Flood Risk Assessment in Cornwall

Our consultants prepare comprehensive flood risk assessments and surface-water drainage strategies for sites across Cornwall, from the Tamar valley in the east to West Penwith. A flood risk assessment in Cornwall shows your local planning authority that a development will stay safe from flooding for its whole lifetime, allowing for climate change, without pushing flood risk onto anyone else. We prepare flood risk assessments in Cornwall for planning, property purchase, mortgage and insurance, and BREEAM, alongside sequential and exception tests and sustainable drainage (SuDS) strategies — dealing directly with the Environment Agency and Cornwall Council on your behalf.

Cornwall is a county of steep, fast-responding catchments and a long, exposed coastline, which makes flooding here distinctive. Atlantic rain falls on hard granite and slate that sheds water quickly, narrow valleys funnel it into historic harbour towns, and spring tides and storm surges push back up the estuaries. The result is some of the most dramatic flash flooding the UK has seen, and a planning system that scrutinises drainage and surface water closely. We translate that local picture into a report the regulators will accept.

From the flash-flood catchments of Boscastle and Coverack to the tidal streets of Looe and Truro, Cornwall floods in ways a generic, desk-based report rarely captures — which is why a site-specific flood risk assessment in Cornwall matters here.

Do you need a flood risk assessment in Cornwall?

You need a flood risk assessment for most development in Cornwall that sits in Flood Zone 2 or 3, for any site of one hectare or more even in Flood Zone 1, and increasingly for smaller Flood Zone 1 sites where the Environment Agency Flood Map for Planning shows surface water risk or the site falls inside one of Cornwall's Critical Drainage Areas. If a report is required but missing, Cornwall Council can refuse the application or hold it pending further information.

• Flood Zone 2 or 3. Any more vulnerable use — housing, for example — in the medium or high-probability floodplain needs a site-specific flood risk assessment for planning, and usually the Sequential Test as well.
• One hectare or more in Flood Zone 1. The national major-development threshold triggers an assessment even where the river and sea risk is low.
• Surface water and Critical Drainage Areas. A Flood Zone 1 site can still need an assessment where the Flood Map for Planning shows surface water risk or the site lies in a Critical Drainage Area — a frequent trigger in Cornwall's towns.
• A drainage strategy on top. Ten or more dwellings, sites over one hectare or 1,000m² of floorspace, or any scheme adding impermeable area, will also need a sustainable drainage strategy.
• Not sure? Send us the site address or postcode and we will tell you quickly, at no cost, whether an assessment is needed and what it has to cover.

Flood risk across Cornwall: rivers, coast and surface water

Cornwall faces flooding from every direction. Short, steep rivers such as the Camel, Fowey, Fal, Cober and the Looe rivers rise within hours of heavy rain; estuary towns from Truro to Looe flood on high spring tides and storm surges; and surface water overwhelms drains in the dense, low-lying cores of towns like Penzance, St Austell and Falmouth. Most Cornish sites face more than one of these sources at once, which is exactly why a generic, desk-based report rarely satisfies the regulators here.

Surface water is now the most widespread source of flood risk in England, and in Cornwall's tightly drained harbour and valley towns it is often the issue that decides whether an application is approved.

Check flood risk anywhere in Cornwall

Cornwall is outlined below. Screen any postcode in the county against Environment Agency data for rivers and sea, surface water and the planning flood zones, with a climate outlook to 2125. It is an indicative first pass for context, not a substitute for a site-specific flood risk assessment.

Critical Drainage Areas in Cornwall

A Critical Drainage Area (CDA) is a place with recognised critical surface-water drainage problems, notified to the local planning authority by the Environment Agency. Inside a CDA, Cornwall Council expects new development to reduce flood risk downstream rather than leave it unchanged — so the bar is betterment, not a neutral impact. A CDA can pull a flood risk assessment and a detailed drainage strategy into play even when a site sits comfortably in Flood Zone 1, because national guidance requires flood risk to be considered from every source, not just rivers and the sea.

This matters more in Cornwall than in most counties, because Cornwall Council has formally designated a long list of CDAs covering many of its main towns. If your site falls inside one, expect close scrutiny of runoff rates, attenuation, exceedance routing and SuDS provision, and budget for it from the start. The Environment Agency Flood Map for Planning shows flood zones but does not show CDAs, so the boundaries have to be checked against Cornwall Council's own mapping.

• Betterment, not neutrality. A scheme in a Cornwall CDA has to show it lowers flood risk downstream, typically by holding runoff below greenfield rates and adding storage — a stricter test than the usual “no worse than before”.
• It can apply in Flood Zone 1. Being clear of the river floodplain does not exempt a CDA site; surface water and drainage are assessed in their own right.
• Householder and minor schemes too. Cornwall Council asks even small extensions in a CDA, or in Flood Zones 2 and 3, to complete its householder and minor-extension flood risk assessment process.
• Check the boundary early. CDA limits come from Cornwall Council's mapping, not the national flood maps, so confirm whether your site is in or out before the layout is fixed.
• Recent local precedent. Cornwall's CDAs shape real decisions — a 527-home scheme at Bodmin reached reserved matters within a designated critical drainage area, which we unpack in what planning policy actually requires.

Local flood risk and drainage policy in Cornwall

Two tiers of authority set the rules. The Environment Agency leads on the main rivers, the sea and reservoirs, while Cornwall Council, as the Lead Local Flood Authority (LLFA), leads on surface water, groundwater and ordinary watercourses. South West Water is responsible for public sewers, and National Highways for the A30 and A38. A flood risk assessment and drainage strategy in Cornwall has to satisfy national planning policy and this specific local framework at the same time.

Cornwall Council has been the LLFA since the Flood and Water Management Act 2010. It adopted its first Local Flood Risk Management Strategy in November 2014, refreshed it in 2020 with a stronger emphasis on climate change, nature-based solutions and property flood resilience, and is due to review it again in 2026. As a unitary authority Cornwall is also the planning authority, the highway authority and the coast protection authority, so on most Cornish sites you are dealing with one council wearing several hats, alongside the Environment Agency and the role the LLFA plays as statutory consultee on surface water drainage.

Cornwall Local Plan Policy 26 (Flood Risk Management and Coastal Change) requires development to avoid flood-risk areas in the first instance, to be consistent with the Shoreline Management Plan and catchment plans, and to decrease surface-water runoff — particularly in Critical Drainage Areas — through sustainable drainage. The Climate Emergency Development Plan Document goes further: Policy CC4 expects SuDS to control surface water as close to source as possible and to be visible, multifunctional and biodiverse, with non-buried systems prioritised in town centres, commercial areas and redevelopment.

Cornwall's SuDS and drainage requirements

Cornwall Council publishes a detailed SuDS for Cornwall guidance suite that a drainage strategy has to engage with directly. It is more prescriptive than many counties', and getting ahead of it is the surest way to avoid an LLFA drainage objection that stalls determination.

• The drainage hierarchy. Discharge must follow the SuDS hierarchy — reuse or infiltrate first, then to a watercourse, then a surface-water sewer, with the combined sewer a last resort — in line with the four pillars of SuDS.
• Greenfield runoff rates. Cornwall's guidance expects post-development discharge no greater than the theoretical greenfield rates for the 1, 10, 30 and 100-year storms, with attenuation sized for climate change.
• Infiltration evidence. Where soakaways are proposed, the LLFA expects BRE 365 infiltration testing rather than assumed rates — often a real constraint on Cornwall's granite and slate. Our free geology and ground-conditions checker and BRE 365 calculator give an early read.
• Groundwater monitoring. Cornwall's LLFA groundwater monitoring advice (updated 2024) sets out when a season of groundwater monitoring is needed to justify infiltration — data that takes months to gather, so it pays to start early.
• Rainfall data. Cornwall has a standardised approach to rainfall model data shared with the Devon LLFAs, so a strategy should use the agreed inputs from the outset.
• Ordinary watercourse consent. Works to culvert, divert or build over an ordinary watercourse need land drainage consent from Cornwall Council, on top of any SuDS approval — a common riparian responsibility on rural and edge-of-settlement sites.

Groundwater and ground conditions in Cornwall

Cornwall's geology cuts two ways for drainage. The granite uplands of Bodmin Moor, Carnmenellis and West Penwith, and the slates and shales (“killas”) that cover much of the county, are largely low-permeability, so infiltration drainage often fails a BRE 365 test and an alternative discharge route has to be ready. The legacy of metal and china-clay mining adds made ground, shafts, adits and altered groundwater pathways that can surface as flooding or undermine soakaway assumptions. Where the Environment Agency or Cornwall Council raises it, groundwater has to be assessed within the flood risk assessment — it appears on no national flood map and is governed by local geology, not a nearby river. On sites where it is a genuine constraint, our groundwater flood risk assessment derives a defensible design water table and tests it against floor and foundation levels.

A history of flooding in Cornwall

Cornwall's flood record is part of why its planning authority scrutinises development so closely, and why an assessment here has to be evidence-led rather than generic. The county has produced some of the most severe flash floods in modern British history, and each event reshaped the defences and policy a developer now designs around.

• Boscastle, 16 August 2004. A stationary storm dropped more than 200mm of rain in around five hours over a tiny, steep catchment, with the Valency reaching a peak discharge of roughly 140 cubic metres per second and a time-to-peak under 45 minutes. We look at the lasting lessons in our Boscastle 2004 analysis.
• Coverack, 18 July 2017. The Lizard village took around 200mm of rain in roughly three hours — among the most intense rainfall ever recorded in the UK — in a flash flood that tore through homes and the seafront.
• Looe, recurring. South-east Cornwall's harbour town floods on high tides up to several times a year and is widely described as the UK's most flood-prone town, which is why Cornwall Council and the Environment Agency are progressing a major tidal flood defence scheme.
• Mevagissey. A steep, rapid-response catchment with a long flood history; the Environment Agency introduced a dedicated flood warning service for the village in 2021.
• Truro, 1988. Heavy rain coinciding with a spring tide on the Fal flooded the city centre; the defences built since include a tidal barrier on the Truro River and flood storage on the River Kenwyn at New Mill.
• Storm Chandra, January 2026. Prolonged rain on saturated ground put dozens of flood warnings and alerts in force across Devon and Cornwall, a reminder that the risk is current, not historic — see our Storm Chandra coverage.

No defence removes flood risk entirely, so a defended Cornish site can still sit in a flood zone and must be assessed for the risk that remains if a barrier is overtopped or fails.

How Unda helps with flood risk assessment in Cornwall

We act as the accurate translation layer between your scheme and the requirements set by the Environment Agency, Cornwall Council as Lead Local Flood Authority, and national planning policy. We are experts on Cornwall's flooding and its local drainage rules, not a generic report mill, and we stay involved past the report to answer consultee questions.

A flood risk assessment in Cornwall from Unda assesses every credible source — the rivers and their estuaries, the tides, surface water, groundwater, sewers and any ordinary watercourse on the boundary — compares modelled flood levels against your proposed floor levels, sets safe access and egress, and recommends mitigation the regulators will accept. We are a CIWEM Business Partner with chartered members, and a senior consultant signs off every report. Where Cornwall Council needs it, a surface-water drainage strategy sits alongside the assessment as a separate report so the two cases stay consistent and a drainage objection has nowhere to land.

Flood risk assessments and related reports for every situation in Cornwall

A flood risk assessment, a sequential or exception test and a drainage strategy are three distinct reports. They often travel together in a planning submission, but they answer different questions: is the site safe, should the development be here, and where does the surface water go. We prepare each across Cornwall.

• Flood risk assessment for planning. The site-specific report prepared to satisfy the NPPF, the Environment Agency and Cornwall Council. Groundwater and, where needed, flood modelling are assessed within it.
• Flood risk survey for property purchase. An independent, property-specific survey for buyers and their advisers when a conveyancing search flags flood risk on a Cornish home.
• Flood risk assessment for insurance. Property-level evidence for brokers and underwriters where cover has been restricted, declined or repriced — relevant given how flood risk affects value and insurability.
• Flood risk assessment for BREEAM. Structured around BREEAM Pol 03 for project teams targeting certification.
• Sequential and exception tests. The locational justification when a Cornish site sits in a higher-risk flood zone.
• Sustainable drainage strategy (SuDS). Surface-water design to Cornwall's hierarchy and greenfield-runoff requirements, with preliminary drainage design at outline stage and discharge of drainage conditions after permission.
• Groundwater flood risk assessment. For chalk-free but mining-affected and low-permeability sites where the water table or made ground is a real constraint.

Not sure which you need? Our flood risk consultants will scope it from the site address, and you can read more about the firm on our about page and in the media.

Cornwall flood resources and contacts

The most reliable Cornwall flood information comes from the lead authorities. For an immediate threat to life during a flood, always call 999; for a power cut during flooding, call 105.

• Check for flooding in Cornwall. Environment Agency live warnings, river levels and maps at check-for-flooding.service.gov.uk; sign up for flood warnings or call Floodline on 0345 988 1188. Our free live river levels map shows nearby gauges.
• Cornwall Council flood risk team. Surface water, groundwater and ordinary watercourses, and reporting a flood, via Cornwall Council — 0300 1234 222 or floodrisk@cornwall.gov.uk.
• Cornwall Council SuDS team. Drainage approval, land drainage consent and the SuDS guidance suite at SuDS for Cornwall, or sudsdrainage@cornwall.gov.uk.
• South West Water. The sewerage undertaker for Cornwall; report flooding from public sewers to them.
• Cornwall Community Flood Forum. Local flood groups, resilience advice and reporting at cornwallcommunityfloodforum.org.uk.

Flood risk assessment in Cornwall: frequently asked questions