These costs include the drainage strategy report, computer modelling, and SuDS design. They do not include infiltration testing (typically GBP 1,000 - GBP 3,000 depending on the number of trial pits) or detailed drainage design (which is usually prepared post-planning).

How Aegaea Can Help

Aegaea’s civils team prepares drainage strategies for developments of all types and sizes across England, Scotland, and Wales. Our Chartered engineers design drainage systems that comply with national and local policy, satisfy the LLFA and water company, and integrate with the site layout.

We offer a full drainage consultancy service, from initial feasibility assessment and infiltration testing through to detailed drainage design and SAB applications (in Wales). We also prepare the supporting Flood Risk Assessments where required.

If you need a drainage strategy for a planning application, contact us for a no-obligation quote.