Flood Risk Assessment in Wiltshire

A flood risk assessment in Wiltshire usually turns on the ground under the site as much as the nearest river. Chalk covers the south and east of the county, where the water table can rise to the surface after a wet winter, while the clay vales of the north flood from the Bristol Avon and from rain running off the land. Our consultants prepare flood risk assessments across Wiltshire for planning applications, property purchases, mortgages and insurance, together with sequential and exception tests and surface water drainage strategies.

This page covers the ceremonial county of Wiltshire, also searched as Wilts: the Wiltshire Council area, from Cricklade and Marlborough down to Salisbury Plain and the Dorset border, and the separate Borough of Swindon. Between them they decide planning applications in towns such as Chippenham, Trowbridge, Devizes, Melksham, Calne and Bradford on Avon, mostly in the SN, SP and BA postcode areas. Wiltshire and Swindon share a resilience forum but have two lead local flood authorities, each with its own plan and drainage rules.

After Storm Chandra in January 2026, groundwater in the Dorset and Wiltshire chalk rose as high as in the 2013/14 floods, and in places higher, with 34 groundwater flood warnings still in force at the end of February.

To talk through a site anywhere in the county, call 01293 214444 or email enquiries@unda.co.uk.

Two Avons, the Kennet and the Thames: where Wiltshire's floods start

Wiltshire has no coast and no tidal risk, so the question on any site is where the rain goes, and that depends on the rock. The county's Level 1 Strategic Flood Risk Assessment, prepared by JBA Consulting in November 2024, places the main river risk on the Hampshire Avon, the Bristol Avon, the Kennet and the Test, and names Bradford on Avon, Malmesbury, Melksham, Salisbury, Royal Wootton Bassett and Cricklade among the towns most exposed to it. Swindon sits in a third system, draining north to the Thames through the Ray and the Cole.

Gauges on all of these rivers, and the groundwater boreholes on the chalk, are on our live river levels map. Our pages on the River Kennet and the River Thames follow both rivers beyond the county, and the risk from an embanked canal is the subject of our article on a canal breach and what it means for planning.

Chalk groundwater: the flood source the planning map leaves out

Groundwater is the flood risk easiest to miss on a Wiltshire site, because the Flood Map for Planning does not show it. The council's flood strategy describes a county split by geology: clay in the north, where floods come from rivers and runoff, and chalk in the south, where the water table can rise into gardens, cellars and roads and stay there for weeks.

• Where the SFRA puts it. The 2024 assessment maps groundwater emergence across the county and rates Marlborough, Pewsey, Salisbury and Warminster among the areas of high or moderate risk.
• What 2026 showed. After Storm Chandra on 26 and 27 January 2026, the Environment Agency's Dorset and Wiltshire groundwater incident response recorded levels as high as, and in places higher than, those of 2013/14. Wiltshire Council warned that about 100 properties near Salisbury Cathedral faced flooding from rising groundwater.
• It reaches the sewers. Wiltshire Council's flood strategy papers record groundwater getting into sewers and flooding rural septic tanks for long periods.
• It sets the soakaway level. Wiltshire Council expects the base of any soakaway to sit at least 1 m above the peak winter groundwater level, measured rather than assumed.

On the chalk, an assessment has to deal with groundwater directly: borehole records, the SFRA emergence mapping and, on larger schemes, a winter of monitoring. Our guide to groundwater flood risk in planning applications explains what a council expects to see, Because the council expects readings from November to May, a scheme that starts groundwater monitoring in the spring can wait the best part of a year for its drainage answer.

Screen any postcode from Cricklade to the Dorset border

Type a postcode below to see the planning flood zone, surface water and reservoir risk for any address in Wiltshire or Swindon, or open the full flood risk map for a wider view. The screening does not show groundwater, which on the chalk is often the deciding risk.

Wiltshire Council or Swindon Borough Council: which one decides?

Wiltshire has two unitary councils, and each is both the planning authority and the lead local flood authority for its own area. Swindon Borough covers the town and the villages around it, from Highworth to Wroughton; Wiltshire Council covers everywhere else. The Environment Agency is consulted on development in Flood Zones 2 and 3 and on main rivers in both.

Wiltshire's position changed in 2026. The council's draft Local Plan to 2038 was submitted for examination in 2024, but in February 2026 the inspectors offered only two routes, withdrawal or a report recommending that it was not adopted, and Full Council chose to withdraw it in May. Applications are therefore still decided under the 2015 Core Strategy and the national policy in chapter 18 of the August 2026 NPPF, read with the 2024 Wiltshire SFRA. In Swindon, EN6 in the Local Plan 2026 requires a site-specific assessment for any proposal of one hectare or more in Flood Zone 1, and for all development in Flood Zones 2 and 3 and in critical drainage areas, including minor schemes and changes of use; the Local Plan 2043 that will replace it draws on the 2026 Swindon SFRA.

What Wiltshire Council's drainage rules ask of a scheme

Wiltshire Council publishes its drainage requirements on its development and planning information page, and the central document is the Drainage Betterment Strategy of June 2024. Its central demand is betterment: on a greenfield site, runoff after development has to be lower than it was before.

• Greenfield sites. Discharge must be 30% below the pre-development rate for every storm from the 1-in-1 to the 1-in-100 year event, and the 1-in-100 year, 6-hour runoff volume must not exceed the greenfield volume.
• Brownfield sites. Peak rates should come as close to greenfield as is reasonably practicable; where that cannot be done, a reduction of at least 30% on the existing rate.
• Infiltration. The likely minimum acceptable rate is about 1x10-6 m/s, using the lowest tested rate, with features half-empty within 24 hours.
• Groundwater monitoring. Expected from November to May, and 12 months of readings for major applications where winter peaks are not already known.
• Minor schemes. Discharge limited to the greenfield rate, with a 10% allowance for urban creep.
• Checklists. A SuDS validation checklist and technical notes on exceedance flows, culverts, discharge to highway drains, and solar farms and battery storage.

In practice that puts ground investigation, done in the right season, at the start of a Wiltshire drainage strategy. Our article on when infiltration testing is required covers the testing, and our BRE 365 infiltration rate calculator gives a first reading from trial pit results. The 2025 National Standards for Sustainable Drainage apply on top, and the NPPF's policy F8 expects every development to follow them.

Floods recorded across Wiltshire and Swindon

The flood record shows the same mix of sources an assessment has to test: river floods in the clay vales, flash flooding in towns and long groundwater floods on the chalk. The events below are drawn from council and Environment Agency records, including the council's Section 19 flood investigations.

1. 1947 to 2008. The Environment Agency's recorded flood outlines for Swindon Borough include March 1947, September 1968, July 2007 and January 2008, and the council records that many Wiltshire communities flooded in 2007 and 2008.
2. December 2013 to March 2014. More than 500 properties flooded across Wiltshire and a number of roads were closed to traffic.
3. September 2014 and 2015. Storms in Trowbridge and West Wiltshire led to Section 19 reports; in September 2015 another 20 properties flooded in Salisbury and the south of the county.
4. October 2021. Flooding in Tisbury and Donhead St Andrew in the Nadder valley was investigated under Section 19.
5. September to November 2023. Intense rain overwhelmed drains across Swindon, recorded in the borough council's Section 19 report.
6. November 2024. Storm Bert brought more than 220 reports of flooding across Wiltshire; the Avon burst its banks in Bradford on Avon, and the council called it the worst flooding in Chippenham and Melksham since the 1960s.
7. January to March 2026. Groundwater warnings covered Chilmark, Teffont and the Bourne and Till valleys, and levels were high in Salisbury city centre, where the River Park was by then protecting more than 350 properties from the Avon.

Sites behind the new walls and embankments of the Salisbury River Park, built from 2022 by the Environment Agency with Wiltshire Council and Salisbury City Council, still carry a residual flood risk. A planning assessment has to address it, because the flood zones are drawn as if the defences were absent.

Bradford on Avon is the opposite case. In July 2025 the Environment Agency, Wiltshire Council, the town council and Wessex Water judged a permanent flood scheme for the town financially unviable: the 2017 design of low walls and pumping stations had risen from £4.5 million to more than £11 million, and would still have been overtopped in Storm Bert. The Environment Agency is offering property flood resilience to homes instead, with surveys due to start in 2027, and has identified potential funding routes for a new pedestrian bridge, because the historic Town Bridge can be cut off in a severe flood. With no scheme planned, a planning application beside the Avon in the town cannot count on future defences, so floor levels, safe access and escape have to be settled in the site's own flood risk assessment.

What triggers a flood risk assessment in Wiltshire?

Most applications in Flood Zones 2 and 3, and larger sites anywhere, need a site-specific flood risk assessment in Wiltshire; on the chalk, even a householder extension can need one.

Wiltshire's SFRA applies the sequential test to surface water and groundwater as well as rivers, in line with the September 2025 change to planning guidance, and treats any development inside the 0.1% chance surface water extent as a candidate for the exception test unless the layout can avoid it. In the Hampshire Avon catchment there is a further hurdle: housing must be shown to be phosphorus neutral under the council's nutrient neutrality strategy, revised in February 2024, which our article on nutrient neutrality in planning explains.

Reports our consultants prepare for sites across Wiltshire and Swindon

Each report below answers a question that Wiltshire and Swindon sites raise, with the local reason it comes up.

• Every source on one site. From a Calne infill plot to a farmyard on Salisbury Plain, a flood risk assessment weighs the rivers, surface water, groundwater, sewers and the canals against the scheme.
• Applications to either council. Either council will judge a flood risk assessment for planning against its own policy, Core Policy 67 or EN6, and the SFRA behind it.
• Buying in a river town. A flood risk survey for property purchase on homes in Bradford on Avon, Melksham or a chalk valley, where groundwater can matter as much as the river.
• Sites inside the SFRA risk zones. A sequential and exception test report where Wiltshire's SFRA zones put a site at risk from rivers, surface water or groundwater, with the area of search set for the scheme.
• Clay or chalk. The ground decides the drainage strategy: attenuation on the northern clays, infiltration on the chalk once the winter water table is known.
• Greenfield betterment. A surface water drainage strategy showing the 30% reduction on greenfield rates that Wiltshire Council's 2024 Betterment Strategy asks for.
• After the decision. The discharge of drainage conditions attached to Wiltshire and Swindon permissions, where a condition may call for winter groundwater readings before infiltration is accepted.
• Salisbury Plain and the Marlborough Downs. Where the SFRA's emergence mapping or the 2026 warnings put a site on rising chalk groundwater, a groundwater flood risk assessment sets out the levels, the timing and what the building and its drainage must allow for.
• Off mains in the downland villages. A foul drainage strategy for rural sites on Salisbury Plain and the downs, where the council records groundwater flooding septic tanks and getting into the sewers.
• The Bristol Avon towns. Site-level flood modelling in Chippenham, Melksham or Bradford on Avon, where national mapping is too coarse to set floor levels beside the Avon.
• Long floods on the chalk. A flood warning and evacuation plan for sites that may be cut off for weeks, as in 2026, when groundwater alerts around Chilmark and Teffont stayed in place until the end of March.
• Homes with a flood history. Owners among the 500-plus properties flooded in 2013/14, or hit by Storm Bert in 2024, can give an insurer a flood risk report for insurance based on the building itself.
• Solar and battery sites. Wiltshire Council now has a technical note of its own on solar farms and battery storage, and a renewable energy flood risk assessment answers it.
• Growth land in Swindon. A flood risk feasibility study before land is promoted, as with the New Eastern Villages, planned for about 8,000 homes around the River Cole.
• National policy in a plan gap. Our NPPF flood risk guidance explains chapter 18 of the August 2026 NPPF, which Wiltshire applications rely on alongside a Core Strategy adopted in 2015.

Town pages and the counties next door

For a site in one of these places, the town page covers its own rivers, flood record and policy context in more detail:

• South Wiltshire. Salisbury, where four chalk rivers meet the Avon, and Warminster on the upper Wylye below Salisbury Plain.
• North Wiltshire. Malmesbury, almost surrounded by the two arms of the Bristol Avon.
• Swindon. Swindon, on the Ray and the Cole under the borough council's own plan.
• Downstream and next door. The Bristol Avon continues to Bath, the Kennet into Berkshire and the Hampshire Avon south through Hampshire; the Thames rises just over the border in Gloucestershire.

Everywhere else in England and Wales is on our list of the areas we cover.

Reporting flooding and getting warnings in Wiltshire and Swindon

If water is entering a building or anyone is in danger, call 999. Otherwise, use these contacts to report flooding, check warnings and find the body responsible.

Our guide to lead local flood authorities sets out what each council does and does not deal with when flooding is reported.

Wiltshire flood risk questions from buyers, builders and planning consultants

Unda Consulting Limited prepares flood risk assessments, sequential and exception tests and drainage strategies for homes, farms, businesses and development land across Wiltshire and Swindon. To discuss a site, call 01293 214444 or email enquiries@unda.co.uk.