Surface water drainage strategy
A strategy for managing rainwater from roofs and hard surfaces so it doesn't increase flood risk.
About this service
When you replace a garden with a building, patio or driveway, rain that once soaked into the ground runs off much faster. Across London, thousands of small developments doing this add up to sewer flooding in heavy storms. That is why councils now ask many developments to show how they will manage surface water. A surface water drainage strategy sets this out: how much rainwater your site will produce, where it will go and how fast it will be released.
We prepare strategies for new houses, small residential schemes, mixed-use buildings and larger extensions where the council has asked for one. The work starts with runoff calculations for the existing and proposed site. We then look at the options in order of preference, from reusing and infiltrating water through to storing it and releasing it slowly to a sewer, and propose a scheme that suits the site, the ground and the budget.
On clay sites, which covers much of London, soakaways often struggle, so attenuation storage with a flow control is common. If the council expects sustainable drainage features, our SuDS strategy goes further into green roofs, permeable paving and rain gardens. Whichever route the site allows, all our drainage calculations go through a three-step quality control process, with final sign-off by our Head of Engineering.
When you need it
- A planning application needs a surface water strategy
- You're adding roofs or hardstanding
- The council requires controlled discharge rates
What's included
- Runoff calculations
- Attenuation and discharge proposals
- Outline layout
How it works
- Assess the existing siteWe measure existing roof and hard surface areas, check site levels, ground conditions and sewer records, and establish how the site drains now and what discharge rate the council expects.
- Calculate runoffWe calculate runoff for the proposed development across a range of storm events, including an allowance for climate change, and work out how much storage is needed to meet the target rate.
- Choose a discharge routeFollowing the London drainage hierarchy, we test whether water can be reused, infiltrated or sent to a watercourse before falling back on a surface water or combined sewer.
- Propose attenuationWe size storage such as tanks, crates, blue roofs or oversized pipes, with a flow control to limit discharge, and position them sensibly on the site.
- Issue the strategyWe issue the report and an outline drainage layout for the planning submission, and can later develop it into a detailed design to discharge drainage conditions.
What you receive
Why discharge rates drive the design
Most of the cost in a surface water strategy comes from the discharge rate the council will accept. Many London boroughs ask new development to reduce runoff as close as practical to the rate the site would produce as undeveloped land, often called the greenfield rate. Getting there on a tight urban plot can require a surprising amount of storage, so it is worth knowing early. Your planning officer or the Lead Local Flood Authority will confirm the target for your site.
Ground conditions are the other big factor. London Clay drains very slowly, so infiltration is often limited, while sites on river terrace gravels may suit soakaways well. Rather than guess, a soakaway and infiltration test gives real numbers. Storage also has to fit around foundations and trees, which is why we look at the drainage and structure together where we can.
Frequently asked questions
What is a surface water drainage strategy?
It is a report showing how rainwater from roofs, paving and other hard surfaces on a development will be managed so it does not increase flood risk. It covers runoff calculations, the chosen discharge route, any storage needed to slow flows down and an outline layout. Councils often request one with planning applications for new homes and larger extensions.
What is the London drainage hierarchy?
It is the order of preference for disposing of surface water set out in London planning policy. Broadly, it favours storing rainwater for reuse first, then letting it soak into the ground, then using green and blue features or open water, then releasing it to a watercourse, and only then to a surface water sewer and finally a combined sewer. Each step must be considered before moving down.
What is a greenfield runoff rate?
It is the rate at which rain would run off the site if it were undeveloped land. Councils use it as a benchmark, often asking developers to limit discharge from new buildings to that rate or close to it. Achieving it usually means storing rainwater on site and releasing it slowly through a flow control. Your council sets the target, and we can confirm it early.
How much does a surface water drainage strategy cost?
Our surface water drainage strategies start from £700 + VAT. The fee depends on the site size, the number of buildings and whether ground testing or a detailed layout is needed. We confirm the fixed fee in writing before starting, and our typical turnaround is 10 working days from instruction, confirmed with the fee.