Flood Risk Assessment in Southampton

A flood risk assessment in Southampton has to start with the tide. The city sits at the head of Southampton Water, where the tidal Test and Itchen meet. About 13% of it lies in Flood Zones 2 and 3, and the council's own flood evidence records no formal raised flood defences along its 35 km of tidal frontage. We prepare flood risk assessments in Southampton for planning applications, property purchases, lenders and insurers, alongside sequential and exception tests and surface water drainage strategies.

On this page, Southampton means the city's built-up area. Beside the Itchen that is the centre, Ocean Village, Northam, Chapel, St Mary's and Bevois Valley, with St Denys, Portswood, Highfield, Bassett and Swaythling further north. Shirley, Millbrook, Redbridge and Lordshill lie towards the Test, and Woolston, Bitterne, Bitterne Park and Sholing across the Itchen, in the SO14, SO15, SO16, SO17, SO18 and SO19 postcode districts. Southampton City Council is both the planning authority and the Lead Local Flood Authority. The rest of the county, from the New Forest to the north Hampshire chalk, is on our Hampshire page.

On a 1 in 200 year tide the sea at Dock Head would reach 3.1 m above Ordnance Datum at 2015 levels and 4.3 m by 2115, while parts of the docks stand as low as 0.5 m. By the end of the century the council expects such a tide to reach much of the city centre and the port.

For a quick view on whether a Southampton site needs a report, ring 01293 214444 or send the address to enquiries@unda.co.uk.

Is Southampton in a flood zone? The tide, two rivers and the brooks

Large parts of it are. The tide is the main source of flooding in Southampton, as high spring tides and Solent surges push up Southampton Water into the Test and Itchen estuaries. The city's double high water holds the sea near its peak for longer, so a surge has more time to coincide with it. Rivers and brooks come second, and surface water can collect almost anywhere in a city this built up.

Both rivers rise on the chalk well outside the city and respond slowly to rain, so here the tide matters more than the flow. The Itchen reaches the city from Winchester, upstream. The Environment Agency's gauges at Woolston on the tide, Riverside Park on the Itchen and Millbrook on the Tanners Brook are on our live river levels map.

Southampton on the flood risk map: Redbridge to Woolston

Type a Southampton postcode into the map below for a first screen against Environment Agency data on the sea and rivers, surface water and reservoirs, or open our full flood risk map. Much of the waterfront, the docks above all, was reclaimed from the estuary, which is why the flood zones follow the Itchen bank and the port so closely.

How high the tide could reach in Southampton by 2115

The Level 2 Strategic Flood Risk Assessment (SFRA) that Southampton City Council published in 2017 modelled the 1 in 200 year tide, a 0.5% chance in any year, at Dock Head and then added sea level rise. Its results show the risk spreading from the Itchen bank towards the city centre and the main line.

These are strategic figures from a 2016 Environment Agency model. A site assessment takes the extreme level nearest the site from the Environment Agency, as the SFRA asks, and applies today's climate change allowances, which have changed since 2016. Even a 1 in 20 year tide, 2.95 m AOD at Dock Head, comes over the frontage at the Town Depot off Endle Street, Shamrock Quay, Drivers Wharf and Priory Road. Our guides to how a storm surge forms and what AOD means for floor levels explain the terms.

When Southampton flooded: Boxing Day 1999 to April 2024

Flooding in Southampton is mostly tidal, and the same streets keep appearing in the record: the council's 2011 preliminary flood risk assessment gathered 194 recorded flood events across the city. The dated events below come from council reports and records.

1. Boxing Day 1999. A high tide flooded the upper Itchen, one of the events the council later set out when making the case for its coastal strategy.
2. 10 March 2008. A spring tide and a storm arrived together. Water reached parts of St Denys and the Town Depot on the Itchen bank. The tide markers on the riverside walk at Priory Hard, Cobden Bridge and Woodmill carry a red line at that day's level.
3. October 2012 to 2015. Council photographs record tidal flooding in Chapel Road in October 2012, at Woodmill in 2013, at Shamrock Quay and Lower William Street on 14 February 2014, in St Denys in 2014 and at Riverside Park in 2015.
4. Winter 2013 to 2014. Away from the tide, the Itchen and Monks Brook flooded the A27 Mansbridge Road and businesses on Woodmill Lane, springs appeared across the city, and Bassett Green, Daisy Dip and Townhill Park reported groundwater.
5. February 2020 and February 2021. High tides flooded Mount Pleasant Industrial Estate, and a year later Crosshouse Hard.
6. 9 April 2024. On the spring tides that came with Storm Pierrick, Endle Street, Albert Road North and a basement in Chapel Road flooded. At Chapel Riverside, a recent development whose site-level protection was only meant to be interim, car parks and one non-residential ground floor block went under.

In St Denys the council fitted flood resistance and resilience measures to 27 properties in Priory Road in 2015 and to 30 more in 2017. Under its flood investigation protocol, a formal investigation follows when, among other triggers, five or more properties flood internally in one incident. Our guide to Section 19 flood investigations explains why some floods are investigated and others are not.

Flood defences in Southampton: what the halted Itchen scheme means

Southampton has no formal raised flood defences. Its waterfront is held by quay walls, sheet piles, embankments and private frontages of mixed height and condition, most of them owned by the businesses behind them. The River Itchen Flood Alleviation Scheme (RIFAS) was meant to be the first stretch of a continuous defence.

• The scheme. A 3.7 km wall on the west bank of the Itchen, from Mount Pleasant Industrial Estate north of Northam Bridge to just south of the Itchen Bridge, developed by the Environment Agency and Southampton City Council.
• Who is at risk now. More than 153 homes, including the Northam Estate, and 288 businesses on that bank face tidal flooding today, rising to over 1,150 homes and 900 businesses by 2120 as the sea rises.
• The cost. Estimates rose from £29.8m at the outline business case in early 2022 to about £96.7m by December 2024, against about £45.5m of identified funding.
• Halted in 2025. The Environment Agency withdrew its funding, confirmed at a meeting on 12 March 2025, which left a shortfall of over £50m. The council has paused the scheme while it looks for other ways to pay for flood resilience on the Itchen.
• The longer plan. The council's 2012 coastal strategy treats RIFAS as the first phase of about 22 km of continuous defence from Woodmill through St Denys, Northam and the city centre to Redbridge. Doing nothing, it estimated, would cost about £1.25bn in damage to homes and businesses over the century.

For a site today, the assessment has to work from the waterfront as it stands, not as the scheme would have left it. Even where a defence does go up, the Flood Map for Planning leaves it out, so the flood zone stays the same. Our guides to residual flood risk and why a site stays in a flood zone after new defences are built explain how that works.

Policies CS 23 and AP 15: what a flood risk assessment in Southampton must show

Southampton City Council decides applications against its Core Strategy, adopted in January 2010 and partly reviewed in March 2015, and, in the city centre, the City Centre Action Plan of March 2015. Both remain in force until the Southampton City Vision Local Plan replaces them. The council's 2025 plan timetable puts that plan's pre-submission consultation in December 2026 and its adoption in about September 2027.

• CS 23 Flood Risk. The council works with the Environment Agency on new development in the flood risk zones of the city centre and Northam, and expects each scheme to reach an appropriate degree of safety, allowing for the standard of defence and for sea level rise over its life.
• CS 20 and drainage. Sustainable drainage goes into every development, minor schemes included, unless it can be shown to be inappropriate on that site.
• AP 15: the defence line. Where the flood defence search zone crosses a city centre site, the development raises the land to form the defence, builds a front-line defence, or keeps land free for one later. Developers also pay towards the strategic defence through the Community Infrastructure Levy.
• AP 15: safe for its lifetime. A flood risk assessment, a dry route to Flood Zone 1 in the design flood, more vulnerable uses on the least exposed part of the site, habitable floors above the design flood level, resistance and resilience to the extreme level, a flood plan for warning and evacuation, and a refuge above the extreme level where floors cannot be raised.
• The SFRA's triggers. An assessment for every site over a hectare, for all new development in Flood Zones 2 and 3 including minor works and changes of use, and for Flood Zone 1 sites with critical drainage problems or exposed to other sources.

The national layer is chapter 18 of the NPPF as revised in August 2026. There, policy F4 decides whether an assessment is required, F5 and F6 hold the sequential and exception tests, and F8 points drainage at the 2025 National Standards for sustainable drainage. Our note on the August 2026 NPPF flood and drainage changes covers what moved.

The 2017 Level 2 SFRA remains the city's own flood evidence. It judges homes over a 100-year lifetime and expects habitable floors above the extreme flood level, with the 300 mm freeboard the Environment Agency asks for. It also says the council will not usually allow new basement homes in flood risk areas. The Partnership for South Hampshire's 2024 Level 1 SFRA covers Southampton among ten authorities, though its Southampton part had not been published by October 2026. As Lead Local Flood Authority, the council's flood risk team reviews applications alongside the Environment Agency and recommends refusal where a scheme is not shown to be safe.

Drainage in Southampton: greenfield rates on brownfield land

Southampton's drainage rules were stricter than the national standards of the day. The council's Local SuDS Design Guidance of 2017 holds every site, land already built on included, to greenfield peak runoff, because surface water risk in the city is so high. Where that cannot be done, the rate has to be agreed with the council before the application goes in.

Infiltration is not a given here. London Clay underlies the north of the city, a sandier band of the Wittering Formation crosses the middle, and tidal flat deposits line the rivers, where reclaimed ground can hold water close to the surface. The SFRA also warns that outfalls in the Millbrook and Portswood catchments sit below high tide and back up when heavy rain and a high tide coincide. Our guide to BRE 365 infiltration testing explains the soakage test, and the ground conditions checker gives a first read for a postcode.

Does a conversion or extension need a flood risk assessment in Southampton?

Very often, yes. Because the SFRA names minor development and changes of use in Flood Zones 2 and 3, a flat conversion in Northam or an extension in St Denys can need an assessment as surely as a block of flats on a wharf. The table covers the usual cases.

Our guides to converting a house into flats, a change of use to an HMO and basement conversions in a flood zone cover three common Southampton cases, and the Environment Agency standing advice applies to many extensions.

Reports for Southampton sites, from Northam's wharves to Lordshill

We work on Southampton sites for developers, architects, planning agents, landlords, homeowners, buyers and marine and port businesses. A waterside scheme here often needs three separate reports: one to show the site is safe, one to show it should be built there at all, and one to show where its rainwater goes.

• Tide, brooks and drains together. Where a Southampton site can flood from the estuary, a brook and a tide-locked drain in the same storm, a flood risk assessment has to deal with each source and with how they combine.
• Applications to Southampton City Council. From a conversion in Portswood to new homes on the Itchen bank, a flood risk assessment for planning sets out what CS 23 and AP 15 ask for, including floor levels, a dry route out and the flood plan.
• Before exchange on a Southampton home. Homes on Priory Road and around Woodmill have flooded from the tide more than once, and a flood risk survey for property purchase reads that record against the property's own levels.
• Unallocated land in the flood zones. New homes or another more vulnerable use in Flood Zone 2 or 3 on a site the plan has not allocated will usually need a sequential and exception test before the council can approve them.
• Clay, reclaimed land and tide-locked outfalls. With London Clay in the north and high groundwater by the rivers, a drainage strategy has to show where the water goes when a high tide shuts the outfall.
• Greenfield rates on brownfield sites. A surface water drainage strategy holds a Southampton redevelopment to greenfield peak runoff for the 1 in 1 and 1 in 100 year storms, with 10% added for urban creep, as the council's guidance requires.
• Conditions on a Southampton consent. Where drainage was left to a condition, discharge of drainage conditions means providing the calculations, details and maintenance plan the council needs before it signs the condition off.
• Getting people out before a tide. AP 15 expects a flood plan covering warning and evacuation, and a flood warning and evacuation plan ties it to the Itchen estuary warning area and a route to dry ground.
• Levels for the exact frontage. The SFRA quotes the tide only at Dock Head, so where the Itchen meets the tide at Woodmill or the Tanners Brook reaches the Test, flood modelling can give levels for the site itself.
• High water tables by the rivers. Reclaimed land near the Itchen and Test, and the springs at Bassett Green and Townhill Park after the 2013 to 2014 winter, make a groundwater flood risk assessment worth having for basements, deep foundations and soakaways.
• Cover for homes on the tidal frontage. Brokers pricing any of the more than 153 homes at tidal risk on the Itchen's west bank need evidence for the individual house, which a flood risk assessment for insurance provides from its own levels and flood history.
• Wharves, quays and industrial land. A buyer or lender looking at Mount Pleasant Industrial Estate, Shamrock Quay or the Town Depot, all flooded by the tide between 2008 and 2020, can have the site's exposure set out in commercial flood due diligence.
• Keeping waterside firms trading. With 288 businesses at risk on the west bank of the Itchen, a business continuity flood assessment plans for a tide that closes the yard, the access road or the power supply.
• Offices and commercial floorspace in the centre. The City Centre Action Plan promotes new offices and other commercial space, much of it inside the tidal flood zone, and a BREEAM flood risk assessment covers the flood risk credits of a BREEAM assessment in Southampton.
• Regeneration land on the Itchen. Before land is bought for the council's Itchen Riverside vision in Chapel, Northam, the Northam Estate or Drivers Wharf, a flood risk feasibility study tests whether tidal risk still leaves a workable site.

Flood warnings in Southampton and who to call

If floodwater puts anyone in danger, call 999, and call 105 if it has cut the power. Beyond that, who to contact in Southampton depends on where the water comes from: the Environment Agency for the tide and its warnings, the city council for surface water and groundwater, and Southern Water for the sewers.

What Southampton buyers, landlords and developers ask about flood risk

Our chartered flood risk consultants take calls about Southampton sites on 01293 214444, and enquiries@unda.co.uk reaches the same team. Each report is signed off by a senior consultant before it is issued, and Unda Consulting Limited is also a CIWEM Business Partner. Nearby, we also cover Portsmouth on Portsea Island and the Isle of Wight across the Solent; the full list is on every area we cover.