Answering a lead local flood authority drainage objection

A refusal that follows a lead local flood authority objection is almost never about flood risk to the site. It is about surface water leaving the site: the discharge rate, the storage volume, the evidence for infiltration, and who maintains the system in thirty years' time. Lead local flood authorities have been statutory consultees on surface water drainage for major development since 2015, and a planning officer will normally adopt their objection without reworking it.

Which means the appeal is a technical exercise against a technical objection. Policy argument does not help. Calculations, test results and a maintenance mechanism do.

What a drainage objection usually says

Read the consultation response rather than the refusal notice, because the response contains the actual complaint. Four versions recur.

Insufficient information, where the strategy lacks calculations, a drainage layout or evidence of the discharge point's capacity. Unacceptable discharge rate, where the proposed rate exceeds what the authority will accept for the site. Unproven infiltration, where soakaways are proposed without testing. And no maintenance arrangement, where the system is designed but nobody is identified to look after it.

The first and last are the easiest to resolve and the most common. An objection on insufficient information is not a judgement that the scheme is undrainable, and saying so in the appeal statement, politely, is a reasonable starting point.

Infiltration testing and discharge rates

The discharge hierarchy is the frame for everything. Infiltration first, then a watercourse, then a surface water sewer, then as a last resort a combined sewer, and each step down needs evidence that the one above was tested and rejected.

Infiltration claims need field testing, not a desk assessment of the geology. Soakaway testing to the established method, with trial pits at the depth and location of the proposed features, three test fills per pit, and the groundwater level recorded, is what a lead local flood authority expects. A strategy claiming infiltration on the basis of a sand and gravel map, with no pits, will be objected to every time.

Discharge rates are the second fight. Greenfield sites are normally limited to the calculated greenfield runoff rate for the relevant return periods. Previously developed sites are expected to reduce peak runoff as close to greenfield as reasonably practicable, and many authorities apply a betterment requirement expressed as a percentage reduction on the existing rate.

So the existing rate matters, and it has to be evidenced. Which brings us to the point most appeals lose on.

Betterment and existing run-off evidence

Where betterment is claimed against an existing connection, the authority will want evidence that the existing connection exists, that it is lawful, and that it has the capacity assumed. A site plan showing a pipe is not evidence.

What works: a CCTV survey of the existing drainage, the sewerage undertaker's records, historic plans showing the connection, and a measured impermeable area for the existing site taken from a survey rather than from mapping. Impermeable area is routinely overstated, and an authority that recalculates it from aerial imagery will arrive at a lower figure and a lower permitted discharge.

Where the existing connection cannot be proven, the fallback is to design to greenfield rates, which usually means more storage. Price that outcome before appealing, because redesigning may cost less than arguing.

Storage, exceedance and climate change

The storage calculation has to be shown, not summarised. The authority expects the critical duration event to be identified, storage sized for the 1 in 100 year event with the current climate change allowance applied, and the half drain time demonstrated so the system recovers before the next event.

Climate change allowances are revised, so use the current published allowance for the relevant river basin district and management catchment and say which version you applied. An appeal running on a superseded allowance invites a request for the whole calculation to be redone.

Exceedance routing is the section most often missing. Where the system is overwhelmed, water has to go somewhere, and the authority wants a plan showing overland flow routes away from buildings and away from neighbouring land. A finished floor level strategy with levels stated completes it. Both belong in the Drainage Strategy, and where the site also sits in a flood zone the Flood Risk Assessment carries the separate sequential and exception test work.

Maintenance and who will do it

A technically sound system with no maintenance plan still attracts an objection. The authority wants to know, for each component, who owns it, who maintains it, what the maintenance schedule is, and where the funding comes from.

Three routes exist and they can be combined:

1. Adoption by the sewerage undertaker under a sewer adoption agreement, which suits conventional pipework and some below-ground attenuation
2. Adoption by the highway authority for features within the highway
3. A private management company funded by estate charges, with a commuted sum where the authority requires one

Attenuation basins, swales and permeable paving are the components that fall between the stools, and they are usually the ones left unallocated. Name the responsible party for each, attach a maintenance schedule with frequencies, and the objection on this ground falls away.

Conditions offered instead of refusal

Before the appeal, test whether the authority would have accepted a condition. Many drainage objections are capable of resolution by a pre-commencement condition requiring a detailed scheme to be approved, and lead local flood authorities frequently recommend exactly that where the principle is drainable and only the detail is missing.

Offer the condition wording in the appeal, drafted tightly: the detailed scheme, the discharge rate, the storage volume, the maintenance plan and the verification report, each with a trigger. An inspector presented with a workable condition and an objection that amounts to missing detail will often grant permission subject to it.

That approach is worth more than a long statement disputing the technical position, and it fits the general discipline of converting a refusal into a plan described in reading a refusal notice and turning it into a plan.

Engaging the flood authority late

If the objection was made without a site meeting, ask for one even after refusal. Lead local flood authority teams are small and generally pragmatic, and a 40-minute conversation about the discharge point often resolves what three rounds of correspondence did not.

Where a revised strategy is produced for the appeal, send it to the authority before submitting. A withdrawn objection, or a letter confirming that a condition would be acceptable, is the strongest document in an appeal of this kind.

One honest uncertainty: standards for surface water drainage vary between lead local flood authorities, and a strategy accepted in one county can be objected to in the next. Ask for the authority's own published guidance and design to it rather than to the national standards alone.

A Planning Statement drafted on PlanScope can set out the drainage policy position and the condition offer as a structured section, with the calculations supplied by the drainage engineer. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval. For specialist technical reports, qualified professional input may be required.

This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it.