Drainage information for a minor planning application

For a single dwelling or a small extension, the drainage information a council can insist on at validation is much less than most local lists imply, and much more than most applications provide. The national requirements do not include a drainage strategy. What holds applications up is a block plan that does not say where the water goes, so the validation officer asks, and two weeks disappear.

The proportionate answer for a minor application is usually a short note and a marked-up plan: existing drainage, proposed impermeable area, which discharge route is being used and why, and the basis for sizing whatever receives the water. That is enough for most sites, and it is far quicker to produce than the abbreviated major-scheme strategy people tend to submit instead.

National and local validation requirements

The national information requirements for a planning application are set by the Development Management Procedure Order 2015 and the associated guidance: the form, the fee, a location plan, a site plan, ownership and agricultural holdings certificates, and a design and access statement where one is required. Drainage is not on that list as a standalone document.

Local lists are where drainage requirements come from, and they differ. A council can require information it has published on its local list, provided the requirement is reasonable having regard to the nature and scale of the development and it is material to the application. That qualification matters: a local list item written for major development does not automatically bite on a rear extension, and it is legitimate to say so in a covering letter rather than commissioning a strategy the site does not need.

Flood risk is the exception that does have national force. A Flood Risk Assessment is expected for development in Flood Zone 2 or 3 and for sites of one hectare or more in Flood Zone 1. Planning Practice Guidance also defines minor development for flood risk purposes, covering householder development, minor non-residential extensions with a footprint under 250 square metres, and alterations that do not increase the size of a building. For minor development and most changes of use the sequential test is not required, which removes a document people often prepare unnecessarily.

When a full drainage strategy is disproportionate

A drainage strategy is a design document for a scheme with a network: multiple plots, adoptable sewers, attenuation sized for a range of return periods, an exceedance plan and a maintenance regime. A Drainage Strategy earns its place on a major application and on constrained minor sites.

It is disproportionate for a rear extension, a garage conversion, a loft, or a single dwelling in a garden plot where the receiving system is established and capacity is not in question. The test I would apply: if the drainage arrangement can be described accurately in three paragraphs and drawn on one plan, it does not need a strategy.

Two situations turn a small scheme into one that needs more. A site in a critical drainage area, or with recorded surface water flooding, where the council's own mapping flags a problem. And a site where infiltration is proposed but ground conditions are uncertain, because a soakaway that does not work is a problem for the occupier and for the neighbours downhill.

Information for a single dwelling

For one new house, the following is normally enough and rarely refused:

• a block plan showing the proposed impermeable areas, with the roof and hard surfacing hatched
• the surface water discharge route, drawn from the building to the point of discharge
• the foul discharge route and the connection point, with the sewer or treatment plant identified
• the position and indicative size of any soakaway, with its distance from buildings and boundaries
• the basis for sizing, whether an infiltration test result or a stated design assumption
• confirmation of which discharge route in the hierarchy is being used, and why the higher options were rejected

The hierarchy is the part that officers and lead local flood authorities look for. Approved Document H sets out that rainwater should be discharged to an infiltration system where reasonably practicable, then to a watercourse, then to a sewer. Saying plainly that infiltration was tested and rejected because the ground is clay, or because the water table is high, or because a source protection zone applies, answers the question before it is asked.

Where there is no mains sewer, the foul side needs its own treatment, and our note on showing foul drainage where there is no mains sewer covers what to submit.

Permeable surfaces and the small site right

Front garden paving is where householders most often create a planning problem without noticing. Under Part 1 of Schedule 2 to the General Permitted Development Order 2015, hard surfacing within the curtilage of a house fronting a highway is permitted development where the surface is porous, or where provision is made to direct run-off to a permeable area within the curtilage. Above five square metres, a non-porous surface draining to the highway needs permission.

The practical consequences for a small application:

1. Specify the surfacing material, not just the extent, on the drawing.
2. Where a permeable block paving system is used, say what sub-base and sub-grade it drains into, because a permeable surface over clay with no storage is a decorative feature rather than a drainage measure.
3. Where run-off is directed to a border or soakaway, show it.
4. Keep the levels working, because a permeable drive that falls towards the pavement fails whatever the specification says.

Conditions that defer detail

Councils will usually accept a condition requiring surface water drainage details before above-ground works, provided the application establishes that an acceptable solution exists. That is the ordinary route on small sites, and it is the right one where a percolation test cannot be done until the site is cleared.

What makes a deferral acceptable is evidence of principle. A note saying "drainage to be agreed" is not evidence of principle. A note saying infiltration is proposed, with the reason it is thought viable, a location that leaves room for it, and an acknowledgement that a test will confirm the rate, is.

Do not accept a condition that defers something you already know. A pre-commencement condition costs weeks at discharge stage, and where the answer is available now, submitting it now is faster. Our note on reading the surface water flood risk map without overstating it covers how to describe the site's surface water position accurately in that supporting note.

Common queries that delay validation

The queries that generate a "further information required" letter on minor applications are predictable:

Each of these takes a sentence. Each of them, left out, costs a fortnight.

An illustrative example. A single dwelling on a 400 square metre garden plot, no watercourse, Flood Zone 1, not in a critical drainage area. The submission includes a block plan with roof and drive hatched, the impermeable area given as 145 square metres, a soakaway shown 5 metres from the dwelling and clear of the boundary, a note that the British Geological Survey mapping indicates sandy soil and that a percolation test to the recognised method will confirm the rate, and the foul connection marked to the sewer in the road. Total additional work: one drawing annotation and half a page. It validated first time.

A proportionate drainage note

The structure that works on a minor application, in about one page:

1. The site, its Flood Zone, and whether any surface water or critical drainage designation applies.
2. Existing drainage arrangements, including what currently happens to roof water.
3. Proposed impermeable areas, existing and proposed, in square metres.
4. Surface water route, with the hierarchy addressed in one sentence per option.
5. Foul route and connection point.
6. Sizing basis, with any test result or the assumption to be confirmed.
7. Exceedance, in one sentence: where water goes if the system is overwhelmed, and confirmation that it flows away from buildings.

Where flood risk requires more, the Sequential Test page explains when that separate exercise applies, which for minor development is usually not at all.

This is general guidance, not legal advice; check your council's published local validation list and the position with your lead local flood authority before relying on it.

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