Surface Water Drainage Strategy

A Surface Water Drainage Strategy is usually required as part of a planning submission. Its purpose is to explain how rainfall will be dealt with across the site once the development is built.

At its core it asks a simple question: what happens to rainwater now, and what will happen once the site is developed?

To answer that, the strategy looks at the existing site conditions alongside the proposed layout. When development introduces new roofs, driveways, patios or other hard surfaces, areas that previously allowed water to soak into the ground can become impermeable. Water that once infiltrated naturally therefore needs somewhere else to go.

The drainage strategy sets out how that will be managed, ensuring the development does not increase flood risk on the site itself or elsewhere.

A typical drainage strategy typically includes:

• Existing drainage review
• Runoff and discharge assessment
• SuDS options
• Hydraulic calculations
• Compliance with NPPF and local guidance

Planning authorities now require a drainage strategy for any development impacting drainage. The inclusion of SuDS is a consideration in the awarding of sustainable certifications, such as BREEAM credits.

Surface water drainage strategies are prepared in line with the National Planning Policy Framework (NPPF) and national guidance on Sustainable Drainage Systems (SuDS).

A key principle within this guidance is the drainage hierarchy. In simple terms, this means dealing with rainwater as close to where it falls as possible, rather than simply piping it away.

Developers are expected to consider options such as reuse or infiltration first, before looking at connections to traditional drainage systems such as sewers.

The National Standards for Sustainable Drainage Systems set out a clear order of preference for where runoff should go. Designers are normally expected to demonstrate that higher priority options have been considered before moving further down the hierarchy.

The hierarchy typically follows this order:

• Priority 1 – Rainwater collected and reused for non-potable uses
• Priority 2 – Infiltration to ground
• Priority 3 – Discharge to a surface water body (such as a ditch or watercourse)
• Priority 4 – Discharge to a surface water sewer or other piped drainage system
• Priority 5 – Discharge to a combined sewer

Most planning authorities now expect drainage to be considered early in the design process, rather than treated as something to resolve at the end of the planning stage.

The objectives are to:

• Prevent surface water problems being pushed elsewhere
• Avoid overloading existing drainage infrastructure
• Ensure the development itself is not exposed to avoidable flood risks

When necessary, the Surface Water Drainage Strategy accompanies a Flood Risk Assessment.

Drainage is now a requirement for almost all planning applications. The application will need to demonstrate how runoff is being managed in line with both national policy and any local authority guidance for the area too.

Paragraph 182 of the NPPF requires developments to incorporate sustainable drainage that:

• Controls flow rates.
• Reduces runoff.
• Improves water quality.
• Supports biodiversity.
• Provides amenity wherever possible.

Paragraph 182 of the NPPF States:

“Applications which could affect drainage on or around the site should incorporate sustainable drainage systems to control flow rates and reduce volumes of runoff, and which are proportionate to the nature and scale of the proposal. These should provide multifunctional benefits wherever possible, through facilitating improvements in water quality and biodiversity, as well as benefits for amenity.”

Pre-Planning Guidance

Pre-application advice may not reference drainage if it pre-dates 2025 and doesn’t reflect the latest policy.
Pre-planning responses provide initial authority views but do not override current policy or validation requirements. Therefore, developments with earlier advice may still need to submit a surface water drainage strategy under updated policies.
It’s always worth checking your pre-application advice against the latest planning requirements before you submit anything.

What Happens After Planning

Once you’ve got planning permission, the strategy is turned into a Detailed Drainage Design. This is where the big ideas become real – with calculations, drawings, pipe levels and everything needed to actually build it.

Planning permission often comes with conditions, like providing detailed drainage designs and a plan for how everything will be managed and maintained – especially since most SuDS aren’t adopted by the local authority.

People sometimes use the terms interchangeably, but calling it a ‘SuDS Report’ can give the impression it’s just a generic template.

A drainage strategy should be a site-specific engineering assessment that considers:

• Existing drainage infrastructure
• Wider catchment context
• Greenfield and post development runoff
• Soil conditions and infiltration potential
• Site layout, levels and engineering constraints
• The scale and nature of the proposed development

When it’s done properly, the drainage strategy isn’t just a tick-box for planning – it’s a key document that guides how water is managed right through the development process.

Effective surface water design can provide benefits beyond merely removing water from the site.

SuDS slow runoff, store water during storms, and release it gradually, closely reflecting natural drainage. This helps reduce peak flows and manage flood risk more effectively.

SuDS on Smaller Developments

Smaller developments often use simpler features such as:

• Soakaways
• Permeable paving
• Rainwater harvesting
• Rain gardens or water butts

Even simple measures can make a big difference to water management, especially if you think about them early. In the past, drainage was often left to the contractor with a vague note like ‘drainage by others’ – and that’s when problems and avoidable flooding crept in.

“As part of our wider flood risk work, we’ve visited many sites where drainage hadn’t been considered properly at the outset or tailored to the conditions of the land they were built on. We were later brought in to investigate, model the site and review the drainage, often after damage had already occurred. It added cost for the client and avoidable disruption, which is why I’m keen to see proper drainage assessment become a requirement from day one.”

Simon Crowther BEng (Hons) FCIWEM C.WEM MIET

Director, FPS Environmental

SuDS on Larger Developments

Larger schemes often incorporate SuDS into the overall masterplan, including:

• Swales
• Attenuation basins and ponds
• Permeable paving
• Underground storage/tanks
• Wetlands and landscaped features

When SuDS are viable, they can boost biodiversity, improve water quality, and create green spaces people want to use. There’s even research showing that good SuDS can make developments more desirable and valuable.

Sustainable Drainage Insights

How SuDS Are Transforming Drainage and Placemaking in Modern Developments.

How Biodiversity Net Gain (BNG) and SuDS Work Together in Development.

A drainage strategy identifies the most suitable SuDS features for each site. Common examples include:

Swales
Shallow planted channels that convey, treat, or store runoff.

Attenuation basins and ponds
Provide temporary storage during heavy rainfall events.

Permeable paving
Allows water to infiltrate or pass into underlying storage where soil conditions permit.

Soakaways and infiltration systems
Used where ground conditions permit natural infiltration.

Underground attenuation tanks
Used on constrained sites to store water before controlled discharge.

A robust strategy requires key information, including:

• Site boundary and location
• Proposed layout
• Existing drainage details
• Levels (where available)
• Soil conditions and infiltration testing
• Discharge locations

It can be tempting to skip infiltration testing in an attempt to save on costs, but in reality it is often a false economy. Without test results, the drainage design has to assume a worst-case position on infiltration, which can push schemes towards more engineered solutions or sometimes limit what can realistically be built on the site.

Carrying out infiltration testing early allows the drainage strategy to be based on the ground conditions. That often leads to simpler, more efficient and more cost-effective drainage solutions – which are cheaper to build too.

On larger sites it is also common for ground conditions to vary across the plot, which means multiple infiltration tests may be needed.

Alex is a qualified Civil Engineer BEng (Hons) with 12 years industry experience in the development sector, with specialisms ranging between Flood Risk, Drainage Design, Planning and Development Constraints, Construction, and Highways.

At FPS he heads up the Flood Risk and Civil Engineering team, with a focus on quality and timely delivery of engineering consulting. Having worked nationally on a number of high-profile development projects, he understands local level requirements and the risks that are applicable on the many projects that FPS are involved with.

Surface Water Drainage Strategies often form part of broader development engineering work.

FPS Environmental also supports:

• Flood Risk Assessments
• Land Development Engineering
• Detailed Drainage Design
• External Works, Level Design & Earthworks
• Construction Environmental Management Plans