Drainage information required at outline stage

At outline stage a council needs to be satisfied that the site can be drained, that enough land has been set aside to do it, and that the rate and point of discharge are achievable. It does not need pipe sizes, manhole schedules or a hydraulic model. Where an outline application stalls on drainage, it is usually because one of those three things was asserted rather than evidenced, or because the applicant submitted a detailed design that fixed a parameter they later wanted to move.

The distinction matters commercially. A parameter plan that commits 0.4 hectares to attenuation in the wrong corner of the site is very hard to unwind at reserved matters.

What has to be fixed at outline

An outline application under Article 5 of the Town and Country Planning (Development Management Procedure) (England) Order 2015 seeks permission for the principle of development, with one or more of access, appearance, landscaping, layout and scale reserved.

Drainage is not one of the five reserved matters. It sits alongside them as a material consideration, so the level of detail required is whatever the decision maker needs to conclude the development is acceptable in principle. Three things normally have to be resolved at that stage.

1. That surface water and foul water can both be dealt with, by a route that exists or can be secured.
2. That the developable area shown can accommodate the drainage infrastructure without displacing the quantum of development applied for.
3. That flood risk on and off the site is acceptable, which for sites in Flood Zone 2 or 3, or of one hectare or more in Flood Zone 1, means a site-specific Flood Risk Assessment at validation.

Everything past that is design, and design follows the layout that has not been fixed yet.

Showing that the site can be drained

The outline-stage Drainage Strategy is a feasibility document. It should establish the discharge hierarchy position with evidence rather than assertion, and that evidence is usually four items.

Preliminary infiltration testing, or a clear statement of why infiltration was ruled out. Underlying clay, a high water table, made ground with a contaminative history or a source protection zone are all legitimate reasons, but say which one applies and show the ground data behind it. Groundwater levels deserve particular care, and the wider question of groundwater flooding evidence and how to assess it often decides whether infiltration is available at all.

A written response from the sewerage undertaker to a pre-development enquiry, giving a point of connection and any capacity constraint for both foul and surface water.

Where discharge to an ordinary watercourse is proposed, confirmation of ownership or access to the outfall and an acknowledgement that consent under section 23 of the Land Drainage Act 1991 will be required from the lead local flood authority. For a main river, the Environment Agency's permitting position.

A greenfield runoff calculation with the method stated, and the resulting storage volume calculated for the design events with the Environment Agency climate change allowance applied. Say which allowance and which epoch you used.

That package tells the lead local flood authority the site works. It does not tell them how, and at outline it does not have to.

Land take for drainage in the parameter plans

This is the part applicants underestimate. Attenuation for a greenfield-rate discharge on a medium-sized residential site is measured in thousands of cubic metres, and if it is delivered in open basins at shallow depth the land take is real.

Calculate the volume first, then decide the form, then decide whether it is shown as a fixed parameter or as an area within which drainage will be provided. The second framing is usually better. A parameter plan that identifies a drainage zone of a stated minimum area, positioned at the low point of the site, gives the authority its certainty and leaves the designer room.

Some practical points that repeatedly cause trouble at reserved matters. Basin side slopes eat area, so a volume that looks like it fits at 1 in 3 slopes may not at the 1 in 4 the council's own guidance expects. Attenuation cannot sit under adoptable highway in some undertakers' standards. And a basin shown in the same location as the required open space or biodiversity net gain area will be counted twice by an optimistic layout and once by the officer.

Show the drainage zone, the developable area and the open space on the same plan at the same scale. If they overlap, resolve it before submission.

Discharge point and rate at outline

The discharge rate is a number the authority will condition, so give it deliberately. Greenfield rate applies on greenfield sites. On previously developed land, policy normally requires betterment against the existing rate, and the size of that betterment varies between lead local flood authorities, so check the local standard rather than assuming a percentage.

The discharge point needs to be deliverable. A proposed outfall on third-party land with no easement is not a strategy, and officers have become quicker at spotting this. Where an easement is needed, say so at outline and explain the route to securing it, because an authority can grant permission subject to a Grampian condition preventing commencement until the outfall is secured.

Flood Zone work runs in parallel. If the site needs the sequential and exception tests, do them at outline. The two limbs of the exception test and the evidence each needs are a matter of principle, not of layout, so leaving them to reserved matters is not an option.

What properly belongs in a condition

The Planning Practice Guidance sets six tests for conditions: necessary, relevant to planning, relevant to the development permitted, enforceable, precise and reasonable in all other respects. A detailed drainage design condition meets all six at outline.

A workable set usually reads along these lines.

Two cautions on pre-commencement conditions. Section 100ZA of the Town and Country Planning Act 1990 requires the applicant's written agreement to a pre-commencement condition, so you will be asked to sign a schedule. Read it. And phrase phased conditions by phase rather than for the whole site, or the first phase cannot start until the last phase is designed.

When the authority asks for more than it needs

Lead local flood authorities are under pressure and some respond to an outline consultation with a request for a full detailed design. That is a disproportionate request and it can be resisted politely.

The reply that works sets out what the strategy demonstrates, points to the conditions offered, and asks specifically what residual question the authority has that the conditions do not answer. Most of the time the honest answer is that the officer wants reassurance on one point, usually infiltration or the outfall, and that point can be met with a single additional piece of evidence rather than a full design.

Where the request persists, the escalation is to the case officer rather than the consultee, because the decision on proportionality is the local planning authority's. An extension of time is usually preferable to a refusal on drainage grounds, since a refusal reason of this kind is expensive to overturn on appeal and easy to avoid.

Parameters that constrain later phases

The last thing to check before submission is what the outline permission will lock in. Read the proposed parameter plans and conditions as though you were the engineer designing phase three in four years.

Finished floor levels expressed as a fixed figure rather than a minimum become a problem when the detailed levels design moves. A drainage zone shown as a precise polygon becomes a problem when the layout shifts by 10 metres. A discharge rate stated for the whole site with no phasing becomes a problem when phase one has to build the whole attenuation volume before selling a house.

State rates and volumes by phase where the development will be phased. Express levels as minima. Describe the drainage zone as an area of at least a stated size within a broader parcel. And make sure the strategy document itself is the one listed in the approved plans condition, because whatever is listed there is what the reserved matters submission has to accord with.

Enter your postcode on PlanScope to see which reports an outline application on your site is likely to need.

This is general guidance, not legal advice. Check the position with your local planning authority and lead local flood authority before relying on it. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.