Drainage Strategy vs Flood Risk Assessment: What's the Difference?
If you're putting together a planning application and someone has asked for both a Flood Risk Assessment and a drainage strategy, it's fair to wonder what the difference is and whether you really need two documents. The short answer to drainage strategy vs Flood Risk Assessment is that they answer different questions: a Flood Risk Assessment looks at all the ways flooding could affect your site (and how your site could affect others), while a drainage strategy deals specifically with how surface water runoff is managed once the development is built. They overlap, they're often submitted together, and on larger schemes you'll usually need both. But they are not the same thing, and confusing the two is a common reason planners come back with questions.
What a Flood Risk Assessment actually does
A Flood Risk Assessment (FRA) is a planning document that assesses the risk of flooding to and from a proposed development over its whole lifetime. That lifetime matters: residential schemes are generally assessed over around 100 years, so the assessment has to account for how risk will change with climate change, not just today's conditions.
Crucially, an FRA considers every source of flooding, not just rivers. That means:
- **Fluvial (river) flooding** — overtopping or breach of watercourses.
- **Tidal and coastal flooding** — relevant near the coast and tidal reaches.
- **Surface water (pluvial) flooding** — water running across the ground during heavy rain before it reaches a drain or watercourse.
- **Groundwater flooding** — rising water tables, which can be a real issue on certain geologies. We cover this in more detail on our groundwater flood risk assessment page.
- **Sewer and infrastructure flooding** — surcharging foul or combined sewers, and failure of artificial sources such as reservoirs or canals.
The FRA has two jobs. First, it shows that the development itself will be safe for its users, with appropriate finished floor levels, safe access and egress, and any mitigation needed. Second, it demonstrates that the proposal won't increase flood risk elsewhere. Depending on the site's flood zone and size, the assessment may also need to work through the Sequential Test and, where required, the Exception Test, which we explain on our Sequential and Exception Test page. You can read more about what goes into one on our main flood risk assessment page.
What a drainage strategy covers
A drainage strategy, often called a surface water drainage strategy or a sustainable drainage (SuDS) strategy, is narrower in focus. It deals with one thing: how rainwater that falls on your site will be managed, controlled and discharged once the scheme is built. It doesn't assess the risk of a river bursting its banks; it sets out the engineering approach to surface water.
A good drainage strategy works through the drainage hierarchy set out in national policy and Building Regulations. You're expected to discharge surface water in this order of preference:
1. Into the ground via infiltration (soakaways), where ground conditions and groundwater levels allow.
2. To a surface water body, such as a watercourse.
3. To a surface water sewer.
4. To a combined sewer, as a last resort.
The strategy then sets out the numbers the LPA and the Lead Local Flood Authority (LLFA) will scrutinise: the proposed discharge rate (how fast water is allowed to leave the site), the attenuation volume (how much storage is needed to hold water back during a storm so it can be released slowly), and the design storm events the system is checked against, including an allowance for climate change.
Betterment and greenfield runoff rates
This is where the idea of **betterment** comes in. On greenfield sites, the aim is usually to limit post-development runoff to the existing greenfield runoff rate so you're not making things worse downstream. On previously developed (brownfield) sites, many authorities expect a reduction on the existing rate — you'll often see targets such as a 50% betterment, meaning runoff is roughly halved compared with the current situation. The specific requirement varies by authority and by the receiving network, which is why early engagement with the LLFA pays off. You can read more about design principles on our SuDS page.
Do I need both, one, or the other?
This is the question most applicants actually want answered. There's no single rule, but the following holds true most of the time.
**Major developments usually need both.** If you're proposing 10 or more dwellings, or a site of 0.5 hectares or more, the LLFA is a statutory consultee on surface water drainage, and you'll typically be expected to provide a drainage strategy. If the site also sits in Flood Zone 2 or 3, or is larger than one hectare in Flood Zone 1, you'll need a Flood Risk Assessment as well. In practice the two are produced together and reference each other.
**Smaller sites may need only one.** A single dwelling or small extension in Flood Zone 1, away from any watercourse, might not need a full FRA at all, but could still need to show how surface water will be dealt with, particularly if you're increasing the impermeable area. Equally, a site might have a flood risk issue from a nearby river but discharge cleanly to an existing system, so the FRA is the main document.
**It can be one document or two.** A drainage or SuDS strategy can be a standalone report, or it can sit as a chapter within the Flood Risk Assessment. For smaller schemes, folding the drainage approach into the FRA keeps things simple. For larger or more technically complex sites, a separate detailed drainage strategy is often clearer for the LLFA to review.
How the two documents relate and overlap
The overlap sits squarely around surface water. The FRA has to consider surface water as one of its sources of flooding, and the drainage strategy is the detailed answer to how that surface water is controlled. So your drainage strategy effectively feeds the surface water section of the FRA, and the two must tell a consistent story. If the FRA says finished floor levels are set 300mm above a certain level, the drainage design has to work with that. If the drainage strategy relies on infiltration, the FRA needs to be comfortable that this won't aggravate groundwater issues.
Get them out of step and you create an easy target for a planning objection. Produced together, they reinforce each other.
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A simple decision guide
Use this as a starting point, then confirm with your LPA or LLFA:
- **In Flood Zone 1, under 1 hectare, minor development:** often a drainage strategy or a surface water statement is enough; a full FRA may not be required.
- **In Flood Zone 1, 1 hectare or more, or 10+ dwellings:** expect to need a drainage strategy, and an FRA covering surface water and other sources.
- **In Flood Zone 2 or 3, any size:** an FRA is required, almost always alongside a drainage strategy for anything but the smallest proposal, and you may also face the Sequential and Exception Tests.
When in doubt, the safest assumption on a major application is that you'll need both, properly coordinated.
Use this as a starting point, then confirm with your LPA or LLFA:
- **In Flood Zone 1, under 1 hectare, minor development:** often a drainage strategy or a surface water statement is enough; a full FRA may not be required.
- **In Flood Zone 1, 1 hectare or more, or 10+ dwellings:** expect to need a drainage strategy, and an FRA covering surface water and other sources.
- **In Flood Zone 2 or 3, any size:** an FRA is required, almost always alongside a drainage strategy for anything but the smallest proposal, and you may also face the Sequential and Exception Tests.
When in doubt, the safest assumption on a major application is that you'll need both, properly coordinated.
Get in Touch
If you're not sure whether your application needs a Flood Risk Assessment, a drainage strategy, or both, the quickest way to find out is to ask. We assess your site, tell you honestly what's required, and prepare what you need with a fast turnaround and fixed competitive fees, with reports from £450.
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Get in touch to discuss your site and receive a quote.
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