Every developer has a story about validation. One council wants a drainage strategy up front; the next takes the application and conditions it; a third asks for something nobody has heard of because a policy officer added it to a list in 2019.
The National Planning Policy Framework published on 17 August 2026 takes a run at that problem — and in doing so quietly moves drainage evidence forward in the programme.
What policy DM2 says
DM2 sits in the new decision-making chapter, alongside DM1 (preparing proposals) and DM3 (determining them). It has two limbs, and both matter:
“1. To ensure a clear and consistent approach to the information required to determine development proposals, local validation lists setting out the information required in support of an application for development should include the information specified in the relevant national decision-making policies (summarised in Annex C).
- Local validation lists should only include additional information requirements if there is a policy in the development plan requiring a specific further assessment. Any such additional information requirements should not be applied equally to all applications but should be proportionate to the scale of development and its potential impact. Where appropriate, the requirements should clearly distinguish between what is required for major, medium and other types of development proposal.”
Limb 1 sets a floor. Limb 2 sets a ceiling — and it is a genuine constraint on local practice. A validation requirement that isn’t in Annex C and isn’t traceable to a development plan policy is now, on the face of national policy, one the council should not be applying. Blanket requirements applied equally to a 200-home scheme and a pair of semis are specifically called out.
Annex C: what’s on the national list
Annex C summarises the information requirements that flow from the Framework’s own decision-making policies. For anyone working on drainage, three entries matter:
- a site-specific flood risk assessment;
- a SuDS statement — a statement showing how the national standards have been met;
- a coastal change vulnerability assessment, where relevant.
That is the shift. A SuDS statement is not a document you produce when a condition is discharged. It is a document the application needs in order to be valid.
It joins up with policy F8, which requires sustainable drainage on development with drainage implications to be designed in accordance with Defra’s 2025 National Standards for SuDS. F8 tells you what the design has to comply with; DM2 and Annex C tell you that you have to show it at the front door.
Why this changes the programme, not just the paperwork
Under the old pattern, a lot of drainage work happened after permission. Outline consent came with a condition requiring a detailed drainage strategy; infiltration testing got done when the site was accessible; if the ground turned out to be worse than assumed, the layout absorbed it.
Front-loading the SuDS statement breaks that sequence in a useful and an uncomfortable way at the same time.
The useful part. Drainage constraints surface before the layout is fixed by a planning permission. If a soakaway won’t work on this site, finding out at application stage costs a drawing revision. Finding out at discharge-of-conditions stage costs a redesign, a Section 73 and a delay.
The uncomfortable part. You need real infiltration data earlier. A SuDS statement that leans on a desk-study assumption about permeability is a statement waiting to be unpicked. BRE Digest 365 soakaway testing is not something you can do retrospectively once the layout is committed.
What a defensible SuDS statement needs to show
Nothing here is new engineering — what has changed is when it has to exist. Expect to need:
- The drainage hierarchy applied and evidenced. Infiltration first, then discharge to a watercourse, then to a sewer — with a reason for each step you rule out. “Not feasible” without test data is not a reason.
- Infiltration testing to BRE Digest 365, in the right locations, at the right depth, with the results and the design infiltration rate stated. If testing is genuinely not possible yet, say so explicitly and say what happens if the assumption fails.
- Compliance with the 2025 National Standards stated against the standards themselves, not against a generic paragraph about sustainable drainage.
- Storage volumes, discharge rates and climate change allowances shown, with the design event and the exceedance route.
- Maintenance and adoption — who maintains what, for how long, funded how.
- Proportionality. DM2 explicitly distinguishes major, medium and other development. A small scheme should not be asked for a major scheme’s evidence pack, and you are entitled to say so.
The other half of the policy: pushing back
DM2(2) is the clause worth keeping in your back pocket. If a council’s validation list asks for a document that is neither in Annex C nor required by a specific development plan policy, national policy says it should not be there.
Two practical caveats before you rely on that:
- Local validation lists take time to catch up. Councils will be revising them over the coming months, and the Planning Practice Guidance published alongside the Framework had topics still referring to the previous NPPF immediately after publication. Check what version you are arguing against.
- “Proportionate” is a judgement, not a threshold. DM2 gives you a principle, not a number. It is a good argument at pre-application; it is a weaker argument once validation has been refused.
What to do about it
- Move the soakaway testing earlier in your programme. If the drainage strategy is now a validation document, the test results that underpin it have to exist before submission.
- Write the SuDS statement against the National Standards explicitly, clause by clause. A statement that references them generally invites a request for more information — which is exactly the trigger DM3 gives an authority for pausing a decision.
- Be honest about assumptions. If you are designing on an assumed infiltration rate, state it, state the sensitivity, and state the fallback. An acknowledged assumption is a far better position than one discovered later.
- Review the local validation list against DM2 before you accept it. Particularly on smaller schemes, where blanket requirements are most likely to be disproportionate.
- Don’t assume a condition will save you. The whole direction of the new Framework is that the evidence arrives with the application.
The bigger picture
Sustainable drainage has spent years in an odd position in England: mandatory in Wales through Schedule 3, non-statutory here, and delivered through whatever the planning system could be persuaded to require. The August 2026 Framework has not commenced Schedule 3 — there is still no SuDS Approving Body in England — but it has done something with a similar practical effect. It has named the standards, required compliance with them, and made the evidence a condition of your application being accepted at all.
For most developers that means one thing: the drainage engineer needs to be in the room earlier.
If you have a site where infiltration is the open question — or a SuDS statement that needs to stand up at validation rather than at condition stage — get in touch and we’ll tell you what testing and evidence we’d want to see before it goes in.
Sources: National Planning Policy Framework, August 2026 — policies DM2, DM3 and F8, and Annex C, GOV.UK · National standards for sustainable drainage systems (SuDS), GOV.UK · NPPF August 2026 changes: flood risk and drainage implications for planning, Unda