Surface water from a neighbouring site and riparian rights

Water that flows naturally from higher land onto lower land is not a wrong, and the owner of the lower land generally has to accept it. What the higher owner may not do is collect, concentrate or artificially increase that flow and discharge it onto a neighbour. That distinction, between natural drainage and an engineered discharge, is the whole of the civil law question when a new development upstream starts putting water through your gateway.

The planning system runs alongside it rather than instead of it. A permission granted with a drainage condition does not give the developer a right to discharge onto you, and your objection to the application is not the same thing as a claim.

Natural drainage and what the law allows

The starting position is a natural servitude. Surface water and percolating water move downhill, and no easement is needed for that. A lower owner who dislikes it may protect their own land, including by raising levels or building a wall, and is not generally liable to the higher owner if water then ponds upstream.

Change the flow artificially, though, and the position changes with it. Regrading a field so run-off collects into one corner, piping a yard to a single outfall at a boundary, culverting a ditch and shortening its route, or paving a large area that previously absorbed rainfall can all amount to a nuisance if the result is damage downstream.

Two practical consequences follow. A claim needs a change, so the evidence has to show what the flow was before. And a claim needs damage or real interference, not merely more water in a ditch that copes with it.

Riparian ownership and maintenance duties

If a stream, brook or ditch runs through your land, you are presumed to own the bed up to the centre line where it forms a boundary, and the whole bed where it runs through. That ownership carries duties rather than just rights.

That last row catches landowners who culvert a ditch to gain field access. Consent under the Land Drainage Act 1991 is needed for works that obstruct or alter the flow of an ordinary watercourse, and doing it without consent is an offence as well as a liability risk.

Lead local flood authorities also have powers to require a riparian owner to remove an obstruction, which is sometimes the fastest remedy available when the problem is a blocked culvert rather than a new development.

Where increased run-off becomes a claim

The pattern that generates most disputes is familiar. A site upstream is developed, the impermeable area grows, the surface water is collected and piped to a single outfall into the ditch at the boundary, and the ditch, which handled diffuse flow from grass, now takes a concentrated discharge during every storm.

Whether that is unlawful turns on the facts: has the rate or the volume increased, has the discharge been concentrated where it was previously spread, and has damage resulted. A development that limits its discharge to the greenfield run-off rate for the design event and attenuates the rest has a good answer. One that discharges unrestricted does not, and a Drainage Strategy that assumes the ditch has spare capacity without checking is where the trouble usually originates.

Outfall design is part of the same question. A pipe discharging at the wrong level or without energy dissipation scours the bank and undermines the channel, and the detail matters as much as the rate, as the note on outfall design, bank stability and scour protection sets out.

Evidence of a change in flows

Build the record before you need it. The useful evidence is unglamorous.

1. Dated photographs of the ditch, the gateway or the field in storm conditions, taken over several seasons, with a fixed reference point in frame.
2. A topographic survey of your land and the boundary channel, so that any later change in levels can be measured rather than argued.
3. Rainfall records from the nearest gauge for the dates of the events you are recording, so that a flood can be matched to a return period.
4. The developer's own submitted drainage calculations, obtained from the planning file, including the discharge rate, the attenuation volume and the design event.
5. Independent calculations comparing the greenfield run-off rate for the site with the proposed discharge, which is where the rainfall dataset used matters, as explained in which rainfall dataset to cite in a drainage calculation.

An illustrative example. A smallholding below a six-hectare field that gains permission for 40 houses. Before development, run-off reached the boundary ditch diffusely along 200 metres of hedge. After, it arrives through one 300 millimetre pipe. The attenuation tank is sized for the one in 30 year event with a restricted outflow, which is policy-compliant, but the exceedance route for larger events crosses the smallholding's yard and nobody modelled it. The dispute is about the exceedance route, not the design event, and that is typical.

Planning conditions that help a neighbour

If the application is still live, ask for specific conditions rather than general reassurance. The ones that make a difference are a limit on the discharge rate tied to the greenfield rate for the site, a requirement for the exceedance flow route to be shown and designed for the extreme event, a construction phase surface water management plan, a restriction on raising ground levels near the boundary, and a long-term maintenance obligation naming who is responsible for the attenuation and the outfall.

Be specific in the objection and attach your evidence. An objection that says "this will flood us" is weighed against a consultant's calculation. One that says "the submitted calculation uses a one in 30 year event and the exceedance route shown on drawing 104 crosses our yard" gets a response from the drainage officer.

Where the site is upstream and the flood risk work is thin, ask whether a Flood Risk Assessment was required at all and whether the lead local flood authority has commented. On a sloping site the run-off behaviour is harder to control, and the issues are covered in managing run-off on a steeply sloping site.

Remedies outside the planning system

Planning permission is not a defence to nuisance. If an engineered discharge causes damage, the remedies are a civil claim in nuisance or negligence, an injunction to stop the discharge, and damages for the harm caused. Those are solicitor matters and they are expensive, so most disputes settle on an engineering fix.

Alongside that, the lead local flood authority can act on obstruction of an ordinary watercourse, the Environment Agency can act on unconsented works affecting a main river and on pollution, and the highway authority has duties where the water comes off a road. Building control has no role once construction is finished.

Send a measured letter early. A note to the developer setting out what you have recorded, what you believe has changed and what you want done is often enough, because an engineering adjustment during construction costs a fraction of a dispute afterwards.

Avoiding the dispute at design stage

If you are the one developing, three steps remove most of this risk. Survey the receiving watercourse and establish its capacity rather than assuming it. Design the exceedance route for events beyond the design storm and show it on a drawing. And talk to the downstream owner before the application goes in, because a neighbour who understands the attenuation is far less likely to spend two years objecting to it.

This is general guidance, not legal advice. Riparian duties, nuisance and easements are legal questions for a solicitor, drainage design should come from a suitably qualified engineer, and consents for works affecting a watercourse should be confirmed with the lead local flood authority or the Environment Agency.

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