Lead local flood authority comments on your drainage

The lead local flood authority is the county council or unitary authority for your area, and since April 2015 it has been a statutory consultee on surface water drainage for major planning applications. Its comments arrive as a written response on the public register, and on a small major they are frequently the only thing standing between the case officer and a recommendation. A holding objection from the lead local flood authority stops an application in its tracks until the drainage evidence is fixed.

Most of those objections are avoidable. They come from missing information rather than from unworkable engineering.

What the lead local flood authority does

The role was created by the Flood and Water Management Act 2010, which made county and unitary authorities responsible for managing local flood risk: surface water, groundwater and ordinary watercourses. Main rivers and the sea remain with the Environment Agency.

In planning terms that produces four functions.

• Statutory consultee on surface water drainage for major development.
• Author or commissioner of the local flood risk management strategy, which sets out the authority's approach and often its own design standards.
• Consenting authority for works affecting ordinary watercourses under land drainage legislation, which is a separate consent from planning permission.
• Holder of local flood records, including historic flooding reports that never appear on a national map.

That last function is worth using before you design anything. A request for flood history at a site address usually gets a reply within a couple of weeks and can reveal a repeated surface water problem that no published dataset shows.

Schedule 3 to the 2010 Act would create a separate SuDS approving body regime with its own approval process. Commencement remains pending as at the date of writing, so check the current position rather than assuming either way.

When your application is referred

Statutory consultation applies to major development: ten or more dwellings, a site of 0.5 hectares or more where the number is not known, or 1,000 square metres or more of floorspace, or a site of one hectare or more.

Below that threshold many authorities are consulted anyway, either through a local agreement or because the council's validation list requires a drainage strategy that the case officer then sends across. So a scheme of six houses may well be reviewed by the same engineer, without the statutory response period that a major attracts.

Referral also happens outside the thresholds where the site sits in a critical drainage area, where the council's strategic flood risk assessment identifies a surface water flow path, or where a watercourse crosses the land. The council's strategic flood risk assessment is where those local triggers are usually mapped.

The discharge hierarchy and infiltration evidence

Every drainage strategy is read against the same hierarchy, in this order.

1. Infiltration to the ground, through soakaways, permeable paving or infiltration basins.
2. Discharge to a surface water body, meaning a watercourse, ditch or pond.
3. Discharge to a surface water sewer, with the undertaker's agreement.
4. Discharge to a combined sewer, as a last resort and only with evidence that the options above were tested.

The word that matters is evidenced. Rejecting infiltration because the soils are assumed to be clay will produce a request for testing. Rejecting it because a soakage test to a recognised method returned an infiltration rate too low to work, with the borehole logs and the test date attached, will not.

Two practical points about testing. Do it in the winter half of the year where you can, because a test in a dry August overstates the rate and the reviewer knows it. And test at the depth and location of the proposed feature rather than wherever the rig could reach, because a test at 1.5 metres tells you little about a basin at 0.8 metres.

Groundwater matters as much as permeability. Where the water table sits within a metre of the base of a proposed soakaway, infiltration is not available regardless of the soil, and the strategy has to say so with monitoring data.

Local standards beyond the national ones

The national position is set by the non-statutory technical standards for sustainable drainage systems, which cover peak flow control, volume control, flood risk within the development, structural integrity and maintenance. They are short and they leave a good deal to local discretion.

That discretion is where the work is. Most lead local flood authorities publish their own guidance, and it commonly goes further than the national standards on the following.

• Greenfield run-off rates, with some authorities requiring the 1 in 1 year greenfield rate rather than the estimated mean annual flood rate for all return periods.
• A minimum discharge rate, since very low rates block small orifices in practice.
• Climate change allowances, applied to rainfall using the current Environment Agency peak rainfall allowances for the relevant river basin district.
• An allowance for urban creep on residential schemes, typically applied as an uplift to the impermeable area.
• Treatment stages for water quality, following the simple index approach in the industry SuDS manual.
• Above-ground SuDS preferred over buried tanks, with a stated proportion of the treatment train at surface.

Download that guidance before designing. A Drainage Strategy designed to the national standards alone, in an authority that requires urban creep and a 1 in 1 greenfield rate, will be sent back for a redesign that changes the site layout.

Holding objections and how schemes recover

A holding objection is not a refusal recommendation. It is a statement that the authority cannot support the application on the information provided, and it converts into a no objection once the gap is filled.

The gaps that generate them are consistent.

No infiltration testing, or testing carried out at the wrong depth or season. No calculations, only a narrative. Attenuation volumes with no supporting model output. No exceedance routing showing where water goes in an event beyond the design standard. No maintenance regime, or one that names an adopting body that has not agreed to adopt. And drawings that contradict the calculations, usually because the layout changed after the drainage was designed.

Recovery is straightforward if you treat it as an information request rather than an argument. Submit the missing material as a formal addendum, ask the case officer to re-consult, and allow three to four weeks. Where the objection is about principle rather than information, for example a discharge to a combined sewer that the undertaker has not agreed, expect a longer negotiation and involve the water company early.

A Flood Risk Assessment and the drainage strategy have to agree with each other. Contradictions between the two documents, most often about groundwater or about the existing runoff regime, are read as a sign that neither was checked.

The conditions the LLFA usually asks for

Even a supportive response comes with conditions, and read them before you accept them because they set your programme.

The standard set runs to three. A pre-commencement condition requiring submission and approval of the detailed surface water drainage design, based on the approved strategy. A condition requiring a maintenance and management plan naming the responsible party for the lifetime of the development. And a pre-occupation condition requiring a verification report, with photographs and as-built drawings, confirming the system was built as approved.

That third one is newer than the others and catches developers who built to a value-engineered design. If the attenuation crate installed is smaller than the one approved, the verification report is where it surfaces, and the remedy is expensive.

Design it properly, evidence the testing, and keep the drainage engineer involved after permission rather than only before it.

Enter your postcode on PlanScope to see which reports your site is likely to need. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval, and for specialist technical reports qualified professional input may be required. This is general guidance rather than legal advice.