What Drainage Information Is Needed for Planning Permission?

For years it was standard practice to secure surface water drainage by planning condition and work up the detailed design after permission was granted. That route still exists. What has changed is how much a planning application has to demonstrate before a condition can be used at all. The drainage information needed for planning permission now has to show that a workable surface water strategy actually exists on the site. Conditions can secure the detail of how a scheme is built; they can no longer stand in for proving that the scheme will work.

So the useful question is not "can drainage be dealt with by condition?" It is "has the application shown enough for the authority to grant permission with confidence?" This article sets out what drainage information you should submit at application stage, what can safely be left to a condition, and where the line between the two now sits.

The question is no longer whether drainage can be dealt with by condition, but whether the application has shown that a workable surface water strategy exists before permission is granted.

How drainage is assessed in a planning application

Planning decisions are not made on the assumption that drainage problems can be sorted out later. The Local Planning Authority (LPA) has to be satisfied that a development will be safe for its lifetime and will not increase flood risk elsewhere. For major development, the Lead Local Flood Authority (LLFA) is a statutory consultee on surface water drainage and reviews whether a viable strategy has been demonstrated as part of the application. Both are looking for evidence that drainage is achievable, not a promise that it will be designed in future.

This is where the quality of what you submit matters. As we explain in why technical expertise matters in flood risk assessment for planning, weak drainage submissions rarely fail on the wording of a policy. They fail because they leave the decision-maker unsure the scheme is deliverable. A thin strategy invites an objection; a clear one closes the question down.

Drainage strategy versus detailed drainage design

The distinction that decides almost everything here is the one between a drainage strategy and a detailed drainage design. They answer different questions.

A drainage strategy establishes whether surface water can be managed on the site in principle. It identifies the discharge destination, applies the SuDS drainage hierarchy, tests whether infiltration is feasible, and shows how runoff will be controlled with an allowance for climate change. It answers: will this approach work on this site?

A detailed drainage design sets out how that strategy is delivered. It covers the hydraulic sizing of features, the layout and specification of SuDS components, and the construction detail. It answers a different question: exactly how do we build it?

Conditions are well suited to securing the right-hand side. They cannot substitute for the left. National guidance on the use of planning conditions is clear that conditions should only be used where they are necessary, relevant and enforceable, and not to defer matters so fundamental they should have been resolved before permission. A condition can refine a strategy. It cannot rescue one that was never shown to work.

What drainage information you should submit at application stage

For most developments, the LPA and LLFA now expect a proportionate but genuinely site-specific level of drainage information with the application. At a minimum it needs to show that a surface water strategy is feasible and policy-compliant, which usually means covering:

• The discharge destination, justified against the hierarchy. Surface water should follow the priority order set out in Defra's 2025 National Standards: reuse, then infiltration, then a watercourse, then a surface water sewer, and only as a last resort a combined sewer. Where discharge to sewer is proposed it has to be realistic rather than assumed.
• The feasibility of the chosen approach, with evidence. If infiltration is proposed there should be a credible basis for it, ideally from BRE Digest 365 testing. If it is not viable, that should be shown, not glossed over.
• How runoff will be controlled, including greenfield or betterment discharge rates and attenuation for events up to the 1-in-100-year storm with a climate change allowance.
• How the drainage fits the layout and levels, so the strategy and the proposed site plan are consistent rather than drawn in isolation.

Many authorities now also ask for a standardised SuDS proforma summarising these points. Every London borough expects one, and the practice is spreading. Getting this evidence right at submission is also what keeps an application valid in the first place; missing drainage documents are a common reason a scheme is returned as invalid before the determination clock even starts.

How much depends on the type of application

The level of detail scales with the application. At outline stage the LLFA generally wants a high-level but site-specific strategy: the proposed method of disposal, whether infiltration is feasible, indicative discharge rates and destinations, and the use of SuDS where appropriate, enough to fix the principle and set parameters for reserved matters. A preliminary drainage design is the usual vehicle for this. For a full application, expect the strategy to be more developed and tied closely to the detailed layout.

What drainage details can be left to a planning condition

Where a strategy has been clearly defined and shown to be feasible, a condition is the right tool for the remaining detail: the detailed hydraulic design, the final sizing of SuDS features, the construction specifications, and the adoption and maintenance arrangements shown on the right of the diagram above. Used this way, conditions give flexibility while making sure the approved strategy is actually delivered.

The essential point is that the underlying approach is already settled, so a condition refines the strategy rather than reopening it.

When drainage cannot be left to condition

Problems arise when an application leans on a condition to resolve uncertainty rather than to add detail. In practice this shows up as:

• Infiltration proposed with no supporting evidence. No permeability testing, no groundwater consideration, just an assumption that a soakaway will work.
• Discharge to sewer assumed without confirmation from the sewerage undertaker that capacity exists.
• Generic drainage proposals that are not tied to the site's actual constraints, levels or flood risk.

In each case the authority cannot be satisfied the development is deliverable, and the likely outcome is an LLFA objection, a validation problem, or refusal. These weaknesses are also harder to fix than they used to be. From April 2026, most planning appeals default to written representations, so the evidence base is effectively fixed at application stage. A drainage strategy that was too thin at submission generally cannot be rebuilt at appeal, where inspectors place real weight on whether drainage was properly addressed at the point of determination.

A simple test: strategy or detail?

When an outstanding drainage point comes up late in an application, one question usually settles whether it can be conditioned. Is the issue how a solution will be designed, or whether it will work at all?

If the question is how a solution will be designed, it can normally be secured by condition. If it is whether the solution will work at all, it has to be resolved before permission.

That single distinction is now central to how drainage is assessed in planning.

Why drainage requirements at planning are getting stricter

The push for clearer drainage information at application stage reflects a wider shift in planning practice, and several recent changes have sharpened it.

The Environment Agency's 2024 assessment put around 4.6 million properties in England at risk from surface water, a 43% increase on the previous figure, and surface water is now the most widespread source of flood risk in the country.

Policy has followed. The September 2025 update to Planning Practice Guidance confirmed that the sequential, risk-based approach applies to all sources of flooding, surface water included, and to future climate risk.

The technical bar has risen too. Defra rewrote the National Standards for Sustainable Drainage Systems in 2025, setting a stricter discharge hierarchy in which cost alone is never enough to justify a lower-priority destination. CIRIA's C823F guidance now pushes SuDS thinking to the earliest design stages through a "four gateways" approach, reinforcing the case for a proper drainage design at outline. And individual authorities are formalising higher expectations: Arun District Council's checklists, for example, now ask for construction-level drainage detail before determination rather than after.

One thing has not changed, though. Schedule 3 of the Flood and Water Management Act 2010, which would make SuDS approval a statutory process in England, has still not been commenced. The 2025 standards therefore remain planning guidance rather than statute. In practice that changes little: the National Planning Policy Framework still expects sustainable drainage unless there is clear evidence it would be inappropriate, so treating the standards as optional is a mistake.

How a clear strategy makes condition discharge easier

The quality of the drainage information at application stage has a direct effect on how smoothly conditions can later be discharged. Where a scheme is approved on an unclear or poorly evidenced strategy, the problems tend to surface at the discharge of conditions stage, often as redesign, extra survey work and delay to a start on site.

A well-defined strategy avoids most of that. Because the principle is already sound, detailed design can be progressed efficiently and the conditioned submission is far more likely to be approved first time. It is also worth remembering that a planning drainage condition, not a building regulations sign-off, is what governs the scheme; the two systems do not automatically talk to each other.

What this means in practice

Getting this balance right is not about submitting more information than necessary. It is about submitting the right drainage evidence at the right stage. A clear, proportionate strategy at application stage reduces the risk of objection, avoids unnecessary redesign, and supports an efficient discharge of conditions later.

The test is straightforward. If the outstanding matter is how a drainage solution will be designed, it can often be conditioned. If it is whether that solution will work at all, it has to be resolved before permission is granted.

How Unda can help

Unda works with developers, landowners and consultants to prepare robust drainage strategies for planning applications and to carry them through to delivery. We can advise on what a specific LPA or LLFA expects, prepare a preliminary or detailed surface water drainage strategy, and manage the discharge of drainage conditions once permission is in place.

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