Flood Risk Assessment in Oxfordshire

A flood risk assessment in Oxfordshire goes to one of five district planning authorities. On major schemes it is also read by Oxfordshire County Council as lead local flood authority, which rewrote its surface water standards in June 2026. Almost all of the county drains to the Thames, and the Cherwell, Windrush, Evenlode, Thame and Ock carry floodwater to Banbury, Witney, Thame, Abingdon and Oxford.

Our consultants prepare flood risk assessments in Oxfordshire for planning, property purchase, mortgage and insurance, alongside sequential and exception tests and surface water drainage strategies. Reports for sites in Oxfordshire come from Unda Consulting Limited, a CIWEM Business Partner. We work on sites in Witney, Carterton, Burford and Chipping Norton in the west, Bicester, Kidlington and Woodstock in the north, and Didcot, Wantage, Faringdon and Thame in the south and east, and anywhere else with an Oxon address.

The Met Office expected autumn 2024 to be Oxfordshire's second wettest since its record began in 1836, and the county's floods that year have so far produced more than 50 Section 19 investigations, from Storm Henk in January to Storm Bert in November.

To talk through a site anywhere in Oxfordshire, call one of our consultants on 01293 214444 or email enquiries@unda.co.uk.

Who decides an Oxfordshire planning application, and who reviews its drainage?

Oxfordshire still has two tiers of local government. The district or city council decides the planning application against its own Local Plan. Oxfordshire County Council is the lead local flood authority for the whole county and is consulted on surface water whenever development is major. Each of the five planning authorities has an adopted plan with a flood policy, and three replacement plans were at examination in October 2026.

On 16 July 2026 the government confirmed that the county council and the five district and city councils, with West Berkshire, would give way to three unitary councils in April 2028: Greater Oxford, Northern Oxfordshire and Ridgeway. In September it said it would review its reorganisation decisions across England. Until anything changes, applications still go to the district or city council and the county stays the lead local flood authority. Our guide to local government reorganisation explains what moves when a county is split.

Ordinary watercourses are the exception to that split. The county holds the power to consent works on them, but in Oxfordshire it has delegated that to the district and city councils, so a new culvert or outfall on a ditch goes to the planning authority's own drainage officers. Our page on flood risk assessment in West Oxfordshire follows how that district's planners apply policy EH7.

Looking up an Oxfordshire address on the flood map

Enter a postcode in the map below to see the Environment Agency's flood zones, surface water extents and reservoir flood areas around it, or open our full flood risk map for anywhere in England. The coral outline is the county council's area, so it takes in all five districts from Banbury down to Henley-on-Thames.

Treat the result as a first look. Flood Zones 2 and 3 show river flooding only, drawn as if no defence existed, while surface water sits on a separate layer and groundwater on none. Our explainer on flood zones in England sets out how each zone is drawn and what it means for an application.

The Thames and the tributaries that flood Oxfordshire's towns

Almost 97% of Oxfordshire drains to the Thames, the county's flood strategy records; the rest runs north-east to the Great Ouse or west to the Severn. The Thames enters the county near Lechlade and passes Oxford, Abingdon and Wallingford before leaving at Henley-on-Thames. Many of the places investigated after the 2024 floods sit on its tributaries and smaller brooks, not on the main river.

The River Thames has a page of its own covering its whole length, from these upper reaches to the tidal river. In October 2026 the Environment Agency listed 89 flood warning and alert areas in Oxfordshire, 85 of them in its Thames area and 26 on the Thames itself, and our live river levels map follows the gauges on the Thames, Cherwell, Windrush and Ock as they rise.

Limestone, chalk and the water beneath Oxfordshire

Groundwater floods parts of Oxfordshire, though no public map shows it well. The county's flood strategy places higher risk on the oolitic limestone of the Cotswolds in the north-west, the sandstones and limestones around Oxford and the chalk in the south, notes that no publicly available groundwater flood maps exist, and records the county's part in Project Groundwater, a monitoring programme led by Buckinghamshire Council.

The Environment Agency runs a groundwater flood alert for the Henley and Assendon area, on the chalk north of Henley. West Oxfordshire's 2025 flood assessment puts parts of Carterton, Witney, Burford, Charlbury, Woodstock and Chipping Norton at moderate groundwater risk on the limestone. In Eynsham in November 2024, saturated ground let water into homes from below as well as from the Chil Brook (Section 19 report).

For a basement, a soakaway or a deep foundation, the seasonal high water level decides what is possible. Our note on checking a site's water table covers the datasets, and the ground conditions checker gives an early idea of whether infiltration could work before anyone digs a trial pit.

Floods across Oxfordshire since 2007, from the official record

Recent floods in Oxfordshire have come from several sources at once: rivers out of bank, water running off fields and roads, and foul sewers surcharging. The dates below come from council, Environment Agency and Met Office records.

1. 20 July 2007. RAF Brize Norton recorded 126.2 mm of rain in a day, the most since its records began. About 1,631 homes and 72 businesses flooded in West Oxfordshire alone (district council summary), and the county's flood partners formed the group that still coordinates their work.
2. 23 and 24 December 2020. Flooding in Witney and nearby Hailey led to Section 19 investigations; the county strategy uses the Witney event as its example of flooding that triggers one.
3. 4 and 5 January 2024. Flooding after Storm Henk led to investigations for Henley-on-Thames and Goring, Clifton Hampden, Nuneham Courtenay, Abingdon, Didcot, Chinnor, Milton, Steventon and East Hagbourne, and for the Kings Meadow site on Oxford's Osney industrial estate.
4. 23 and 24 September 2024. Intense rain flooded homes from Bicester, Kidlington and Begbroke to Wheatley, Thame and Didcot. The county council described that September as Oxfordshire's wettest month since records began in 1836.
5. 22 to 26 November 2024. Storm Bert put 89.8 mm on South Newington and 72.4 mm on Banbury's Grimsbury gauge, more than a whole November's average at both (Met Office). Witney, Bampton, Eynsham, Brize Norton, Hook Norton, Bloxham and more than a dozen other places now have Section 19 reports for those days.

Those reports are Section 19 flood investigations, and the county lists every one on its Flood Toolkit: of the 64 shown in October 2026, more than 50 deal with flooding in 2024. For a site near any of these places, the report is the first local evidence to read. Our account of the Pitt Review explains how the 2007 floods changed who manages surface water in England.

What Oxfordshire's June 2026 drainage standards ask of a major scheme

In June 2026 the county council replaced its 2022 guidance with version 2.0 of its Local Standards and Guidance for surface water, a full revision written around the 2025 national standards. It covers major development, which the county reviews as statutory consultee, and the earlier version is archived in October 2026. A major scheme has to show the following.

• Discharge rates. Runoff is limited to greenfield rates, using the 50% annual exceedance probability rate wherever there is doubt, and a previously developed site may discharge no more than five times greenfield.
• Volumes and creep. Rates and volumes after development must not exceed those before it, from the 100% to the 1% annual exceedance probability event. The upper end climate change allowance applies to rainfall, and 10% is added for urban creep on house plots, though not on flats.
• Everyday rain. The first 5 mm of any rainfall must leave no runoff from the site, and calculations set pipe storage and the MADD factor to zero.
• Into the ground. Infiltration is tested three times in succession to BRE 365 and the lowest rate used; groundwater is monitored, usually over winter and spring, to show its highest level stays at least 1 m below the base; and infiltration features half-empty within 24 hours.
• Where water goes. Reuse first, then the ground, a watercourse, a surface water sewer and, last, a combined sewer. Foul sewers and highway drains are not accepted for new surface water.
• Looking after it. A maintenance schedule naming who is responsible, 3 m easements for access, and, once built, a verification statement, CCTV survey and as-built drawings.

Policy F8 of the NPPF now expects all development to follow the 2025 national standards for sustainable drainage, so they apply to smaller schemes as well, even though the county does not comment on them. Oxford City Council points minor applicants to its own SuDS guide. The county's pre-application drainage advice is charged, from £583 for a written response on up to 24 homes. A scheme that misses these tests invites an LLFA objection, and our BRE 365 infiltration rate calculator turns trial pit readings into an infiltration rate before the drainage layout is fixed.

Flood schemes in Oxfordshire, and what a defence leaves unchanged

Two Environment Agency schemes protect Oxfordshire's largest towns from their rivers, and in Abingdon the Agency is trying a community approach instead.

• Oxford. The Oxford Flood Alleviation Scheme is a new channel about 5 km long west of the city, from north of Botley Road to the Thames near Kennington, with more than 20 hectares of new wetland. Its compulsory purchase order was confirmed in May 2025, the Environment Agency's September 2026 update reported the full business case approved and a construction partner appointed, and work is expected to start in late 2026. Our article on the scheme's approval covers the route and the expected benefits.
• Banbury. The £18.5 million scheme finished in 2012 stores up to 3 million cubic metres of Cherwell floodwater behind a 3 km embankment by Banbury Country Park and releases it to the river at a controlled rate.
• Abingdon. Under its Abingdon Together programme the Environment Agency has tested a temporary barrier for the homes at highest risk, including those flooded in 2024, and is helping residents form a community flood group.

Flood zones are drawn as if defences were not there, so none of these schemes takes land out of Flood Zone 3. A site behind them still needs its residual risk assessed: overtopping, a breach, and water from sources the scheme does not touch. Our notes on residual flood risk and on staying in a flood zone after new defences set out how that is done.

Where Oxfordshire's growth meets its drainage limits

The 2024 investigations reached newer estates and business parks as well as old riverside streets. Didcot alone has reports for Harrier Park, Dart Drive and Sutherland Beck, Derwent Avenue and the Trident Business Park, and there are others for Hanover Gardens in Bicester and for Kidlington, Yarnton and Begbroke.

• Sewage treatment in Oxford. In March 2025 the Environment Agency, Oxford City Council and Thames Water agreed an interim wastewater scheme for 2027 and a full upgrade by 2031, so that development could be occupied from 2027; the deal covers around 18,000 homes in and around Oxford. Our article on the Thames Water crisis looks at what treatment capacity means for planning.
• Sewer flooding records. Thames Water's records since 1989 show 293 sewer flooding incidents in South Oxfordshire, 240 in Cherwell, 223 in the Vale of White Horse, 192 in West Oxfordshire and 153 in Oxford, according to the county strategy.
• Ditches beside new estates. Brooks and ditches belong to the owners of the land beside them, and works on them need the district or city council's consent; our guide to main rivers and ordinary watercourses explains the split with the Environment Agency.

Does your proposal need a flood risk assessment in Oxfordshire?

The national triggers come from policy F4 in chapter 18 of the NPPF: land in Flood Zone 2 or 3, a site of a hectare or more in Flood Zone 1, and any site the Environment Agency's maps or the council's own evidence show at risk from surface water or another source. Each district applies those triggers through the flood policy listed in the first table on this page, and our guide to the NPPF's flood policies sets out chapter 18 in full.

A site at risk only from surface water can sometimes avoid the sequential test: under policy F5 it is not needed where the assessment shows the development will be safe, and our note on the surface water exemption explains when it applies.

Reports for Oxfordshire sites, from Cotswold barns to Thames-side plots

A flood risk assessment, a sequential and exception test and a drainage strategy answer three different questions: whether the development will be safe, whether it should be built there, and where the rain that falls on it will go. Our chartered consultants prepare each of them for sites across the county, and a senior consultant signs off every report.

• Every source. Thames floodwater, a field-edge brook, rising groundwater and a surcharged sewer can all reach the same Oxfordshire plot, and a flood risk assessment weighs them together.
• Planning applications. Whether the case officer works for Oxford, Cherwell, South Oxfordshire, the Vale or West Oxfordshire, a flood risk assessment for planning answers that council's flood policy and chapter 18 of the NPPF.
• Buying a home. Streets in Witney, Abingdon, Eynsham and Kidlington named in the 2024 investigations can be flagged by a search or a lender, and a flood risk survey for property purchase explains what the flag means for one house before exchange.
• Homes in a flood zone. Where housing is proposed in Flood Zone 2 or 3, a sequential and exception test looks for reasonably available sites at lower risk across the search area the district accepts and, where the exception test applies, shows the scheme stays safe for its lifetime.
• Major schemes. On ten homes or more, the county council checks the drainage strategy against its June 2026 Local Standards before the district decides the application.
• Greenfield rates. A surface water drainage strategy works out the greenfield runoff, storage and infiltration evidence the county standards call for.
• Conditions before work starts. Where an Oxfordshire permission reserves the drainage details, discharge of drainage conditions supplies the calculations, verification plan and maintenance schedule the council needs.
• Homes flooded in 2024. Owners in streets flooded that year, from Abingdon's Nash Drive to Bampton and Witney, can use a flood risk assessment for insurance to show an insurer the risk the building faces now.
• Commercial land in the floodplain. Kings Meadow on Oxford's Osney industrial estate flooded in January 2024, and flood risk due diligence tests land in the Thames and Cherwell floodplains before it is bought or refinanced.
• Business parks. The Trident Business Park in Didcot flooded in November 2024, and a business continuity flood risk assessment plans how a business keeps trading when water reaches the site.
• Wastewater capacity. With Oxford's treatment works upgrade due in 2031 and more than 1,100 sewer flooding incidents recorded across the five districts since 1989, a foul drainage strategy shows where a scheme's wastewater will go.
• Limestone and chalk. On the Cotswold limestone and the southern chalk, where the county strategy places its higher groundwater risk, a groundwater flood risk assessment tests basements and soakaways against the seasonal high.
• Older river models. West Oxfordshire's 2025 assessment says its river models predate the latest climate allowances and that sites need detailed modelling, and flood modelling provides the levels for a plot on the Windrush, Evenlode or Thames.
• Care homes and campsites. Care homes, campsites and homes inside the county's 60 flood warning areas may need a flood warning and evacuation plan setting out when and how people leave.
• Land for the new plans. With replacement plans for Oxford, Cherwell, South Oxfordshire and the Vale going through examination, a flood risk feasibility study tests land before it is promoted.

Oxfordshire towns with pages of their own, and the counties around it

Five Oxfordshire towns have their own pages, which go further into their rivers, defences and local policies:

• Oxford. Where the Cherwell joins the Thames, with the floodplain west of the station and the alleviation scheme being prepared.
• Abingdon. Where the Ock joins the Thames, with homes flooded more than once in 2024.
• Banbury. On the Cherwell and the Sor Brook, protected since 2012 by the storage reservoir upstream.
• Henley-on-Thames. On the Thames at the Berkshire border, investigated after the January 2024 flooding.
• Wallingford. A Thames town in South Oxfordshire, covered by the same joint flood assessment as Abingdon and Henley.

For West Oxfordshire as a planning authority, our West Oxfordshire page covers Witney, Carterton and the Cotswold villages. Over the county boundary there are pages for Berkshire, Buckinghamshire, Gloucestershire, Wiltshire and Warwickshire, and every area we cover is listed on our areas page.

Reporting flooding in Oxfordshire, and who handles what

If floodwater puts anyone in danger, ring 999; for a power cut, ring 105. Otherwise the right body depends on where the water comes from.

The county's Local Flood Risk Management Strategy, approved in January 2025, runs for five years and sets out how these bodies work together, including a pilot of local flood wardens and emergency plans for towns and parishes. Our guide to which lead local flood authority covers a site explains the role nationally. The Buckingham and River Ouzel board is the only internal drainage board with land in Oxfordshire.

Common questions on flood risk assessment in Oxfordshire

From a barn above the Evenlode to a riverside plot in Henley, our consultants can tell you what a site needs. Call 01293 214444 or email enquiries@unda.co.uk.