Arun’s SuDS Design Checklists: From Outline Application to Occupation

The Arun SuDS design checklists are five separate documents, published by Arun District Council in October 2025, and between them they govern a scheme from its first outline drawing to the day someone moves in. They align the district with the National Standards for Sustainable Drainage Systems, published by Defra on 19 June 2025, and with CIRIA’s SuDS Manual (C753). The guidance behind them has not stood still. Arun revised parts of it as recently as 26 August 2026. If you are designing sustainable drainage for a site in the district, these are the documents Arun’s engineers will hold your submission against.

The practical effect is that work most councils are content to condition, Arun expects before it determines the application. Infiltration testing, groundwater monitoring and construction-level drawings all move forward in the programme, and a scheme that has not planned for that loses a winter. A whole winter, on a site that only tests well between December and February. The full picture of what the council expects, including its adopted Local Plan policies, sits on our page covering Arun District Council’s flood risk and drainage requirements.

Arun’s engineers are consulted on every planning application in the Lidsey treatment catchment and on every application of more than two units elsewhere in the district, so the checklists reach far smaller schemes than the major-application threshold does.

What Arun’s five SuDS design checklists are

The five checklists are separate documents, each covering a different stage or a different kind of evidence. Four concern the design. The fifth concerns what was actually built, which is rarer, and it is the one that catches people out at the end of a job when there is no time left to fix anything. Arun publishes them all as Word documents on its designer checklists page, dated October 2025, and sets them against its own general SuDS design guidance.

Critical items run through the first two. They are printed in bold, and Arun is explicit about the consequence: on a discharge of condition application a failure to address one will lead to an objection, and on a full application an objection is likely. The council’s stated reward for getting them right is the ordinary one, quicker reviews and fewer requests for further information. The cost of getting them wrong is a delay you cannot make up.

Who actually reviews drainage on an Arun planning application?

Two bodies look at drainage on an Arun site. Applicants routinely send the wrong evidence to the wrong one. West Sussex County Council is the Lead Local Flood Authority and the statutory consultee on surface water for major applications. Arun’s own Engineering Services team is the drainage consultee to the planning authority, and its remit reaches much further down the scale.

• West Sussex County Council, as LLFA. The statutory consultee on surface water drainage for major applications across the county, and the body that issues Ordinary Watercourse Land Drainage Consent.
• Arun District Council, as planning authority and drainage consultee. Its engineers are consulted on all applications in the Lidsey treatment catchment area, on all applications of more than two units elsewhere in the district, and wherever a watercourse is affected.
• Arun’s own Land Drainage Byelaws. A separate consent from the county’s, administered by the district. Arun has confirmed it no longer undertakes land drainage consenting on West Sussex’s behalf.
• Southern Water, as sewerage undertaker. Where a connection to a public sewer is proposed, Arun expects a hydraulic capacity check and written permission from the asset owner before it will accept the destination.

That two-unit threshold is why the checklists bite harder here than a comparable document elsewhere, and why a surface water drainage strategy for an Arun planning application has to be evidenced to a standard most minor schemes never face. On a three-house scheme in Arun a district drainage engineer reads your calculations. In many authorities nobody technical would look at all. It is also why an objection on drainage grounds arrives on Arun schemes that would sail through in a neighbouring district.

Outline stage: proving the principle before the layout is fixed

The Outline SuDS Design Checklist applies where the scale and layout of the development have not yet been decided. The bar is lower here than at full application. It is not low. What Arun wants proved is that a workable drainage solution exists for the site as drawn.

That means the ground investigation work has to start early. Where infiltration is proposed but not yet proven at outline stage, Arun expects a secondary sustainable discharge route to be identified as well, so the site remains deliverable if the infiltration case later fails. That is sensible and awkward at once. It obliges a designer to carry two solutions through a stage where the layout is still moving.

The layout itself moves too. Arun’s advice to applicants is unambiguous that surface water drainage must be adequately considered when the site layout is being formulated, not fitted into whatever space is left over. Where a scheme cannot show it can be drained, the council’s position is that it cannot recommend approval, and it points to appeal decisions in which inspectors agreed with its reasons for refusal on exactly that basis. Applying the SuDS hierarchy properly at this stage is what keeps the later checklists answerable, and Arun’s first standard sets out the evidence it wants before it will accept a piped destination. Deciding early whether the site needs infiltration testing at all saves a season.

Full and reserved matters: construction-level detail before determination

The Full SuDS Design Checklist covers full applications, reserved matters and the discharge of a SuDS design condition. Here Arun’s expectation is set out plainly, and it is a construction standard rather than a planning one.

That is why the checklist asks for gradients, levels, structural specifications and a construction method statement rather than an indicative layout. The design parameters behind it sit on Arun’s own standards pages, and several are tighter or more specific than the national baseline.

Three of those repay a closer look. Arun has moved to annual exceedance probability throughout its extreme rainfall standard, so a submission still written in one-in-a-hundred-year language needs translating before anyone can check it. The FEH22 point data files are not simply referenced in the report but sent to the council’s land drainage inbox, so the greenfield runoff rate can be reproduced. And interception applies to every site, whatever its scale. That is where the first few millimetres of rainfall stop being a detail and start deciding whether a design passes.

A runoff coefficient of 1 on every impervious surface means permeable paving and hard landscaping earn no allowance in the greenfield calculation unless the contributing area plan says otherwise, which usually makes attenuation volumes larger than a designer expects.

Ground investigations: the evidence Arun will not take on trust

The Ground Investigations Checklist exists because, in the council’s own words, it is the designer’s responsibility to demonstrate what the seasonal maximum groundwater levels are wherever SuDS features are proposed. A desk study will not do it. Arun says plainly that it does not regard desk-based work as sufficient evidence of groundwater levels at a local scale.

1. Agree the methodology first where anything unusual is proposed. Arun does not generally accept borehole infiltration testing and wants the approach discussed before the application is made.
2. Install monitoring wells and record levels at least weekly, with extra readings around rainfall events, so the seasonal maximum is measured rather than inferred.
3. Test to BRE Digest 365 or CIRIA R156, three times in quick succession, at the proposed base level of the feature and at least a metre above peak groundwater.
4. Fill each trial pit to the design head of water and record it from full to nearly empty, which Arun defines as below 25% full and suggests should be under 10%.
5. Work the results through to a rate you can defend. Our BRE 365 infiltration rate calculator handles the arithmetic, and the underlying geology tells you early whether the exercise is worth commissioning at all.

The number that decides most infiltration schemes in the district is the separation to groundwater. Arun requires a metre of unsaturated ground between the base of an infiltration component and the expected peak groundwater level. Where the monitoring does not deliver that metre, infiltration stops being viable and the design has to find another destination. That is exactly why the secondary route matters at outline stage, and why a designer who leaves the question open until the ground investigation report lands has already given up several months of programme to find out something the geology could have suggested at the start. Arun prefers testing between December and February, so a site that misses the window can wait most of a year for evidence the council will accept.

Critical items, and what an objection actually costs

The bold entries on the outline and full checklists are the ones with consequences attached. Arun’s position is that failing to address a critical item will lead to an objection at condition discharge stage, and is likely to at full application stage. Neither is a refusal. Both are expensive.

Since April 2026 most planning appeals proceed by written representations, which means thin drainage evidence submitted at application stage is very hard to repair afterwards. Once an objection lands there are two realistic options. Supply the missing evidence and wait for the next committee cycle, or redesign. Missing documents can also cost you the application before it is even considered, which is a separate and avoidable way for a scheme to fail at validation.

Designing drainage alongside landscape, ecology and biodiversity

Arun’s guidance follows the National Standards structure of seven standards, and three of them, water quality, amenity and biodiversity, only work if the drainage layout is developed with the landscape and ecology strategies rather than after them. Basins, swales and rain gardens that have to deliver amenity and habitat value as well as storage cannot be squeezed into leftover land. There is never enough of it, and it is never in the right place.

Arun’s seventh standard makes the same point from the maintenance side. Existing trees and hedging need careful thought, because roots damage liners and pipes, and a failed component increases flood risk rather than managing it. The same standard expects overland exceedance routes to be kept clear, which is a landscape decision as much as a drainage one, and it is worth having a topographical survey good enough to prove where the water goes. The council sets no minimum easement width for open SuDS features. It expects applicants to demonstrate the features can be safely maintained, and notes that a clear working strip of at least three metres from the top of bank usually satisfies that.

Maintenance and verification: the checklists that outlive the permission

The last two checklists are the ones most applicants have never opened. They bite after permission is granted. Both are worth reading at design stage. The decisions they rest on are made years before anyone checks them, and a badly framed maintenance regime is one of the commoner reasons a drainage condition takes a second round to discharge.

The SuDS Maintenance Checklist sets out what Arun expects from a maintenance and management manual. Its stated purpose is the long-term functionality of the drainage system and the protection of habitats and ecology, which in practice means naming who maintains what, how often, and with what access. The SuDS Verification Checklist is the genuinely unusual one, and there is no equivalent in most neighbouring districts.

• An independent engineer, appointed early. Arun advises engaging one at the earliest opportunity to avoid a conflict of interest with the designer or the contractor.
• Inspections throughout construction. Not a single visit at the end. The report records what was inspected and when.
• Evidence that the built system matches the approved design. Including the buried elements nobody can inspect once the site is finished.
• A completed verification report before occupation on larger applications, which makes it a programme item rather than a piece of paperwork.

A verification report assembled retrospectively from site photographs after the contractor has left is not what Arun is asking for, and finding that out at handover is the most expensive way to learn it.

Where Arun sits against its neighbours

Checklists are spreading across West Sussex, but they are not all doing the same job. Mid Sussex District Council issued version 2.0 of its detailed drainage design checklist on 7 November 2025. That one is a post-permission document, aimed squarely at discharging drainage conditions, and laid out as requirement, expectation and reference location across surface water and foul water. Chichester District Council has published a surface water drainage proposals checklist since 2021. Neither reaches as early in the process as Arun’s outline checklist does, and CIRIA has been arguing for SuDS to be planned earlier for exactly that reason.

Arun’s set is broader in two ways that change how a scheme is programmed. It runs across the whole life of the development rather than sitting at one stage, and it reaches minor applications that elsewhere would never see a drainage engineer. Add the winter testing preference and the metre of unsaturated ground, and the district’s practical bar is among the higher ones in the south east. If you work across the county, our page on flood risk and drainage in West Sussex covers how the other authorities compare.

What this means for your programme

The workable response is to treat Arun’s drainage requirements as a fixed input to the site layout rather than a submission task at the end of it. Commission the ground investigation before the masterplan is settled. The monitoring window and the infiltration result between them decide what the layout can be, and a masterplan drawn before either is known is a masterplan that will move. Identify the fallback discharge route at the same time. Read the maintenance and verification checklists at design stage, not at handover. None of that is expensive. Doing it late is.

None of this is unique to Arun in principle. The August 2026 NPPF made sustainable drainage to the 2025 National Standards mandatory through Policy F8, and other authorities are working out their own version of the same thing, mostly through the new national SuDS strategy templates rather than bespoke checklists. Arun got there first, and wrote it down in more detail than most.

Frequently asked questions

If you are working on a site in the district and want the drainage evidence sequenced properly from the start, we prepare surface water drainage strategies for planning and handle the wider drainage strategy where foul drainage is involved too. Call one of our experienced consultants on 01293 214444 or email enquiries@unda.co.uk, and we will tell you what your site needs before you commit to a layout.