Securing planning permission is often the most challenging hurdle in any development project. While architectural aesthetics and density figures often grab the headlines, technical constraints frequently cause the most significant delays. Among these technical requirements, the management of surface water and foul water is paramount. A comprehensive drainage strategy is no longer an optional extra but a critical validation requirement for most major and minor planning applications across the UK.
At AJF Engineering Consultants, we understand that a well-prepared drainage strategy does more than tick a box for the Local Lead Flood Authority (LLFA). It unlocks site potential, ensures long-term viability, and protects local communities from flood risk. As a North West based firm of chartered engineering consultants, we provide innovative, director-led solutions to developers and contractors nationwide. This guide outlines the essential steps to preparing a drainage strategy that satisfies planning officers and paves the way for a smooth approval process.
Get in touch with us today by calling 0151 374 0545 or emailing us at info@ajfengineering.co.uk to see how we can help.
What Is a Drainage Strategy?
A drainage strategy is a technical report that details how a proposed development will manage surface water and foul water. It demonstrates to the Local Planning Authority (LPA) and the LLFA that the new development will not increase flood risk on the site or elsewhere.
The document must prove that the proposed drainage methods align with national standards and local policies. It serves as a bridge between the conceptual masterplan and the detailed engineering design that follows planning consent. A robust strategy assesses the site’s current hydrological characteristics and outlines how the altered landscape will handle rainfall, particularly during extreme weather events exacerbated by climate change.
Why Planning Approval Hinges on Drainage
In recent years, the regulatory landscape regarding water management has tightened significantly. The National Planning Policy Framework (NPPF) mandates that major developments must incorporate Sustainable Urban Drainage Systems (SuDS) unless there is clear evidence that this would be inappropriate.
Planning officers and LLFAs are under pressure to ensure that new developments are resilient. They will scrutinise your application to ensure that surface water runoff is managed at the source rather than simply being discharged into an overwhelmed sewer network. Without a credible drainage strategy, your application is likely to face objections from the LLFA or the water company, leading to costly delays or outright refusal.
Our portfolio of drainage projects across the UK demonstrates our ability to deliver successful outcomes even in challenging, high-risk environments.
Step 1: Site Assessment and Due Diligence
Before any design work begins, you must understand the existing conditions of your site. This initial data gathering is the foundation of a successful strategy.
Topography and Ground Conditions
You need to know how water currently flows across the land. Is the site steep, causing rapid runoff, or flat and prone to pooling? A topographic survey is essential. Furthermore, understanding the geology is critical for determining if infiltration (soaking water into the ground) is feasible. If the ground consists of heavy clay, infiltration may not be possible, whereas sandy soil suggests it should be the primary method of disposal.
Flood Risk Identification
Review the Environment Agency’s flood maps to determine if the site lies within a high-risk flood zone. If it does, a site-specific Flood Risk Assessment (FRA) will be required alongside the drainage strategy. This assessment must demonstrate that the development remains safe for its lifetime without increasing flood risk elsewhere.
Existing Infrastructure
Identify the location of existing public sewers and watercourses. You will need asset plans from the local water authority to understand connection points and capacity. This is vital for assessing the feasibility of foul and surface water connections later in the process.
Step 2: Following the Sustainable Drainage Hierarchy
When proposing how to dispose of surface water, you cannot simply choose the most convenient option. You must adhere to the drainage hierarchy set out in the Planning Practice Guidance. You must investigate discharge methods in the following order of priority:
- Into the ground (Infiltration): This is the preferred method. It replenishes groundwater and mimics natural processes.
- To a surface water body: If infiltration is not possible, you should discharge to a river, stream, or canal.
- To a surface water sewer or highway drain: If no watercourse is available.
- To a combined sewer: This is the last resort and requires strong justification, as combined sewers are prone to overflowing during storms.
Your drainage strategy must provide evidence for why a higher-priority method was discounted. For example, if you plan to connect to a sewer, you must provide percolation test results proving that the ground is unsuitable for soakaways.
Step 3: Calculating Runoff Rates and Volumes
A core principle of modern drainage design is that a new development should not discharge water faster than the site did prior to construction.
Greenfield vs Brownfield Rates
For greenfield sites (undeveloped land), the discharge rate should aim to match the natural greenfield runoff rate. For brownfield sites (previously developed land), the aim is usually to achieve a significant reduction in runoff rates, typically bringing them as close to greenfield rates as possible, or at least achieving a betterment of 30 percent to 50 percent depending on local policy.
Climate Change Allowances
Your calculations must account for the future. Current guidelines require drainage systems to accommodate a 1 in 100-year storm event plus a percentage allowance for climate change. This figure is typically 40 percent, but some authorities may require different values based on local catchment data. Failing to include these allowances is a common reason for LLFA objections.
Urban Creep
You must also factor in ‘urban creep’. This is the assumption that over the lifetime of a development, residents will pave over gardens or build extensions, increasing the impermeable area. An allowance of 10 percent is standard for residential developments.
Step 4: Integrating SuDS Features
The days of relying solely on underground plastic crates are fading. Planners now expect to see high-quality SuDS that provide multifunctionality. This means the drainage features should also offer amenity and biodiversity benefits.
Source Control
Manage water where it falls. This can be achieved through permeable paving on driveways, green roofs, and rain gardens. These features trap pollutants and slow down the flow of water before it enters the main system.
Site Control and Regional Control
As water moves through the site, it can be managed via swales, detention basins, or retention ponds. These features provide attenuation (storage) during heavy storms. They hold the water back and release it at the agreed restricted discharge rate. At AJF Engineering Consultants, our team of chartered engineers specialises in designing bespoke SuDS solutions that integrate seamlessly with the site layout, enhancing the visual appeal while performing a vital engineering function.
Step 5: Foul Water Management Strategy
While surface water often dominates the discussion, foul water strategy is equally important. You must demonstrate how wastewater from toilets, kitchens, and bathrooms will be disposed of.
The hierarchy for foul drainage is strictly connection to the public foul sewer. If this is not feasible, a package treatment plant may be considered. Septic tanks are generally the least preferred option.
If you propose connecting to the public sewer, you may need to submit a pre-development enquiry to the water company to confirm they have sufficient capacity at the wastewater treatment works. If capacity is an issue, we can assist in negotiating sewer requisitions or reinforcement works.
Step 6: preparing the Report and Drawings
The final output is a technical report accompanied by engineering drawings. This package should be clear, concise, and easy for the planning officer to interpret.
The Drainage Layout Plan
This drawing should show the proposed location of all drainage runs, manholes, attenuation features, and flow control devices. It should clearly differentiate between foul and surface water networks.
Hydraulic Modelling Results
You must include summary outputs from hydraulic modelling software (such as MicroDrainage or InfoWorks). These results prove that the system does not flood during the 1 in 30-year event and keeps flood water contained within the site boundary during the 1 in 100-year plus climate change event.
Maintenance and Management Plan
An often-overlooked component is the maintenance strategy. Who will adopt the drainage? Will it be offered to the water company under a Section 104 agreement, or will a private management company maintain it? The LLFA needs assurance that the SuDS features will be maintained for the lifetime of the development.
How AJF Engineering Consultants Can Help
Preparing a drainage strategy that withstands scrutiny requires a blend of technical expertise and regulatory knowledge. At AJF Engineering Consultants, we offer a refreshing, director-led service that places technical excellence at the forefront.
Expertise and Innovation
Our team of chartered engineers is experienced in delivering all types of civil and structural engineering schemes. We use the latest industry-standard software to model complex drainage networks, ensuring efficiency and accuracy. Our bespoke approach means we do not just copy and paste standard details. We analyse your specific site constraints to engineer a solution that maximises developable area while meeting all compliance targets.
Navigating Regulatory Hurdles
We deal with LLFAs and water companies daily. We know how to avoid LLFA objections by presenting the right information at the right time. Whether it involves negotiating discharge rates or justifying a specific SuDS component, we act as your advocate. We also have extensive experience with Sewer connections (S106) and Sewer Diversions, ensuring that even complex infrastructure challenges are managed professionally.
Comprehensive Service
Our capabilities extend beyond just the planning phase. We support developers through the detailed design, technical approval, and construction phases. From initial flood risk assessments to final adoption, our goal is to ensure your project is delivered efficiently and professionally.
Conclusion
A well-prepared drainage strategy is the backbone of a successful planning application. It requires early engagement, technical rigour, and a creative approach to problem-solving. By prioritising SuDS and understanding the specific requirements of your local authority, you can transform a potential constraint into an asset for your development.
Don’t let drainage issues delay your project. Trust the experts to guide you through the process.
Get in touch with us today by calling 0151 374 0545 or emailing us at info@ajfengineering.co.uk to see how we can help.