Flood Risk and Drainage at Reserved Matters Stage

Outline planning permission establishes the principle of development. Reserved matters approval turns that principle into a buildable scheme. For flood-constrained sites, the gap between outline and reserved matters is where things can go seriously wrong.

The outline flood risk assessment demonstrates that the site can be developed safely in principle. But it necessarily relies on assumptions — about finished floor levels, compensatory storage locations, drainage discharge points, and site layout. At reserved matters stage, those assumptions must be reconciled with a detailed layout that has been designed around real constraints: topography, access, ecology, noise, residential amenity, and commercial viability.

If the outline FRA and the reserved matters layout do not align, the whole flood risk case can unravel. This article explains how to navigate flood risk and drainage at reserved matters stage, what to submit, and how to avoid the common pitfalls.

Outline vs Reserved Matters: What Flood and Drainage Evidence Is Needed?

The level of flood risk and drainage evidence required at each stage is fundamentally different, though they must tell a consistent story.

At Outline Stage

The outline application demonstrates that the development is acceptable in principle on flood risk grounds. The evidence typically includes:

A flood risk assessment. The FRA establishes the flood risk to the site from all sources (fluvial, tidal, surface water, groundwater, sewer, artificial sources), applies the Sequential and Exception Tests where required, and sets out the proposed mitigation strategy in principle. At outline stage, the FRA may use indicative finished floor levels, conceptual compensatory storage volumes, and broad-brush drainage discharge rates.

A conceptual drainage strategy. This demonstrates that the site can be drained sustainably, typically showing indicative SuDS features, approximate attenuation volumes, and proposed discharge rates to the receiving watercourse or sewer. It does not need to include pipe-level design or detailed calculations, but it must be credible and demonstrate that the drainage approach is achievable within the site constraints.

The Sequential and Exception Tests. Where the site is in Flood Zone 2 or 3, the outline application must pass the Sequential Test (demonstrating that there are no reasonably available alternative sites at lower flood risk) and, where required, the Exception Test (demonstrating that the development provides wider sustainability benefits and will be safe for its lifetime).

At Reserved Matters Stage

Reserved matters bring the detail. The evidence required is more granular and must be tied to the specific layout being proposed:

A detailed drainage strategy. This moves from the conceptual to the specific. It must include a full drainage layout tied to the proposed site plan, showing pipe networks, SuDS features (permeable paving, swales, attenuation basins, soakaways), flow control devices, and connections to the receiving system. The strategy must be supported by detailed hydraulic calculations (typically MicroDrainage or InfoDrainage modelling) demonstrating that the system performs for all storm events up to the 1% annual probability event plus climate change allowance.

SuDS details. The reserved matters submission should include construction details for SuDS features, cross-sections, levels, and a SuDS management and maintenance plan setting out how the system will be managed for the lifetime of the development.

Finished floor levels. Where the outline FRA specified minimum finished floor levels (typically a freeboard above the design flood level plus climate change), the reserved matters drawings must demonstrate compliance. This is a common failure point — see below.

Updated flood risk information. If the outline FRA relied on flood data that has since been superseded (e.g., if the EA’s Flood Map for Planning has been updated, or new climate change allowances have been published), the reserved matters submission should address the updated information and confirm that the outline conclusions remain valid.

How Outline Conditions Defer Detail to Reserved Matters

The conditions attached to an outline planning permission typically defer the detailed drainage design to a later stage. This is standard practice and reflects the fact that detailed design cannot be finalised until the layout is fixed.

A typical condition might read:

“No development shall take place within any phase until a detailed surface water drainage scheme for that phase, based on sustainable drainage principles and an assessment of the hydrological and hydrogeological context of the development, has been submitted to and approved in writing by the local planning authority.”

This condition is usually a pre-commencement condition, meaning it must be discharged before construction begins. In some cases, the condition is attached to the reserved matters approval itself; in others, it requires a separate condition discharge application after reserved matters are approved.

The important point is that the outline permission creates an expectation: the detailed drainage must be consistent with the drainage principles established at outline stage. If the outline conceptual drainage strategy assumed a particular discharge rate, attenuation volume, or SuDS approach, the reserved matters design must deliver on those assumptions — or provide a justified reason for departing from them.

The Trap: When the Outline FRA Does Not Go Far Enough

This is the single most common source of problems at reserved matters stage. An outline FRA that makes overly optimistic assumptions, or that does not interrogate the site constraints in sufficient depth, sets a trap that springs at reserved matters.

Finished Floor Levels

The outline FRA may have specified that finished floor levels should be set 300mm above the 1% AEP plus climate change flood level — say, 15.3m AOD. But when the reserved matters layout is designed, the ground levels across parts of the site are at 14.5m AOD. Raising the floor levels by 800mm creates problems with accessibility, DDA compliance, and the relationship between the development and surrounding land. The cost of imported fill escalates. The layout that was assessed at outline stage no longer works.

Compensatory Storage

If the site is in the functional floodplain (Flood Zone 3b) or partially within Flood Zone 3a, the outline FRA may have proposed compensatory floodplain storage to offset the loss of floodplain volume caused by development. But compensatory storage must be provided on a level-for-level, volume-for-volume basis. If the outline FRA assumed that compensatory storage would be located in a particular area of the site, and the reserved matters layout allocates that area to housing or landscaping, there is nowhere to put the storage without redesigning the scheme.

Drainage Discharge Rates

The outline drainage strategy may have assumed a discharge rate of 5 l/s/ha, based on a preliminary greenfield runoff calculation. But when detailed infiltration testing is carried out at reserved matters stage, the ground proves to be largely impermeable clay. The attenuation volume needed to store the difference between the incoming rainfall and the restricted discharge rate doubles or triples. There is insufficient space within the layout for the attenuation features.

The Solution: Do Enough at Outline Stage

The lesson is clear: invest in sufficient flood risk and drainage analysis at outline stage to avoid these traps. This does not mean preparing a full detailed drainage design at outline — that would be premature and wasteful. But it does mean:

• Using topographic survey data to set realistic finished floor levels
• Carrying out preliminary ground investigation (at least trial pits with soakage assessment) to understand infiltration potential
• Modelling compensatory storage requirements with enough accuracy to identify viable storage locations
• Preparing drainage calculations that are based on tested assumptions, not optimistic estimates

LLFA Consultation at Reserved Matters

The LLFA will be consulted on the reserved matters application and will review the detailed drainage strategy in depth. This is a more rigorous review than at outline stage, where the LLFA may have accepted the conceptual approach in principle.

At reserved matters, the LLFA will scrutinise:

• Whether the detailed design is consistent with the drainage principles approved at outline
• Whether the SuDS hierarchy has been properly applied (infiltration first, then attenuation and discharge)
• Whether the hydraulic calculations demonstrate adequate performance for all design storms
• Whether the exceedance strategy (what happens when storms exceed the design capacity) is appropriate
• Whether the management and maintenance plan is credible and enforceable

If the LLFA raises concerns, the reserved matters approval may be delayed or refused. This is why early engagement with the LLFA — ideally through pre-application discussion — is strongly recommended at reserved matters stage, not just at outline.

Avoiding Condition Discharge Bottlenecks

Even after reserved matters are approved, there are typically further conditions to discharge before construction can begin. Drainage conditions are almost always pre-commencement, meaning they must be formally approved before any site works commence (including in some cases site clearance and demolition).

The risk of a bottleneck is acute where:

• The reserved matters approval is granted but conditions remain outstanding. The developer may be eager to start construction, but the conditions cannot be discharged until the LLFA and water company have reviewed the detailed design.
• The LLFA requests amendments. Each round of amendment and re-consultation adds 4-8 weeks. Two rounds of amendments can delay the start of construction by 3-4 months.
• Water company capacity is constrained. If the drainage strategy relies on discharge to a public sewer, a formal capacity check and Section 104 or Section 106 agreement with the water company may be needed. These processes have their own timescales, which are outside the LPA’s control.

How to Mitigate Bottlenecks

Front-load the technical work. Prepare the detailed drainage design in parallel with the reserved matters application, not sequentially. If possible, submit the condition discharge application immediately after (or even concurrently with) the reserved matters submission.

Engage the LLFA and water company early. Do not wait for the formal consultation. Pre-application discussions with the LLFA and pre-development enquiries with the water company should be initiated as soon as the reserved matters layout is sufficiently advanced.

Build programme contingency. Allow 12-20 weeks from submission of the condition discharge application to formal approval, including at least one round of amendments. This is realistic based on typical timescales across England and Wales.

Reserved Matters and Pre-Commencement Conditions

There is a subtle but important distinction between conditions attached to the reserved matters approval and pre-commencement conditions attached to the original outline permission.

Conditions on the outline permission apply to the entire development. They must be discharged in accordance with the outline permission’s requirements, regardless of what the reserved matters approval says. If the outline permission has a pre-commencement drainage condition, that condition must be discharged even if the reserved matters approval includes its own drainage conditions.

In practice, this means that the developer may need to discharge drainage conditions on both the outline permission and the reserved matters approval — potentially involving two separate sets of submissions, consultations, and approvals. Careful coordination is essential to avoid duplication and delay.

Phased Developments: Maintaining Drainage Consistency

Large developments are often delivered in phases, with separate reserved matters applications for each phase. This creates a particular challenge for drainage, because the drainage system must function as a coherent whole, not just within each phase.

Key Considerations for Phased Drainage

Downstream infrastructure. The drainage infrastructure serving later phases (attenuation features, outfalls, connections to watercourses or sewers) may need to be constructed as part of the first phase, even if those later phases have not yet received reserved matters approval.

Interim drainage arrangements. During the construction of the first phase, the drainage system for later phases does not yet exist. Temporary drainage arrangements must be in place to manage surface water from undeveloped phases without causing flooding or pollution.

Consistency of design parameters. Each reserved matters application must use consistent design parameters — the same greenfield runoff rate, the same climate change allowance, the same receiving watercourse capacity. If Phase 1 assumes a 5 l/s/ha discharge rate but Phase 3 has been designed to 8 l/s/ha, the receiving system will be overloaded.

Cumulative modelling. The drainage model should assess the cumulative impact of all phases, not just the individual phase being applied for. The LLFA will want to see that the system works as a whole.

As we demonstrated in our full project lifecycle case study, maintaining technical consistency across phases requires careful management from the outset.

Timeline Considerations

Reserved matters applications are subject to the same statutory determination periods as full planning applications: 8 weeks for minor developments, 13 weeks for major developments. In practice, reserved matters applications for major developments frequently take longer, particularly where flood risk and drainage are contentious.

A realistic timeline from outline permission to construction start, accounting for reserved matters and condition discharge, might look like this:

Stage	Typical Duration
Prepare reserved matters submission (including drainage)	8-16 weeks
Reserved matters determination	13-26 weeks
Condition discharge preparation	4-8 weeks
Condition discharge determination (including LLFA review)	8-16 weeks
Total: outline to construction start	33-66 weeks

These are indicative ranges. Complex sites, phased developments, or sites requiring additional modelling or investigation will be at the upper end. The key message is that the period between outline permission and construction start can easily exceed 12 months, and much of that time is consumed by drainage and flood risk matters.

How Aegaea Can Help

Aegaea bridges the gap between outline consent and buildable detail. We work across the full project lifecycle — from outline FRA and conceptual drainage through to reserved matters drainage design, LLFA consultation, and condition discharge. Our approach ensures that the assumptions made at outline stage are robust enough to withstand the rigour of reserved matters, and that the detailed design flows logically from the approved principles.

Whether you are preparing a reserved matters submission for a flood-constrained site or trying to unpick a mismatch between your outline FRA and your proposed layout, contact us to discuss how we can help.