Flood Risk & Drainage Requirements for Arun District Council

Arun District Council tests flood risk against Policy W DM2 and drainage against Policy W DM3 of the Arun Local Plan 2011–2031, adopted in July 2018, with West Sussex County Council advising on major schemes as Lead Local Flood Authority. The requirement with the longest lead time is W DM3's call for winter groundwater monitoring and winter infiltration testing, evidence that can only be gathered between October and March. On two or more homes it is a validation item, so it has to be in hand before the application goes in.

Below, each Arun flood and drainage policy is quoted and dated, with the validation triggers, the council's own drainage guidance and the county's August 2026 standard, and what our reports put in front of officers to meet each one. To talk through a site, call 01293 214444 or email enquiries@unda.co.uk.

District, county, Environment Agency or National Park: who decides on an Arun application?

Arun District Council makes the decision. On a major scheme two drainage reviews run side by side, the district's own engineers and the county as LLFA, and each will object if its standard is missed. Ordinary watercourse consent sits with the county; Arun stopped handling it for West Sussex County Council, though works that need no county consent may still need Arun's byelaw consent (council watercourses guidance). Our guide to Lead Local Flood Authorities and our note on what a local planning authority checks explain how the two roles divide.

Inside the National Park the South Downs National Park Authority decides, applying Policies SD49 (Flood Risk Management) and SD50 (Sustainable Drainage Systems) of the South Downs Local Plan adopted in July 2019. Its replacement plan finished its Regulation 19 consultation on 23 June 2026, and the Authority is due to decide on 8 December 2026 whether to submit it.

The 2018 Local Plan policies every Arun scheme is measured against

Requirements checked November 2026.

Arun Policy W DM3 (Sustainable Urban Drainage Systems): the winter evidence

Policy W DM3 asks every development, major or minor, to plan SuDS from the start and, where water is to soak away, to prove it with winter data. Two of its tests are set out in plain terms: developers must "Undertake up to six months groundwater monitoring within the winter period" and "Undertake winter percolation testing in accordance with BRE365" (Arun Local Plan, Policy W DM3).

The policy goes on to require no flooding in a 1 in 30 year storm and, for the 1 in 100 year storm, a 30% climate change allowance on stored volumes with no flooding of homes, the highway or the foul sewer. That 30% dates from 2018. Arun's guidance now asks for the upper end allowance for the lifetime of the development from the Environment Agency's climate change allowances (last updated 27 May 2022), and current calculations should follow it.

It applies to any scheme that adds hard surfaces or roofs. The bar is lower for small sites in practice, because the council scales its monitoring expectations to roof area, but the policy itself has no lower size limit.

Officers and the council's drainage engineers look for test pits where the SuDS will actually go, at the depth they will sit, run three times and observed to empty; and for groundwater readings that prove a metre of unsaturated ground below each infiltration feature. Averaging results from pits some distance from the features does not satisfy them. In an appeal at Roundstone Lane, Angmering (APP/C3810/W/24/3349316, 31 March 2025), the Inspector would not approve a soakaway scheme designed on averaged pit results without long-term groundwater monitoring, and found the details did not show the site would drain in line with Policies W SP1, W DM1, W DM2 and W DM3.

Our drainage strategies for Arun applications start with the ground. We set out the monitoring and testing the site needs, and when, before the layout is fixed, then write the strategy around the results: infiltration where the winter data supports it, a restricted outfall where it does not, and the storage, exceedance routes and maintenance plan the policy names. Background on the delays this causes is in our article on groundwater monitoring for planning.

Arun Policy W DM2 (Flood risk): six criteria, all of them

Policy W DM2 permits development in areas at risk of flooding only where every one of six criteria is met. The second is the core of any FRA: it must show the development "will be safe, including access and egress, without increasing flood risk elsewhere and reduce flood risk overall" (Arun Local Plan, Policy W DM2).

Areas at risk are those on the latest Environment Agency maps and in the council's SFRA, so the policy reaches land at risk from surface water and groundwater as well as Flood Zones 2 and 3. Criterion (e) is easy to miss: "Appropriate flood warning and evacuation plans are in place". Criterion (f) asks for site drainage that copes with storms beyond the design standard, through flood routing and temporary storage. The policy also asks schemes to take account of the Lower Tidal River Arun Strategy and allows the council to seek contributions where a strategy names works.

An FRA written to W DM2 deals with each lettered criterion in turn. Ours set floor levels and a safe route out against the climate change flood level, include the sequential test as a separate report where (a) applies, and add a flood warning and evacuation plan wherever (e) is engaged rather than leaving it to a condition.

Arun Policy W DM1 (Water supply and quality): the Lidsey catchment

Policy W DM1 is a water supply policy with a flood rule inside it. In the Lidsey Wastewater Treatment Works catchment, majors need "a full Drainage Impact Assessment", and "all development within this area must also be accompanied by a Drainage Impact Assessment" covering foul disposal, flood storage and surface water flows (Arun Local Plan, Policy W DM1).

The catchment takes in Barnham, Eastergate, Flansham, Fontwell, part of Middleton-on-Sea, Norton, Aldingbourne, Slindon, Walberton, Westergate, Woodgate and Yapton (council Lidsey page). Groundwater getting into the sewers and surface water reaching the foul network have flooded roads and homes there. The validation list makes the assessment item 11, and a site in one of the Lidsey Surface Water Management Plan's Local Flood Risk Zones must name its zone and the interventions it will carry out. We write the Lidsey assessment into the FRA where that keeps the submission tidy; our Lidsey drainage impact assessment guide covers the content.

Policy W SP1 (Water) and Policy ECC SP1 (Adapting to climate change)

These two strategic policies sit behind the detailed ones; W SP1 was cited with W DM1, W DM2 and W DM3 in the Angmering appeal decision. W SP1 supports development that is "appropriately located, taking account of flood risk" and uses SuDS to cut runoff and improve water quality. ECC SP1 asks designs to account for location against flood risk and coastal erosion, and for drainage capacity. A flood risk assessment and drainage strategy that meet W DM2 and W DM3 meet both, provided climate change runs through the levels and the storage figures.

Policy W DM4 (Coastal protection)

W DM4 governs sea defence and coast protection works themselves, not the homes behind them. Development on the frontage still needs a flood risk assessment under W DM2, and at Pagham Beach the council applies an interim framework while it considers a Coastal Change Management Area (see the coast section below).

How far has the Arun Local Plan update got?

Not far. The Arun Local Plan 2023–2041 update has reached only its Regulation 18 Issues and Options stage, consulted on from 25 March to 13 May 2024 (council plan update page). The Local Development Scheme 2025–2028 timetabled draft policies for consultation between April and September 2026 and adoption in summer 2028, but no draft plan has been published and there are no draft flood or drainage policies.

The plan-making reforms and West Sussex local government reorganisation have since overtaken that timetable. On 3 September 2026 the council's Planning Policy Committee recommended that Full Council, meeting on 11 November 2026, let the committee settle on 30 November how the council responds to the requirement to give notice of intention to start a new plan by 31 December 2026 (committee papers). Until a new plan is adopted, Policies W DM2 and W DM3 remain the tests.

Validation items 9 and 10: what triggers each report in Arun

Arun's validation requirements, in force since 17 October 2024, add local triggers to the national ones. Missing items have to be supplied within two weeks of the council asking; if they are not, the application is returned and Arun keeps £40 of the fee. Our article on why applications are invalidated over flood risk and drainage runs through the common gaps.

Item 10 applies well below the major threshold. Two houses on a garden plot need the same winter ground data as a larger scheme, and outline applications are not excused the ground investigation. Not sure which rows apply? Check the address on our flood risk map, then on the council's validation interactive map for the SFRA zones and the Lidsey catchment. The sequential test is a separate report from the FRA; our explainer on the sequential and exception tests shows how the two fit.

Arun's own drainage guidance: monitoring, testing, flood areas and discharge rates

Behind W DM3 sits a set of council guidance notes, last updated in October and November 2025, and five designer checklists dated October 2025. Critical items on the Outline SuDS Design Checklist are in bold, and the council says a failure to address them will result in an objection to any outline application. The checklists apply to minor and major applications alike.

• Groundwater monitoring. Arun's Ground Investigations Checklist (October 2025) asks for professionally installed boreholes at each proposed SuDS feature or within 2 m of it, read at least once a week, over a suggested six consecutive months between 1 October and 31 March. Where less than 250 m² of new roof is added, one monitoring point (an excavated hole or a borehole) over a suggested two consecutive months between 1 December and the end of February will do, and data from within 50 m may stand in with the owner's permission; under 20 m², the suggested period is one month. The council's groundwater guidance adds that desk studies are not accepted, and that if monitoring stops early the council assumes groundwater at ground level for flotation checks.
• Infiltration testing. Strictly to BRE Digest 365 or CIRIA R156, in pits where the features will go and at their base level, three tests in quick succession, run from full to empty, preferably between December and February. Borehole tests are not generally accepted (council infiltration guidance). Our BRE 365 explainer and BRE 365 infiltration rate calculator cover the method.
• SuDS in flood areas. No SuDS storage in fluvial Flood Zones 2, 3a or 3b or the 0.1% AEP surface water extent unless the whole site lies within them or an exception applies, such as tidal-only risk, an evidenced defended zone, conveyance features, green or blue roofs and sealed tanks. Never in flood storage or flood compensation areas (position statement, 24 October 2025).
• Discharge rates. Since the National Standards were published on 19 June 2025, Arun accepts QBAR or 3 l/s/ha as the minimum discharge rate below which the impact of discharge volumes need not be assessed (council Standard 3 guidance). Previously developed sites are assessed against greenfield runoff equivalents or 3 l/s/ha, with a relaxation of up to five times the greenfield rate agreed only where evidence shows those rates cannot be achieved (council guidance on previously developed sites, 24 October 2025).
• Drain-down and interception. Infiltration must half-empty the 3.3% AEP event within 24 hours, and an interception assessment for the first 5 mm of rainfall belongs in every drainage statement.

The engineers also want FEH 22 point data files sent direct to the council's land drainage inbox, and suggest modelling storms from 15 minutes to 7 days. Our geology and soakaway checker gives an early read on whether infiltration is worth pursuing, and our drainage calculators help size storage before a layout is drawn. Our article on the October 2025 Arun checklists goes through them item by item.

West Sussex County Council's August 2026 standard on major Arun schemes

On major applications the county reviews drainage against its guidance Complying with the National Standards for SuDS (August 2026). Its SuDS page (updated 28 August 2026) says the county "will not accept high density housing or lack of space as reasons to exclude source control SuDS".

On a major scheme the county's greenfield rule sets the rate, and Arun's engineers review the same figure against the National Standards, so the strategy should show the greenfield calculation and say where any minimum rate has been applied. The county also says its SuDS proforma and validation checklist are under review against the National Standards. Our summary of the 2025 National Standards for SuDS and our note on answering an LLFA objection cover both sides.

The March 2026 SFRA and the sequential test in Arun

The current evidence is the Arun Level 1 Strategic Flood Risk Assessment, JBA Consulting's final report of March 2026, which supersedes the 2016 SFRA and covers the district outside the National Park. It defines Flood Zone 3b as the 3.3% AEP extent and, outside the detailed models, uses Flood Zone 3a as a cautious stand-in to be refined by a site-specific FRA. No Critical Drainage Areas are designated. Most of the district is at risk of groundwater emerging within 0.5 m of the surface, with Bognor Regis, Walberton, Angmering, Ford, Barnham and Aldingbourne most exposed.

The council's flood risk planning policy page adds two updates: the Environment Agency's long-term risk data now carry a 2070 to 2125 climate change layer much larger than the SFRA's, and surface water climate change extents joined the Flood Map for Planning on 28 May 2026. The council says the latest data should always be used for the sequential and exception tests. Its coast page still points developers to the 2016 SFRA; the March 2026 report is the one in force. Our guide to strategic flood risk assessments explains Level 1 and Level 2.

National policy behind Arun's requirements: NPPF Chapter 18

The National Planning Policy Framework published on 17 August 2026 sets flood risk out in Chapter 18, Managing flood risk and coastal change, as policies F1 to F9. F4 says when a site-specific FRA is needed, F5 and F6 set the sequential and exception tests, F8 expects SuDS to the National Standards with LLFA advice on major schemes, and F9 governs development in Coastal Change Management Areas. The government's flood risk and coastal change guidance dates from its 17 September 2025 revision. Our summary of the August 2026 NPPF changes and our NPPF guidance service cover the detail.

Climate change allowances come from the Environment Agency, last updated on 27 May 2022; the SFRA places the district in the Arun and Western Streams management catchment. Our article on climate change allowances for planning explains which epoch applies.

The coast, the River Arun and the rifes: Arun's flood risk in brief

Three things drive flood risk here. The tidal River Arun runs through Arundel to the sea at Littlehampton, and the SFRA rates it the district's largest river risk; in April 2024 Storm Kathleen overtopped the west-bank defences and flooded homes on Rope Walk (West Sussex flood reports). The rifes (Aldingbourne, Lidsey, Ryebank and Ferring) drain a flat coastal plain whose outfalls lock shut at high tide. And the sea reaches Bognor Regis, Felpham, Middleton-on-Sea and Pagham, and Storm Ciarán brought flooding to Bognor Regis, Shripney and Yapton in November 2023.

The low ground beside the lower Arun is the River Arun Internal Drainage District, the only one in West Sussex, where the Environment Agency acts as the drainage board. At Pagham Beach Estate the council is considering a Coastal Change Management Area and, since 6 October 2021, has applied an interim framework that refuses all new development and sets floor levels at least 300 mm above predicted flood levels on permitted redevelopment (council coast page). On the coast we prepare flood risk assessments and due diligence.

For each town's history see our pages on flood risk assessment in Littlehampton, Arundel, Bognor Regis and Pagham, the River Arun, and the West Sussex county overview.

Our reports for an application to Arun District Council

A flood risk assessment asks whether the site is safe, the sequential test asks whether it should be there, and a drainage strategy shows where the water goes. Arun can ask for all three on one application, and the rows below give the Arun requirement behind each.

• Every source of flooding. Policy W DM2 and validation item 9 set when Arun needs an assessment, and our flood risk assessment work covers each type.
• With the application. Flood Zone 2 or 3, over a hectare, or an SFRA groundwater or future-risk zone: a flood risk assessment for planning.
• Buying on the coastal plain. Where a search flags the tidal Arun, a rife or the sea, a flood risk survey for property purchase looks at the building itself.
• Criterion (a). W DM2 requires the sequential test on land at risk, and the exception test where national policy calls for it: a sequential and exception test.
• Two homes or more. Validation item 10's drainage statement and the Lidsey rules are met in a drainage strategy.
• Major schemes. Both reviewers, Arun's engineers and West Sussex County Council, are answered in one surface water drainage strategy.
• After permission. Arun's pre-commencement drainage and verification conditions are cleared through discharge of drainage conditions.
• The water table. W DM3's winter monitoring and the SFRA's groundwater emergence mapping make a groundwater flood risk assessment worth scoping early.
• Criterion (e). W DM2 asks for flood warning and evacuation arrangements on land at risk: a flood warning and evacuation plan.
• Outside the detailed models. Where the SFRA uses Flood Zone 3a as a stand-in for 3b, flood modelling can define the functional floodplain on the site.
• Lidsey and the foul network. W DM1 asks for foul disposal in the Lidsey assessment and resists non-mains drainage in sewered areas: a foul drainage strategy.
• National policy. Our NPPF flood risk guidance covers Chapter 18 on an Arun scheme.

Arun shares West Sussex County Council as its LLFA with Horsham, Crawley, Mid Sussex and Adur & Worthing; the full list is on our local authorities page.

Arun District Council flood and drainage questions, answered

Unda Consulting Limited's chartered consultants prepare flood risk assessments, sequential tests and drainage strategies for applications to councils across England, Arun District Council among them. Reports for applications to Arun District Council come from Unda Consulting Limited, a CIWEM Business Partner. Call 01293 214444 or email enquiries@unda.co.uk for a fixed quote within the hour.