Flood Risk & Drainage Requirements for Mid Sussex District Council

Mid Sussex District Council treats high and medium surface water flood risk as if it were Flood Zone 3, and low surface water risk as Flood Zone 2, so a site the national flood map leaves in Flood Zone 1 can still need a flood risk assessment and a sequential test. The policy behind it is DP41 (Flood Risk and Drainage) in the Mid Sussex District Plan 2014–2031, adopted in March 2018, read with the council's own drainage checklists and West Sussex County Council's August 2026 SuDS guidance.

Below are the Mid Sussex flood risk and drainage requirements set by Mid Sussex District Council (MSDC), West Sussex County Council and the Environment Agency, policy by policy, with what our reports put in front of each of them. To talk through a Mid Sussex application, call 01293 214444 or email enquiries@unda.co.uk.

The Mid Sussex District Plan in November 2026: adopted plan, examination resumed

Two plans are in play on a Mid Sussex application. The adopted District Plan 2014–2031 is the development plan, with Policy DP41 as its flood and drainage policy, and the Site Allocations Development Plan Document (adopted 29 June 2022) adds site-by-site requirements through Policy SA GEN. Its replacement, the Mid Sussex District Plan 2021–2039, was submitted for examination in July 2024 and is still at examination.

The Inspector held hearings in February and March 2026 and recommended main modifications, which the council consulted on from 27 July to 7 September 2026. Hearings resumed on Tuesday 3 November 2026 under the Inspector's updated programme (IDJB-17, issued 30 October 2026), with the sessions running to 6 November, according to the council's examination page. The flood modification is MM4, which rewrites proposed Policy DPS4 (Flood Risk and Sustainable Drainage); MM5 deletes proposed Policy DPS5 (Water Neutrality). DP41 is still part of the adopted plan. In recent delegated reports, such as DM/26/1593 (September 2026) and DM/26/1886 (October 2026), officers have given DP41 very limited weight as materially inconsistent with the NPPF and have also considered Chapter 18 policies F4, F7 and F8. Both reports still found that the schemes complied with DP41.

District, county or Environment Agency: who decides what on a Mid Sussex scheme

Mid Sussex has a district drainage team of its own, which reads the drainage on minor schemes that a county LLFA would never see. The council's Flood Risk and Drainage for Planning page says it can be consulted on any type of development. West Sussex County Council, as Lead Local Flood Authority, comments on major schemes, so a large Mid Sussex application has its drainage read twice: once by the district and once by the county.

Mid Sussex Local Plan flood and drainage policies, and how we meet each one

Requirements checked November 2026.

Mid Sussex Policy DP41 (Flood Risk and Drainage): what it asks of an application

Policy DP41 asks every proposal to take a sequential, risk-based approach, stay safe for its lifetime and not raise flood risk elsewhere. It points to the council's Strategic Flood Risk Assessment as the source for present and future risk from rivers, surface water, groundwater, infrastructure and reservoirs. In the District Plan's own words, "Particular attention will be paid to those areas of the District that have experienced flooding in the past and proposals for development should seek to reduce the risk of flooding by achieving a reduction from existing run-off rates" (District Plan, March 2018).

The drainage half is specific. SuDS are expected on every scheme of 10 or more dwellings, or the non-residential equivalent, unless shown to be inappropriate, with arrangements for their long-term maintenance. On brownfield redevelopment, any surface water reaching the foul sewer has to be disconnected and managed through SuDS. Surface water then follows a three-step order: infiltration first, attenuation and discharge to a watercourse second, and a surface water only sewer last.

Three gaps leave a DP41 case open to objection: screening that stopped at the national flood map, a drainage route chosen before infiltration was ruled out, and SuDS with nobody named to maintain them. Our flood risk assessment screens the site against the 2024 SFRA mapping and the surface water data the council treats as Flood Zone 2 and 3 equivalents. The drainage strategy then works down the hierarchy, with the evidence for ruling out each step, and says who will maintain each feature. Because officers now give DP41 itself very limited weight in some delegated reports, the same reports also answer the Chapter 18 tests officers are now considering alongside it.

Policy DP42 (Water Infrastructure and the Water Environment) and the sewer question

DP42 makes sewer capacity the applicant's problem. A proposal that adds demand on foul and surface water infrastructure is permitted where it can show that capacity already exists off site, or how upgrades approved by the sewerage undertaker will be finished before anyone moves in. The council's application checklist asks for public sewer records and, where relevant, an outfall agreed in principle with the water company. Our foul and surface water strategies set out the connection points, the flows and the undertaker's position, so the DP42 case sits in the same document as the drainage.

Policy SA GEN (General Principles for Site Allocations) on an allocated site

For the sites allocated in 2022, SA GEN requires "a site-specific Flood Risk Assessment (FRA)/surface water drainage strategy in areas at risk from fluvial or surface water flooding to inform the site layout" (Site Allocations DPD, June 2022). It also asks for a sequential approach to layout that allows for climate change. An allocation is exempt from a fresh sequential test only where the flood risk has not materially changed since it was allocated, so our assessment checks the current mapping against the evidence the allocation relied on before the layout is drawn.

Proposed Policy DPS4 (Flood Risk and Sustainable Drainage) after main modification MM4

Main modification MM4, consulted on from July 2026, strips most of the submitted wording out of proposed Policy DPS4. Gone are the Qbar discharge rule with its 2 l/s/ha ceiling, the bars on highly and more vulnerable development in the 1 in 100 and 1 in 30 extents, the drainage hierarchy and the safeguarding of the 1 in 30 functional floodplain. Most of what remains points applicants to national policy: "Development proposals should assess relevant sources of flood risk in accordance with national policy and guidance". Where a site-specific FRA is required, Environment Agency or site-specific mapping and the SFRA are still to be used to identify current and future risk from all sources, with their cumulative impacts considered, and any flood mitigation should prioritise natural flood management and nature-based solutions. The modified policy also asks for SuDS that "provide multiple benefits" and a lifetime maintenance and management plan "to be secured by condition at planning application stage" (main modifications schedule, July 2026). If the Inspector confirms MM4, the detailed local standards will live in the council's checklists and the county's SuDS guidance rather than in the plan itself.

Two neighbourhood plans add drainage policies of their own. The Haywards Heath Neighbourhood Plan (made December 2016) Policy E7 asks new housing and commercial development to include SuDS where practical and to say how they will be managed. The Hassocks Neighbourhood Plan, made by the district council on 24 June 2020, asks proposals under Policy 4 (Managing Surface Water) to reduce existing run-off rates first and to infiltrate close to source where it is technically feasible.

Surface water as Flood Zone 2 and 3: when Mid Sussex asks for each report

The council's Validation Criteria (Local List), last reviewed in September 2026, sets the triggers, and it reaches well into minor development. A single new or replacement dwelling normally needs a drainage assessment, the same as a major scheme.

The validation list exempts five cases from the sequential test. They are householder work with no new dwelling, non-domestic extensions under 250 m², most changes of use, sites allocated in the adopted or submission District Plan where flood risk has not materially changed, and sites at risk only from surface water where a site-specific FRA shows the development will be safe for its lifetime. The council also warns that minor extensions inside the 1 in 30-year flood extent will be objected to on flood risk grounds. Every FRA also has to include a statement that the SuDS will not increase the bird strike risk to the airport, with the Civil Aviation Authority's guidance on wildlife hazards around aerodromes given as the reference.

Where a sequential test applies, the council's application checklist expects "a defined search area as agreed with the Local Planning Authority" and a table of alternative sites, and tells applicants to treat all land in Flood Zone 3 as functional floodplain (Zone 3b) unless robust modelling or other site-specific evidence shows otherwise. Our guide to the sequential and exception tests and our note on the surface water sequential test exemption set out the national position these local rules sit on.

The 2024 SFRA, the October 2026 update and the validation list

Mid Sussex's evidence base is the Level 1 Strategic Flood Risk Assessment (final issue July 2024) and the Level 2 SFRA (August 2024), which assessed 18 proposed allocation sites with a risk of flooding. The council says its Level 1 SFRA was reviewed and signed off by the Environment Agency and the county. The SFRA is the reason for the surface water rule: the district sits at the head of four river catchments (the Ouse, Adur, Medway and Mole), many of its streams have never been modelled, and their floodplains show up only as surface water risk on national mapping. The council's checklist makes the SFRA mapping, including the interactive SFRA map, the primary evidence for screening a site.

For the examination, the council added ENV23 Flood Risk Update (October 2026), which scores the 35 extra site allocations in the main modifications and finds 33 of them in Flood Zone 1, most with some surface water risk. It does not replace the 2024 SFRA for anything else. Our explainer on what a strategic flood risk assessment does covers how an SFRA feeds a site-specific report.

The validation list is the document an application is checked against at the door, and in Mid Sussex it points straight to the two drainage checklists below. A missing flood or drainage document can stop an application at validation; our article on why applications are invalidated for flood risk and drainage lists the usual gaps.

The council's own drainage checklists: application stage and condition discharge

MSDC publishes two checklists, both version 2.0 issued on 7 November 2025 and aligned with the June 2025 national SuDS standards. Each has a right-hand column for the applicant to cross-reference where every item is answered, and the council asks for the completed checklist with the submission.

• Application stage. The Flood Risk and Drainage Information Checklist asks for screening against the SFRA, the Flood Map for Planning and the surface water dataset. It also wants a basic drainage strategy with every attenuation feature inside the red line, the hierarchy of infiltration, watercourse, surface water sewer and combined sewer followed with each higher option ruled out, and the sequential test evidence.
• Infiltration testing. BRE 365 testing is not needed at application stage unless infiltration is the only way surface water leaves the site, with no secondary outfall.
• Condition discharge stage. The Detailed Drainage Design Checklist sets greenfield runoff (Qbar) by FEH or an approved method, watercourse discharge restricted to Qbar, sewer discharge at Qbar unless the undertaker agrees more, FEH rainfall with FSR refused without justification, a Cv of 1.0 and 10% urban creep.
• Storms and levels. Calculations for the 1 in 1, 1 in 30, 1 in 30 plus climate change and 1 in 100 plus climate change critical storms with no flooding; soakaways sized for the 6-hour 1 in 100 plus climate change storm and half-empty within 24 hours; finished floors at least 150 mm above surrounding ground.
• Avoiding the condition. The council says full drainage details at application stage make it more likely that drainage conditions can be avoided; schemes without them discharge the matter after permission.

Both checklists also send applicants to a companion document, MSDC's Technical Summary for the condition discharge stage (July 2025), which sits behind the Detailed Drainage Design Checklist but is not linked from the council's flood page. It sets out local rules the checklists only summarise or leave out.

• Discharge rate. Peak discharge limited to the greenfield rate (Qbar) for the area attenuated; on brownfield land, a 50% reduction on the existing runoff rate or Qbar where achievable. The council "does not accept the automatic argument that 2 l/s is the lowest achievable discharge rate", so any rate, including one below 2 l/s, needs site-specific hydraulic modelling, and orifices as small as 15 mm are acceptable where protected from blockage.
• Runoff volume. The post-development runoff volume for the 1 in 100-year, 6-hour event has to be shown against the greenfield volume, with any long-term storage drained without bypassing treatment.
• Ground testing. BRE 365 tests at the location and depth of each infiltration feature, carried out in winter unless the ground is shown to be at its worst case; at least three months of groundwater monitoring where shallow groundwater is likely; a stability check for any soakaway within 5 m of a structure.
• Basins and ponds. At least 300 mm of freeboard above the design high water level, side slopes no steeper than 1 in 3 without safety measures, and piped tanks used only where above-ground SuDS cannot perform within the site.
• Pumps and outfalls. Pumped surface water is "strongly discouraged"; an outfall that can surcharge has to be modelled; and the submission needs written acceptance in principle from whoever owns the receiving sewer, watercourse or IDB channel.
• Who maintains it. A named body for each part of the system and the legal mechanism behind it, such as a Section 106 agreement, covenants or a sewerage undertaker's Section 104 confirmation.

The district and county documents do not always agree, and our reports meet the stricter figure in each case. On major schemes West Sussex County Council asks for greenfield runoff whatever the sewerage undertaker would accept, which is tighter than the 50% brownfield reduction in the Technical Summary and the sewer allowance in the checklist. The county's half-drain test is the 1 in 30 plus climate change event, where the Technical Summary uses the 1 in 30 without it, and the November 2025 checklist adds soakaway sizing for the 6-hour 1 in 100 plus climate change storm. On minor schemes the district documents govern; the November 2025 checklist tells applicants to follow the Technical Summary, so where the two differ, our reports meet the stricter figure.

At both stages the council applies the Environment Agency's May 2022 climate change allowances to every application validated after 10 May 2022, condition discharges included, with no transition. Our free BRE 365 infiltration rate calculator, drainage calculators and geology and ground conditions map give an early read on whether a soakaway is realistic before anyone digs a trial pit.

West Sussex County Council's SuDS standard on major Mid Sussex schemes

On major development the county's August 2026 guidance, Complying with the National Standards for SuDS, applies on top of the district's checklists. The county's SuDS page (updated 28 August 2026) says it expects every major application to meet the national standards, and that it will not accept high density or lack of space as a reason to leave out source control.

• Discharge rate. Greenfield runoff, whatever higher rate Southern Water or Thames Water might accept for a sewer connection.
• Interception. The first 5 mm of everyday rainfall held on site.
• Ground and groundwater. BRE 365 or an approved method, three tests per pit with the lowest rate used; groundwater monitored to find its maximum level, with 1 m between that level and the base of an infiltration feature.
• Outline applications. At least one viable way of draining the site, the attenuation and outfalls on a layout, and winter groundwater monitoring with infiltration results where infiltration is proposed.
• Water reuse. West Sussex is described as seriously water stressed, so major applications are expected to consider rainwater harvesting from outline stage.

The county also offers paid pre-application advice on major development, and its pre-application page says the SuDS proforma and its flood risk validation checklist are being reviewed against the national standards, with the current versions still in use. The checklist and the county guidance overlap but are not identical, so our drainage strategies for major Mid Sussex schemes are checked against both. Our summary of the 2025 national standards for sustainable drainage explains where the county's figures come from.

NPPF Chapter 18 behind the Mid Sussex requirements

National policy is the August 2026 National Planning Policy Framework, Chapter 18 (Managing flood risk and coastal change). Policy F4 asks for a site-specific flood risk assessment in Flood Zones 2, 3a and 3b and on defined Flood Zone 1 sites. F5 sets the sequential test, with an exemption for sites at risk of surface water flooding only; F6 sets the exception test; and F8 asks for sustainable drainage designed to the National Standards for Sustainable Drainage Systems. Alongside it sits the flood risk and coastal change Planning Practice Guidance, most recently revised on 17 September 2025.

The council's checklist and validation list still cite the superseded December 2024 Framework by paragraph, but the tests they apply match Chapter 18. Our summary of the August 2026 NPPF changes covers what moved.

Flood risk across Mid Sussex in brief

Surface water is the dominant source across the district, with river flooding confined to a few Main River corridors near the heads of their catchments. The River Ouse rises in the High Weald near Lower Beeding and passes Ardingly and Lindfield, with the Scrase Bridge Stream through Haywards Heath. The eastern branch of the River Adur runs west from the Burgess Hill area towards Henfield, and the upper Medway rises around East Grinstead.

For the picture town by town, see our pages on flood risk assessment in Haywards Heath, East Grinstead and Hassocks. Our flood risk map shows the flood zones and surface water extents for any postcode, and the live river levels map shows current readings at Environment Agency gauges.

Our reports for an application to Mid Sussex District Council

A flood risk assessment asks whether a Mid Sussex site is safe, the sequential test asks whether the scheme should be on that land at all, and a drainage strategy shows where the water goes; each is its own report. We have prepared reports for applications to the district, and every service below is written to its rules.

• Every report we write. Each Mid Sussex report is prepared by our consultants against DP41, the council's checklists and, on major schemes, the county's standard, starting from a flood risk assessment. Reports for applications to Mid Sussex District Council come from Unda Consulting Limited, a CIWEM Business Partner.
• Planning applications. A site-specific flood risk assessment for planning screened against the 2024 SFRA and the surface water data the council treats as Flood Zone 2 and 3.
• Buying in the district. A flood risk survey for property purchase where a search flags surface water risk on a Mid Sussex home or site.
• Testing alternative sites. A sequential and exception test built on a search area the council's checklist expects to be defined and justified.
• Drainage at application stage. A drainage strategy that answers DP41's hierarchy and the validation list's drainage assessment for new and replacement dwellings.
• Surface water on larger schemes. A surface water drainage strategy set to Qbar, with the 50% brownfield reduction and justified flow controls the council's Technical Summary asks for, and to the West Sussex August 2026 guidance on major development.
• After permission. Discharge of drainage conditions against MSDC's condition discharge checklist and its July 2025 Technical Summary, with winter BRE 365 results, head-discharge curves, freeboard and the maintenance plan.
• Foul water and sewer capacity. A foul drainage strategy where DP42 or the checklist's foul section asks for capacity evidence or a non-mains system.
• Groundwater. A groundwater assessment where the county's winter monitoring and 1 m clearance decide whether infiltration works.
• Emergency planning. A flood warning and evacuation plan where the application checklist asks for the warning, access and evacuation arrangements.
• National policy questions. NPPF guidance and advice on how Chapter 18 applies to a Mid Sussex scheme while the District Plan is at examination.

The same county standards apply in the neighbouring West Sussex districts; see our pages on Crawley, Horsham, Arun and Adur & Worthing, or the full list of council requirements pages.

Mid Sussex flood and drainage questions

For anything on this page, call 01293 214444 or email enquiries@unda.co.uk. Unda Consulting Limited, flood risk and drainage consultants.