Checklist: what a lead local flood authority asks for

A lead local flood authority requirements checklist is short in principle and long in practice. The LLFA, which is your county council or unitary authority, is a statutory consultee on surface water drainage for major development, and what it wants is proof that the proposed drainage works for the right rainfall events, discharges as high up the hierarchy as the ground allows, and will still be maintained in 30 years. Give it less than that and you get a holding objection, which stops the determination rather than slowing it.

Major development here means 10 or more dwellings, a site of 0.5 hectares or more where the number is not known, or 1,000 m squared of floorspace or a site of one hectare or more for other uses.

What the consultee role actually covers

The LLFA comments on surface water drainage. It is not the Environment Agency, which deals with flood zones, main rivers and the sequential and exception tests, and it is not the sewerage undertaker, which deals with capacity and connection. Applicants send the same document to all three and then wonder why each asks for something different.

Its remit comes from the Flood and Water Management Act 2010 and from the consultation arrangements introduced in 2015, backed by national policy expecting sustainable drainage on major development unless it can be shown to be inappropriate. The practical effect is that an LLFA objection on drainage grounds is difficult for a case officer to approve over.

Many authorities publish their own local guidance setting out discharge rate policy, required return periods and preferred SuDS components. Read it before designing anything, because the local standard is frequently tighter than the national default and the LLFA will apply its own.

The information expected at application stage

A drainage submission that does not get bounced normally contains the following.

• Existing site drainage: where water goes now, with evidence rather than assumption, including any records of surface water flooding.
• Greenfield runoff rate calculation for the site, with the method stated and the catchment area used.
• Proposed discharge rate and the justification for it, including any minimum rate agreed with the LLFA.
• Storage volume calculations for the design events, usually including the 1 in 1, 1 in 30 and 1 in 100 year events with the current climate change allowance applied.
• Exceedance flow routing for events beyond the design standard, shown on a plan with flow arrows and safe conveyance to a place where water does no harm.
• A drainage layout drawing with pipe sizes, invert levels, control structures and the location of every SuDS component.
• Ground investigation results, including infiltration testing where infiltration is proposed.
• Groundwater level evidence, because a soakaway with a water table a metre below it does not work.
• Water quality treatment appropriate to the land use, using the simple index approach or the local equivalent.
• A maintenance and adoption plan naming who does what and who pays.

That list is what the Drainage Strategy has to deliver. Where the site is also in Flood Zone 2 or 3, or is one hectare or more, a separate Flood Risk Assessment addresses the flood risk question and the drainage strategy sits inside or alongside it.

Ground investigation and infiltration testing

Infiltration testing is where programmes slip. The test has to be done in the right place, at the right depth and under the right method, and it cannot be done usefully in a waterlogged winter if the result is then used to claim good infiltration.

Three things make results acceptable. Tests at the proposed depth of the infiltration feature rather than at a convenient trial pit depth. Enough test locations to represent the site rather than one near the gate. And a recorded groundwater level with a stated seasonal correction, because the design needs a minimum unsaturated depth beneath the base of the feature.

Where infiltration fails, say so and move down the hierarchy rather than quietly designing a tank. An LLFA officer who sees a rejected infiltration option with test data behind it will accept the next option. One who sees no test data at all will ask for it.

The discharge hierarchy and justifying your position

The order is infiltration first, then a surface water body, then a surface water sewer, then a combined sewer. Each step down needs evidence that the step above was examined and rejected.

Discharge to a watercourse requires the consent of the riparian owner and, for an ordinary watercourse, land drainage consent from the LLFA for any structure in or over it. Discharge to a sewer requires the undertaker's agreement and usually a restricted rate, and the undertaker's capacity assessment is a separate conversation from the LLFA's.

Betterment is the usual expectation on brownfield sites. Where the existing site discharges at an unrestricted rate from hardstanding, the LLFA will look for a meaningful reduction, often toward the greenfield equivalent, and will want the existing rate evidenced rather than estimated.

Maintenance, adoption and the long view

The question that catches small developers is who maintains the attenuation basin in year 25.

Three routes exist. Adoption by the sewerage undertaker, which applies to the pipework and only to components that meet its adoption criteria. Adoption by the highway authority for highway drainage. Or a management company funded by an estate charge, which is where most residential SuDS end up.

Whichever applies, the submission needs a maintenance schedule listing each component, the operation, the frequency and the responsible party, with an access route for the machinery that will do it. A basin that can only be reached across private rear gardens will be flagged.

Schedule 3 to the Flood and Water Management Act 2010 would create SuDS approving bodies and a separate approval regime. That regime had not been brought into force in England at the time of writing, so check the current position before assuming the planning route is the only one.

Conditions and avoiding the holding objection

Where the LLFA is broadly content, it will usually recommend conditions rather than refusal. The familiar set covers detailed drainage design approval before work starts, verification that the system was built as approved, and a maintenance plan secured for the lifetime of the development.

A holding objection means the LLFA cannot assess what it has been given. The commonest reasons are an absent exceedance plan, calculations with no stated method, infiltration claimed without test results, and a maintenance section that names nobody.

Work backwards from that list. Four weeks spent on ground investigation and a properly annotated layout before submission is less expensive than a holding objection that sits on the file for three months while the eight week period expires and the applicant agrees extension after extension. Our checklist for discharging a drainage condition covers the second round of the same argument.

A drainage strategy is a report type where councils expect a suitably qualified author, typically a civil or drainage engineer, and professional review is strongly advised. An AI-assisted draft can structure the document and carry the policy narrative, but the calculations and the ground data are the engineer's work.

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

Enter your postcode on PlanScope to see whether your site triggers a drainage strategy, a flood risk assessment or both.

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.