Flood Risk Assessment in Plymouth

A flood risk assessment in Plymouth starts with the drains as often as with the sea. The city council treats most of the built-up area as a critical drainage area, where the sewers are already at or near capacity, while the tide in Plymouth Sound and the streams running off Dartmoor decide the risk along the waterfront and in Plympton. For sites across the city we prepare flood risk assessments for a planning application, a purchase, a mortgage or insurance, along with sequential and exception tests and surface water drainage strategies.

On this page Plymouth means the city as the Office for National Statistics maps its built-up area, kept within the city council's boundary. That takes in the centre, the Barbican and Sutton Harbour, Stonehouse and Millbay, Devonport and Keyham, Estover and Crownhill in the north, and Plympton and Plymstock to the east, across the PL1 to PL7 and PL9 postcode districts. Tamerton Foliot, Hooe, Oreston and Turnchapel answer to the same council and are covered here too. Saltash and Torpoint, across the Tamar, are in Cornwall and covered on our Cornwall page; the rest of the county is on our Devon page, and other places are listed among the areas we cover.

Plymouth City Council puts 5,047 properties at risk from surface water in a 1 in 100 year storm, against 962 from the sea and 441 from main rivers. Across most of the city, new development has to let rain leave the site no faster than a field would in a 1 in 2 year storm.

Call 01293 214444 or email enquiries@unda.co.uk with a Plymouth address, and one of our flood risk consultants will tell you which reports the site is likely to need.

Sea, streams and sewers: where flooding reaches in Plymouth

Plymouth floods from four sources, and they often arrive together. High tides shut the outfalls, so rain cannot drain to the sea; steep streets send runoff into combined sewers that were built for less; and the Plym, the Tory Brook and the Long Brook rise within hours of heavy rain on the moor. The council's flood strategy, reissued in September 2026, sets out where each one does most harm.

The council's figures put up to 6% of homes in Plymouth at risk in a 1 in 100 year event. Its strategy also counts 21 km of the city's resilient road network in Flood Zone 3, and 86.6 km exposed to surface water. The Tamar, the Plym, the Tory Brook, the Long Brook, the Boringdon stream, the Forder Valley stream and the Tamerton stream are main rivers, looked after by the Environment Agency, while 106 of the city's 137 km of watercourse are ordinary watercourses under the council. Our explainer on main rivers and ordinary watercourses sets out why that split matters for an application, and our River Tamar page covers the river along the city's western edge.

Screening a PL postcode on the city map

Type a Plymouth postcode into the map below to see the Environment Agency's river and sea, surface water and reservoir layers for that spot; our flood risk map runs the same screen anywhere in England. The coral line is the ONS built-up area named Plymouth, trimmed where it crosses into South Hams at Woolwell and east of Plympton. It runs from Devonport and Stonehouse on the Tamar to Plympton, and from Plymstock up to Estover. Bear in mind that the flood zones ignore the Sutton Harbour gates and the Marsh Mills banks, and that the surface water layer shows where water will pond, not whether the sewers can take it.

A 1 in 2 year discharge limit: drainage rules across Plymouth's critical drainage area

The Environment Agency has identified most of Plymouth's urban area as a critical drainage area, where the sewers are at or close to capacity. The council's Technical Guide turns that into firm numbers, and they apply to brownfield redevelopment as much as to new estates on the edge of the city.

• Discharge at QBAR. All new development is held to the 1 in 2 year greenfield runoff rate. For small and single-property schemes the council accepts 1.5 l/s, the lowest rate a flow control can practically deliver.
• A drainage strategy for every site in the critical drainage area. Policy DEV35 and the Technical Guide both ask for one, setting out the chosen option, the evidence for it and who will maintain it.
• Design storm. No flooding above ground in the 1 in 100 year storm with 50% added for climate change, and runoff from neighbouring land checked so it cannot overwhelm the system.
• Urban creep. On major developments, 10% extra impermeable area is added to allow for patios and extensions over the life of the scheme.
• Infiltration tested, with a fallback. Soakage rates have to come from BRE 365 tests on the site. Where a strategy leans on an uncertain element, such as an assumed infiltration rate, a green roof or a pond without detail, the council can ask for a second, workable option.
• Sewer acceptance in writing. A piped connection needs an agreed maximum rate, a named point of discharge and written acceptance from South West Water.
• Coastal outfalls cleaned first. DEV35 asks for particulate and dissolved pollutants to be removed from surface water discharged straight to coastal waters. The council's strategy also links runoff to bathing water quality at East and West Hoe.

Those rules catch small schemes that would pass elsewhere. A pair of houses on a garden plot in Mutley or Peverell still needs its runoff held back and its exceedance kept on the site. Our explainer on critical drainage areas covers the planning side, and our guide on when infiltration testing is needed covers the timing of the BRE 365 tests.

Building where the tide reaches: Sutton Harbour, Millbay and the Plym estuary

Plymouth has more than 60 km of shoreline, from Tamerton Lake round to Jennicliff Bay, and the council identifies 15 places at risk from the sea. Most of the city sits well above tidal levels. The low ground at the heads of the inlets is the exception, and a scheme there is judged against fixed levels.

The Environment Agency's coastal modelling study of 2018 found that waves coming over the walls add to surface water flooding behind them. It also found that raising the Sutton Harbour defences to 4.81 m AOD is the only height that keeps every climate change scenario out. The harbour's lock gates are the main defence for the Barbican; the Environment Agency led a £3 million refurbishment between September 2022 and April 2023, and they failed again on 8 September 2025. The council lists Sutton Harbour, Turnchapel, Hooe and Oreston as the places with the lowest standard of protection, and Embankment Road, the Hoe, Oreston and Hooe as having the defences in the poorest condition. Our guides to residual flood risk and sea level rise explain how an assessment deals with a defence that may be overtopped or fail.

Warning areas and river gauges from the Barbican to Plympton

Six Environment Agency flood warning areas and two alert areas cover parts of the city. Each is named by its streets, which makes it a quick check for a buyer or an applicant. The river gauges show the levels behind them.

The Plym and Long Brook gauges appear on our live river levels map, and anyone in a warning area can sign up for flood warnings. Because the Plympton streams rise so fast, a warning may come with little notice. A building there needs a flood plan that does not depend on hours of lead time: who receives the warning, where people go upstairs and which route stays dry.

Plymouth's flood record, from the Plym in 2000 to the August 2025 cloudburst

The dates below come from the Environment Agency's gauges, the council's own investigation and strategy, and public notices about the harbour defences. Between them they cover all four kinds of flooding the city deals with.

1. 31 December 2000. The River Plym at Carnwood, upstream of Marsh Mills, reached 2.546 m, still its highest reading.
2. 22 December 2012. The Long Brook gauge in Plympton recorded 1.611 m, its highest since it opened in 1997.
3. February 2014. Winter storms sent waves over the sea front at the Hoe. The council's strategy uses a photograph of that day to illustrate wave overtopping.
4. September 2022 to April 2023. The Sutton Harbour lock gates had a £3 million refurbishment, meant to keep the barrage working for about another ten years.
5. 29 August 2025. Just after midnight, 18 mm of rain fell in 15 minutes and the storm was over in half an hour. The council's Section 19 investigation recorded 128 reports of property flooding: 37 in central Plymouth, 37 in Devonport, Milehouse and Keyham, 26 in Plympton, 22 in Plymstock and 17 in the north. The water came off the roads and overwhelmed the public sewers, and watercourses spilled at Honicknowle Lane and Weston Mill. None of it came from the sea.
6. 8 September 2025. The Sutton Harbour lock gates failed again, the council reported, leaving fishing boats unable to get in or out.
7. 10 December 2025. Storm Bram brought down part of the eastern pier arm at West Hoe, a structure already under repair after earlier storm damage.

For a planning application, August 2025 matters most. A short storm with the rivers low and no tidal factor still flooded more than a hundred properties, because the sewers could not take the water. That is what the critical drainage area rules are written to prevent, and why a drainage strategy carries so much weight in Plymouth. Our guide to surface water flooding and planning explains how that risk is assessed.

DEV35, the Technical Guide and a new local plan: the rules a Plymouth application meets

Plymouth City Council is both the planning authority and the lead local flood authority, so one council judges the flood risk and the drainage. It does so against national policy, the Joint Local Plan shared with South Hams and West Devon, and its own flood strategy.

The council is preparing a new Plymouth Local Plan, for the city alone. Its published timetable has the next consultation in summer 2027 and adoption expected in April 2029. On adoption it will replace the Joint Local Plan as it applies to Plymouth; until then DEV35 is the policy applications meet. Single houses away from the rivers and the sea may be handled with the council's pre-application standing advice, as its guidance for developments explains. Our guide to climate change allowances shows which figure applies to which kind of building.

South Yard, Millbay and the eastern suburbs: where Plymouth is building

Much of Plymouth's new building is on previously developed land by the water, which brings tidal levels and sewer limits into the same application. Infill in the older streets meets the critical drainage area rules instead.

• South Yard and Oceansgate in Devonport. South Yard is the city's tax site in the Plymouth and South Devon Freeport, for an innovation centre, a mobility hub and new factories next to Oceansgate. The Devonport frontage faces tidal and wave risk at the Torpoint ferry slipways, and the dockyard catchment has a high surface water risk around St Levan Road.
• Millbay and Union Street. Millbay is a largely reclaimed tidal inlet. New developments there have raised ground levels to defend against the sea, and storage tanks have added sewer capacity. The council still expects sea level rise to threaten Bath Street, Martin Street and the Octagon if the defences are overtopped.
• The western bank of the Plym. The council's strategy marks land along Embankment Road and the Laira for possible development, inside an area predicted to flood to 1.5 m or more in a 1 in 200 year tide in 2110.
• Plympton and Plymstock. Infill and estate extensions here sit in rapid-response stream catchments and around locally significant surface water areas such as Market Road, The Broadway and Billacombe Road, where sewer capacity sets the limit.
• Conversions in the older centre. Changes of use to flats in Stonehouse, on the Barbican and along Union Street often put bedrooms on ground floors in tidal areas. Around Sutton Harbour the Environment Agency's guidance rules that out; elsewhere the floor levels and a safe way out have to be shown.

Langage and Sherford, the other Freeport sites, lie in South Hams, outside the city, and are not covered by Plymouth's Technical Guide.

Does a Plymouth site need a flood risk assessment, a drainage strategy or both?

In Plymouth, the drainage question catches more sites than the flood zone question. The council asks every new development for a surface water drainage strategy, and a flood risk assessment wherever the rivers or the sea reach. The table gives the usual position.

If a site falls between two rows, send us the address and we will say which applies. Our guide on when an FRA is needed covers the national triggers, and our explainer on flood zones in England sets out what each zone means for an application.

Reports for Plymouth's waterfront, valleys and terraced streets

A buyer on the Barbican, a housebuilder in Plympton and an occupier at South Yard each need something different from us. Every report is prepared by our chartered consultants and signed off by a senior consultant.

• All four sources on one site. A flood risk assessment for a Plymouth site deals with the tide and waves, the Plym and its tributaries, the combined sewers and what happens if the harbour gates or the Marsh Mills banks do not hold.
• Applications by the water. The council asks for a flood risk assessment for planning with any new development, including a change of use, where the rivers or the sea reach the site.
• Buying on the Barbican or by the Long Brook. When a search flags a home in a Plymouth warning area, a flood risk survey for property purchase checks its floor and threshold levels against the flood levels.
• Homes in tidal and river zones. New homes at Sutton Harbour or by the Tory Brook have to pass the sequential and exception test, and the Sutton Harbour guidance requires both parts of the exception test.
• The council's drainage officers. In a critical drainage area the council reads the drainage strategy first: where the water goes, how it is held back and who maintains it.
• Held to QBAR. A surface water drainage strategy for a Plymouth site sizes storage for the 1 in 100 year storm plus 50% while keeping discharge to the 1 in 2 year greenfield rate or 1.5 l/s.
• Conditions on Plymouth permissions. The council uses planning conditions to deal with surface water, and discharge of drainage conditions supplies the calculations, sewer acceptance and maintenance plan it asks for.
• Where the Plym meets the tide. At Marsh Mills and along the Laira, river flows and tide levels act together and the depths rise sharply by 2110, so flood modelling sets levels where national mapping is too coarse.
• Occupants around Sutton Harbour. The harbour guidance asks for warning and evacuation arrangements, and a flood warning and evacuation plan sets them out for the building and its residents.
• Homes flooded in August 2025. After 128 reports of property flooding in a single night, owners facing higher premiums can use a flood risk assessment for insurance to show insurers how their property actually stands.
• Waterfront transactions. Before buying or lending on a wharf, a dockside unit or land on the Plym estuary, flood risk due diligence puts the tidal levels and defence condition in front of the deal.
• Businesses in the warning areas. Shops and restaurants on the Barbican and retailers at Marsh Mills can plan for closures and recovery with a business continuity flood risk assessment.
• Commercial floor space at South Yard. New Freeport buildings seeking BREEAM credits need a BREEAM flood risk assessment for the Pol 03 issue.
• Combined sewers and treatment capacity. DEV35 withholds permission until sewage treatment capacity is confirmed, and a foul drainage strategy shows South West Water how a scheme's flows can be taken.
• Land marked for development by the Laira. For land on the western bank of the Plym that the council's strategy flags for possible development, a flood risk feasibility study tests the tidal and drainage limits before an offer is made.

Lock gates, gullies and sewers: who to call about flooding in Plymouth

Several bodies share flood risk in Plymouth, and the right one depends on where the water comes from. If life is in danger, ring 999.

For planning, the council's flood risk management pages hold both parts of its September 2026 strategy, and its emergency flood guidance says who deals with what. Our explainer on the lead local flood authority covers the council's role on applications, and why LLFAs object covers the drainage points that most often hold one up.

Questions about Plymouth sites, from the Barbican to Plympton

Send us the postcode of a flat on the Barbican or a plot in Plympton and our consultants at Unda Consulting Limited will tell you which reports it needs. Call 01293 214444 or email enquiries@unda.co.uk.