Objecting to surface water drainage proposals

Surface water drainage objections work when they give the lead local flood authority something it does not already have. That body, usually the county or unitary council, is the statutory consultee on surface water drainage for major development, and it reviews the applicant's calculations against its own published standards. What it cannot supply is local knowledge of where water actually goes during a storm. You can.

So the useful objection is not "this will cause flooding". It is a dated photograph, a depth, a direction of flow and a location on a plan.

Who advises the council on drainage

Three bodies may be involved and they do different jobs.

The lead local flood authority deals with surface water, groundwater and ordinary watercourses. It is consulted on major development, which broadly means ten or more dwellings, a site of one hectare or more, or 1,000 square metres or more of floorspace, and it comments on the drainage strategy, the discharge rate and the maintenance arrangements.

The Environment Agency deals with flood risk from main rivers and the sea, and with Flood Zones 2 and 3. If the application is not in a flood zone, the Agency will usually have nothing to say about it, and expecting otherwise wastes time.

The sewerage undertaker comments on capacity in the public system and on connections. Its response is often the one that matters most where an existing network is already surcharging.

Minor applications fall outside the statutory consultation, so for anything below the major thresholds your comments go to the case officer and nobody else reviews the drainage in detail. Say so explicitly in your representation, because officers do not always realise it.

Recording existing flooding evidence

Evidence beats assertion every time, and the standard is lower than people assume. You do not need a hydrologist. You need a record.

1. Photograph the flooding while it is happening, with something of known size in the frame, a kerb, a gatepost or a wheelie bin, so depth can be estimated.
2. Note the date and the approximate time, and keep the original image file so the metadata survives.
3. Mark the extent on a plan or an aerial image, with arrows showing the direction of flow.
4. Record how long the water stood and where it drained to.
5. Report the incident to the lead local flood authority and the highway authority at the time, so it enters the official record rather than existing only in your photographs.

Where flooding has been serious, the lead local flood authority may have carried out an investigation under section 19 of the Flood and Water Management Act 2010 and published a report. Ask whether one exists. It is a document the case officer has to take seriously.

The national surface water flood risk maps are a useful cross-check and they are modelled rather than observed, so they will sometimes miss the exact spot where your road floods every February.

Runoff rates and what is being promised

The heart of most drainage strategies is a single number: the rate at which surface water will be allowed to leave the site, usually expressed in litres per second.

Read the Drainage Strategy for three things. What the existing runoff rate is said to be. What the proposed discharge rate is. And whether that rate is the greenfield rate or something higher. On previously developed land the authority will often require betterment, meaning a reduction against the existing rate rather than simply matching it, and a strategy that proposes to maintain the existing rate on a brownfield site is worth questioning against the authority's published standards.

Then look at what happens in events larger than the design event. An exceedance flow route should be shown on a drawing, taking water safely across the site to somewhere it can do least harm. If no exceedance route is shown, ask where the water goes when the system is full, because the answer is usually downhill towards the existing houses.

Our note on finding your lead local flood authority's drainage standards explains where the published requirements live.

Infiltration assumptions and ground conditions

Where a scheme relies on soakaways, permeable paving or infiltration basins, the whole design rests on how fast water soaks into the ground at that site.

Ask whether infiltration testing has been carried out, where, at what depth and in what season. Tests dug in a dry August in the upper metre of soil can produce rates that bear no relation to winter conditions two metres down. Ask what groundwater level was recorded. Ask whether the site has made ground, a former quarry, clay, or a high water table, and whether that is reflected in the design.

On many sites infiltration is not feasible at all, and the correct response is to move down the drainage hierarchy to a watercourse or a sewer. Our note on proving infiltration is not feasible sets out what that evidence should look like. An assumed rate taken from a soil map, with no testing behind it, is a legitimate thing to object to.

Maintenance and adoption

A drainage system performs as designed only for as long as somebody maintains it, and this is where objections most often find real weaknesses.

Ask who will own each element, who will maintain it, how it is funded, and for how long. Common answers are a management company funded by estate charges, adoption of sewers by the water undertaker, adoption of highway drainage by the highway authority, or transfer to the council. Each has a different level of security behind it.

Then ask the practical questions. Is there vehicular access to the control chamber and the basin for a jetting lorry? Is there a maintenance schedule with frequencies? Who inspects the trash screen, and how often? An attenuation basin behind a locked gate with no access track is a basin that will silt up.

Conditions and their discharge

Councils frequently approve a drainage principle and leave the detail to a pre-commencement condition. That is where the substance of the design is decided, and the discharge of conditions application is not publicised in the way the planning application was.

If you want to follow it, say so in your representation and ask the case officer to notify you when details are submitted. Then check the public register periodically. Discharge applications appear there with their own reference number, and the submitted documents are usually available to view.

The stronger ask, made at the right time, is for the drainage strategy to be approved as part of the application rather than deferred. Where a site has a known flooding history, that argument carries weight, and in a designated critical drainage area it carries more. Our note on critical drainage areas explains what that designation adds.

Where to send your evidence

Write to the case officer, within the consultation period, in a single document rather than several emails. Head it with the application reference and the site address.

Copy it to the lead local flood authority drainage team, to your parish or town council, and to your ward councillor. The lead local flood authority's response to the council is a public document, so read it when it appears and, if it raises concerns the applicant then answers, read the answer too. That exchange is often where the design actually gets fixed.

Keep the tone factual and keep to drainage. Flood risk and drainage are material planning considerations. The developer's reputation, the effect on house prices and the number of people who signed a petition are not, and including them dilutes an otherwise strong case.

Understanding which technical documents the scheme has to produce helps you know where to look, and the full list of report types is a reasonable starting point.

This is general guidance, not legal advice. Consultation periods are short and strictly applied, so check the deadline on the council's public register as soon as you become aware of an application.