Flood Risk & Drainage Requirements for Mid Sussex District Council

Getting a development through Mid Sussex District Council turns on two questions that are decided by different people: is the site safe from flooding, and where does its surface water go. Mid Sussex is the planning authority that determines your application, but West Sussex County Council, as the Lead Local Flood Authority, signs off the drainage, the Environment Agency leads on the main rivers, and the council runs its own in-house Flood Risk and Drainage Team on top of all three. Knowing which body wants what, and getting the flood risk assessment and drainage strategy right first time, is the difference between a clean validation and an application that stalls for months.

We prepare the flood risk assessments and drainage strategies that Mid Sussex validates and its officers, the county drainage team and the Environment Agency accept — for planning, property purchase, mortgage and insurance, and BREEAM — alongside sequential and exception tests and sustainable drainage design. We are experts on how this authority reads an application: its adopted Policy DP41, its 2024 Strategic Flood Risk Assessment, its published information checklists, and the West Sussex discharge standards a drainage scheme has to hit.

If you want to talk a Mid Sussex site through or need a quote, contact one of our experienced flood risk consultants on 01293 214444 or email enquiries@unda.co.uk.

The trap in Mid Sussex is surface water. The council treats high and medium surface-water flood risk as the equivalent of Flood Zone 3, and low surface-water risk as the equivalent of Flood Zone 2 — so a site the national flood map shows as low risk can still need a full flood risk assessment and a sequential test here.

Flood risk and drainage for a Mid Sussex application at a glance

• Two councils, two roles. Mid Sussex District Council decides the application; West Sussex County Council, as Lead Local Flood Authority, is the statutory consultee on surface water drainage for major schemes.
• An in-house district flood team. Unusually, Mid Sussex runs its own Flood Risk and Drainage Team that reviews both major and minor applications — so drainage scrutiny here reaches further down the scale than in many districts.
• Adopted Policy DP41. The Mid Sussex District Plan 2014–2031 (adopted 28 March 2018) sets flood risk and sustainable drainage through Policy DP41, backed by the 2024 Strategic Flood Risk Assessment.
• Greenfield discharge, and no automatic 2 l/s. Surface water must be restricted to the greenfield runoff rate (Qbar), and the council will not accept the old 2 litres-per-second minimum without site-specific justification.
• Surface water dominates. Mid Sussex is generally low fluvial risk; the main flood source across the district is surface water, followed by the Ouse, the Adur and the upper Medway.

Who decides flood risk and drainage on a Mid Sussex application?

Four bodies have a say on a Mid Sussex scheme, and they answer different questions. Mid Sussex District Council determines the application and applies its Local Plan flood and drainage policy. West Sussex County Council, as the Lead Local Flood Authority, is the statutory consultee on surface water and drainage. The Environment Agency leads on the main rivers. And the council's own Flood Risk and Drainage Team reviews the flood and drainage case in-house — a layer many neighbouring districts do not have.

Because Mid Sussex is a two-tier area, a single application is often being read on two fronts at once: a flood-risk test at the district, and a drainage test at the county. A submission that satisfies one but not the other is the most common reason schemes here get held up, which is why the flood risk assessment and the drainage strategy have to tell one consistent story.

Does Mid Sussex require a flood risk assessment or a drainage strategy?

A Mid Sussex application needs a site-specific flood risk assessment, a surface water drainage strategy, or both, whenever national triggers or the council's own criteria are met. The Environment Agency's triggers apply — any part of a site in Flood Zone 2, 3 or 3b on the Flood Map for Planning; sites of a hectare or more in Flood Zone 1; land in a critical drainage area; or a Flood Zone 1 site the council's SFRA shows to be at risk. On top of that, Mid Sussex publishes its own list of development types that must provide expanded flood and drainage information.

The council's review criteria catch far more than major housing. They pull in any new build (including demolition and rebuild that creates a new unit), large extensions that increase the built footprint by roughly 50% or more, any subterranean development including a change of use, works that create or alter surface water features such as ponds or weirs, and any development in an area at risk of flooding from any source. In practice, that means a great many minor applications in Mid Sussex are expected to submit flood and drainage evidence that would slip through elsewhere.

If you are not certain which of these applies to your site, our guide on when a flood risk assessment is required and our flood risk map are a fast first read — or send us the address and we will tell you within the hour.

Mid Sussex's Local Plan flood and drainage policies

Mid Sussex judges flood risk and drainage against its adopted Local Plan, and an application has to answer the right policy by its code. The adopted plan is the Mid Sussex District Plan 2014–2031, adopted on 28 March 2018, in which Policy DP41: Flood Risk and Drainage is the controlling policy. A new Mid Sussex District Plan 2021–2039 is at examination, carrying its flood and drainage rules in Policy DPS4; because it is not yet adopted it holds limited weight, but it signals the direction of travel and is worth designing to now.

Policy DP41 — flood risk and drainage

Policy DP41 in the adopted Mid Sussex District Plan requires that development does not increase flood risk on the site or elsewhere, and that surface water is managed through sustainable drainage systems designed to deliver water-quality and ecological benefit, not just to move water off site. In practice DP41 is the hook the council hangs a drainage objection on, so a strategy that follows the drainage hierarchy and holds discharge to greenfield rates is answering DP41 directly. It is read alongside the NPPF and its Planning Practice Guidance.

Policy DPS4 — the emerging position

Policy DPS4 in the emerging District Plan 2021–2039 carries the same principle forward and sharpens the drainage half: a site-specific flood risk assessment where the site is at risk, sustainable drainage on developments of 10 or more dwellings unless it can be shown to be inappropriate, and a SuDS maintenance and management plan secured by planning condition. It cannot yet be given full weight, but designing to DPS4 now — particularly the maintenance and adoption evidence — future-proofs a scheme and rarely does any harm to a submission under DP41.

Mid Sussex's drainage and SuDS requirements

Drainage is where Mid Sussex applications most often come unstuck, because the standards are specific and the council enforces them. Surface water must follow the drainage hierarchy — infiltrate first, then discharge to a watercourse, then to a surface water sewer, and only to a combined sewer as a last resort — and each step up the hierarchy has to be assessed and, if ruled out, justified. Discharge is restricted to the greenfield runoff rate (Qbar), calculated by the Flood Estimation Handbook method, with brownfield sites expected to achieve at least a 50% reduction on the existing rate. This mirrors the West Sussex Lead Local Flood Authority policy, and it is set out in the council's own published drainage guidance.

Mid Sussex has explicitly closed the old shortcut: it will not accept 2 litres per second as the automatic minimum discharge rate. Any rate, including one below 2 l/s, has to be justified with site-specific hydraulic modelling showing the flow control will stay safe and maintainable for the life of the development.

Beyond the discharge rate, the council's Detailed Drainage Design Checklist and its Technical Summary set out precise, locally-applied expectations aligned to the June 2025 national SuDS standards and the CIRIA SuDS Manual (C753). Getting these right at the outset is what keeps a scheme out of an LLFA drainage objection.

How to use the council's drainage checklists

Mid Sussex has done something genuinely useful: it has published exactly what it wants to see, at each stage. There are two documents worth building your submission around, and the smart move is to treat them as the marking scheme.

• The application-stage checklist. The Flood Risk and Drainage Information Checklist lists, section by section, the flood screening, surface water strategy, foul strategy and sequential-test evidence the council expects with the application. Its right-hand column asks you to cross-reference where each item is answered — report name, page or drawing number. Completing it forces a submission that leaves the reviewer nothing to chase.
• The detailed design checklist and technical summary. The Detailed Drainage Design Checklist and its Technical Summary govern the condition-discharge stage: Qbar calculations, BRE 365 test logs, flow-control head-discharge curves, the Simple Index Approach treatment case, freeboard, side slopes and the maintenance plan.
• Design conditions out, not in. The council states plainly that submitting full detailed drainage design with the application can avoid pre-commencement drainage conditions altogether. That is the single biggest programme saving available in Mid Sussex — it moves the drainage argument to before permission, when there is time to resolve it, rather than after, when it blocks a start on site.

Our notes on what drainage information planning needs, the SuDS hierarchy, the four pillars of SuDS and when infiltration testing is needed explain the thinking the checklists ask you to evidence. For an early read on whether the ground will take water, our free geology and ground conditions map, BRE 365 infiltration calculator and drainage calculators give an indicative answer before a trial pit.

How to avoid an invalidated Mid Sussex application

An application that arrives without the flood and drainage information the council expects gets held up before it is even assessed — and in Mid Sussex the bar is set out in writing, so there is no excuse for guessing. The council's checklists are not the statutory validation list on their own, but much of what they ask for also sits in the validation requirements, and the Flood Risk and Drainage Team uses them to decide whether it can even provide a consultation response. Submit less than they ask for and, at best, you invite a request for further information that adds weeks; at worst, the application is invalidated.

The reliable way through is to answer the checklist in full, cross-reference every item to where it is evidenced, and pre-empt the two things that most often trip Mid Sussex schemes: the surface-water-as-flood-zone rule that pulls apparently low-risk sites into needing an assessment, and the drainage discharge rate. Our guide on why applications get invalidated for flood risk and drainage walks through the documents most often missing, and the council's own flood risk and drainage guidance is the authoritative source for the local requirements.

• Screen against the SFRA, not just the flood map. The council expects the 2024 Strategic Flood Risk Assessment and the surface-water dataset to be used as the primary evidence base, because many watercourses in the district are unmodelled by the Environment Agency and show up only as surface-water risk.
• Treat surface water seriously. High and medium surface-water risk is read as Flood Zone 3-equivalent, and low risk as Flood Zone 2-equivalent, so screening cannot stop at the fluvial flood zone.
• Apply the current climate change allowances. The council applies the Environment Agency's May 2022 climate change allowances to every application validated after 10 May 2022, with no transition provisions, at the condition-discharge stage too.
• Cross-reference everything. Name the report, page or drawing where each checklist item is answered — the reviewer should never have to hunt for it.

The sequential and exception tests on a Mid Sussex site

Where a site sits in Flood Zone 2 or 3, or in Flood Zone 1 but at risk from surface water or another source, Mid Sussex expects the sequential test to be applied before the application is judged on its merits. The council publishes an applicant guidance note on applying the sequential and exception tests to windfall sites, and its expectations are demanding: a policy-led search area agreed with the council, a site-by-site comparison of reasonably available lower-risk alternatives — including land you do not own or control — and a clear justification for the chosen site. Passing is not achieved simply by showing the application site can be made safe.

The sequential and exception tests are a separate report from the flood risk assessment. A flood risk assessment asks whether the site is safe; the sequential test asks whether the development should be there at all. Mid Sussex adds two local wrinkles worth knowing: all land in Flood Zone 3 is treated as functional floodplain (Zone 3b) unless robust modelling shows otherwise, and where a lower-risk alternative falls in the High Weald National Landscape or Green Belt you have to show why that constraint outweighs the flood risk. A clean, site-specific assessment showing the site is not actually at risk can remove the need for the test at application stage.

• Sequential test triggers. Development in Flood Zone 2 or 3, and Flood Zone 1 sites the flood map or SFRA shows to be at surface-water or other risk.
• Exception test triggers. More vulnerable uses in Flood Zone 3a, highly vulnerable uses in Flood Zone 2, and essential infrastructure in Flood Zone 3a or 3b.
• Exemptions. Householder development, small non-residential extensions under 250 m², most changes of use, and sites allocated in the District Plan where flood risk has not materially changed.

The tests are a live and shifting area of planning law, as our note on what the Yatton judgment means explains. Getting the vulnerability classification right is where many arguments are won or lost.

The 2024 Strategic Flood Risk Assessment

Mid Sussex's Strategic Flood Risk Assessment is the local evidence base an applicant is expected to build from. The council holds a Level 1 SFRA (2024) covering the whole district and a Level 2 SFRA (2024) prepared for allocation sites outside Flood Zone 1 or with significant risk from other sources, both signed off by the Environment Agency and West Sussex County Council. It maps flood risk from every source, informs where development is steered, and must be used to screen a site and inform the site-specific flood risk assessment.

The most important thing the SFRA does for a Mid Sussex applicant is capture risk the national flood map misses. Because many of the district's watercourses have never been modelled by the Environment Agency, their flood risk appears only as surface water — which is exactly why the council converts surface-water risk into flood-zone-equivalent categories. Reading the SFRA early, rather than relying on the Flood Zone map alone, is what stops a site being under-assessed.

West Sussex County Council as Lead Local Flood Authority

On surface water and drainage, the body to satisfy is West Sussex County Council. As Lead Local Flood Authority it is the statutory consultee on surface water drainage for all major applications across the district, reviewing flood risk assessments and drainage strategies against the national SuDS standards, the CIRIA SuDS Manual and its own surface water management policy. It runs a pre-application advice service that is worth using on any site where the drainage is likely to be tight.

It reviews schemes against the national SuDS standards, the CIRIA SuDS Manual and its own surface water management guidance. Because the county and the district both review drainage, a Mid Sussex scheme is effectively being marked twice on the same work — so a strategy that follows the discharge hierarchy, holds runoff to greenfield rates, and evidences maintenance and adoption from the outset satisfies both at once. Where either reviewer objects, the fix is almost always a document that was missing or a discharge rate that was not justified. Our note on the interaction between LPAs, LLFAs and SuDS sets out how the two tiers fit together, and once permission is granted the detailed discharge of drainage conditions is a distinct piece of work again.

Flood risk across Mid Sussex

Mid Sussex is generally a low fluvial-risk district whose defining flood problem is surface water. The high Wealden ground around Haywards Heath, Burgess Hill, Cuckfield and Hurstpierpoint sheds intense rainfall quickly, overwhelming drains and ordinary watercourses well away from any mapped river. Where rivers do matter, three systems shape the risk: the River Ouse, which rises in the Weald and passes Ardingly Reservoir and Lindfield; the Eastern Arm of the River Adur, which rises on Ditchling Common and runs between Haywards Heath and Burgess Hill; and the upper River Medway around East Grinstead and the north-east.

Recent events show the pattern. In November 2022 the Ouse ran high after around 50 mm of rain fell at Ardingly in 48 hours, with flooding reported at Hassocks, Burgess Hill and Haywards Heath; in January 2025 the upper Medway flooded roads and low-lying land in the north-east of the district. For the full flood-risk picture in the main towns, see our area pages for Haywards Heath and East Grinstead, and the wider West Sussex overview. You can check whether the Ouse or Adur are running high right now on our live river levels map.

How Unda helps with a Mid Sussex application

We are the accurate translation layer between your scheme and what Mid Sussex, West Sussex County Council and the Environment Agency will accept. We know how this authority reads an application — the surface-water-as-flood-zone rule, the Qbar discharge expectation, the refusal to take 2 l/s on trust, and the checklists the Flood Risk and Drainage Team marks against — and we build the flood risk assessment and drainage strategy to answer all of it before an objection has anywhere to land.

That means assessing every credible source on the site, screening against the 2024 SFRA rather than the flood map alone, restricting discharge to the greenfield rate with the modelling to prove it, and, where it saves you time, submitting detailed drainage design with the application so the drainage argument is settled before permission rather than after. Every report is overseen by a chartered flood risk consultant who stays involved past the report to answer the council's questions.

Flood risk and drainage services for every situation in Mid Sussex

A flood risk assessment, a sequential or exception test and a drainage strategy are three distinct reports. They often travel together in a Mid Sussex planning submission, but they answer different questions: is the site safe, should the development be here, and where does the surface water go. We prepare all three, alongside the right form of assessment for buyers, lenders and BREEAM.

Flood risk assessments

• Flood risk assessment for planning. The core report, site-specific and prepared to satisfy the NPPF, the Environment Agency and Mid Sussex. Groundwater and, where needed, flood modelling are assessed within it.
• Homebuyer and property-purchase survey. For conveyancing, where a search has flagged surface-water or river risk and you need the actual building assessed, not a broad map.
• Flood risk assessment for insurance. Property-specific evidence for brokers and underwriters where premiums have risen or cover has been restricted.
• Flood risk assessment for BREEAM. Structured around BREEAM Pol 03, which is credit-driven rather than safety-driven, so a planning assessment alone rarely satisfies it.
• Groundwater flood risk. A defensible design water table for Wealden sites and any basement or soakaway — assessed as a source within the assessment, or as a standalone hydrogeological study on complex sites.

Sequential and exception tests

The sequential and exception tests are a separate, locational report about whether a development should be in a higher-risk location at all. The sequential test defines a search area, identifies reasonably available lower-risk sites and justifies the proposed site; the exception test is the second gate where development still needs to proceed.

Drainage strategy and SuDS

• Sustainable drainage strategy (SuDS). The surface-water engineering — existing versus proposed runoff, and SuDS to hold discharge to the greenfield rate West Sussex expects.
• Preliminary drainage design. The early, outline read that flags infiltration issues before they affect layout and land value.
• Discharge of drainage conditions. The detailed stage to discharge a pre-commencement drainage condition against the council's checklist.
• Flood modelling. 1D and 2D hydraulic modelling where national mapping is not enough — the output feeds the assessment, sequential test or drainage design.

Every report is overseen by a chartered flood risk consultant, and you can screen a postcode on our flood risk map before you commit. Neighbouring authorities work the same two-tier way — see our pages for Crawley and Horsham.

Flood risk and drainage in Mid Sussex: frequently asked questions