Checklist for a drainage strategy submitted to the LLFA

A drainage strategy checklist for the LLFA is really a list of things a drainage engineer at the lead local flood authority will look for before deciding whether to object. They read in a fixed order, and a strategy that answers their questions in that order gets a clean response. One that buries the infiltration test results in an appendix and never states the discharge rate gets a holding objection, which on a self-build programme costs weeks.

The checklist below is the order they read in. Work through it before you submit, not after the first response arrives.

Who you are writing for

The lead local flood authority is a statutory consultee on surface water drainage for major development, which means ten or more dwellings, a thousand square metres of floorspace or a site of half a hectare or more. Below that threshold the planning authority usually handles drainage itself, often by condition.

That matters for a self-builder with a small site. If you are below the major threshold, you are writing for a planning officer who may have no drainage background, so clarity beats completeness. If you are at or above it, you are writing for an engineer who will check your calculations, so the arithmetic has to be reproducible from what you submit.

Either way, the document itself follows the same structure as a full Drainage Strategy, scaled to the site. Ask for a pre-application response. Most lead local flood authorities offer one, and the response tells you the discharge rate, the design events and the local allowances they expect. Including that response in the submission removes the single biggest source of disagreement.

Existing conditions and the discharge hierarchy

Start with what the site does now, with evidence rather than description.

Set out the site area, the existing impermeable area, the ground conditions from a desk study and any intrusive work, the groundwater levels encountered with the date, the position of any watercourse, ditch or culvert on or near the site, and the location of existing drainage including anything crossing the site.

Then work down the discharge hierarchy explicitly: infiltration, then a surface water body, then a surface water sewer or highway drain, then a combined sewer. Reject each option above the one you are proposing, with evidence attached. A strategy that opens at the sewer connection without explaining why infiltration was ruled out will be sent back.

Three records are worth obtaining before you write. An asset plan from the water undertaker showing public sewers. The authority's surface water and critical drainage information. And a topographic survey, because every level in the strategy depends on it.

Infiltration testing and the method used

Where infiltration is proposed, the test results are the foundation of the whole document.

State the method, which should be the recognised digest approach, with three fills per trial pit and the slowest result used. State the location of each pit on a plan, its depth, the date of testing and the groundwater level encountered. Give the calculated infiltration rate and the factor of safety applied.

Then say whether the test position matches the proposed component position. A rate measured at the site entrance and applied to a soakaway at the far corner invites a question you cannot answer.

Where infiltration has been rejected, submit the failed test results. "Clay soils" without a test is not evidence, and the authority knows that an afternoon of site work would have settled it.

Runoff rates and the storage calculation

The discharge rate is the number the engineer looks for first. Put it in the summary.

For a greenfield site, the rate is derived from the catchment characteristics using a recognised method, with the inputs stated: site area, soil type, standard annual average rainfall and the resulting rate. Where the calculated rate is very low, a minimum practical rate is normally applied to avoid blockage of the control device, and five litres per second is the figure generally used. Say which you have adopted and why.

For a previously developed site, the position is set by the authority's requirement for betterment against the existing rate, and the existing rate has to be evidenced from the as-built drainage rather than assumed.

The storage calculation then needs to be reproducible. Submit the model output, not a summary of it, including the rainfall data source, the storm durations tested, the critical duration identified, the volumes required and the freeboard. State which software was used and its version.

Half-drain time and the behaviour of the system in back-to-back events are both checks the engineer may apply, so run them before they do.

Climate change and urban creep allowances

Two allowances apply and they are applied differently.

The climate change allowance is applied to rainfall intensity, using the published peak rainfall allowances for the relevant management catchment and the appropriate epoch for the development's lifetime. State the catchment, the epoch, the percentage used and whether you have taken the central or upper end allowance. Authorities differ on which they require, which is another reason for the pre-application question.

Urban creep is an allowance for future paving over of permeable areas by occupiers, applied to the impermeable area rather than to the rainfall. It is usually expressed as a percentage uplift varying with development density, commonly around ten per cent for lower density residential, and the figure to use is the one in your authority's own guidance.

Applying one and forgetting the other is a routine error. So is applying urban creep to commercial development where the authority's guidance does not require it.

Exceedance routes and on-site flood risk

The design event is not the worst event, so the strategy has to say what happens beyond it.

Show the exceedance route on a plan: where water goes when the system is full, the path it takes across the site, and where it ends up. Demonstrate that it does not enter buildings and does not flow onto third party land or the highway.

Then set the finished floor levels with freeboard above the design flood level and above the surrounding ground, and show the levels on a drawing rather than stating them in text. Where the site is in Flood Zone 2 or 3 or exceeds one hectare, this work sits within a Flood Risk Assessment and the drainage strategy cross-references it rather than repeating it.

Basements, lightwells and below-ground parking need specific attention, since they are where exceedance flow collects. A ramp falling towards a garage door is a flood route into the building.

Water quality treatment stages

For any discharge other than to a foul or combined sewer, water quality has to be addressed, and the method expected is the index approach in the industry drainage manual.

Present it as a table: each contributing surface, the pollution hazard level assigned to it, the treatment components in series, and the mitigation indices achieved for suspended solids, metals and hydrocarbons. Show that the indices meet or exceed the hazard level.

Two design points carry the argument. The components must be in series rather than bypassed in larger events through an untreated overflow. And the first stage needs a forebay or silt trap that can be physically dug out, because construction silt arrives regardless of the method statement.

Where the ground is contaminated, infiltration raises a separate question about mobilising pollutants, and the strategy should say whether a risk assessment has been carried out.

Maintenance, adoption and the documents to submit

The last section is the one most often thin, and it is where conditions come from.

Set out each component, the maintenance tasks for it, the frequency, who carries them out and who pays. Say what happens in perpetuity: adoption by the water undertaker where the asset qualifies as a sewer, adoption by the highway authority where it is in the highway, or private management by a company or a management agreement with a funded service charge. Include access arrangements and easements for every component, including anything on third party land.

The submission bundle that normally satisfies a lead local flood authority contains the following.

1. The drainage strategy report, with a summary stating the discharge rate, the design events and the allowances used.
2. A drainage layout drawing with pipe sizes, invert levels, manhole references and component positions.
3. A catchment plan showing contributing areas, hatched and scheduled.
4. Infiltration test results with pit locations, depths, dates and the three readings per pit.
5. Hydraulic calculations as model output, with the software and version named.
6. The exceedance plan and a levels drawing showing finished floor levels and freeboard.
7. The water quality table.
8. The maintenance plan with responsibilities, frequencies and access arrangements.
9. Evidence of any third party agreement, consent or easement needed, or confirmation that one is being pursued.

Where foul drainage is also in issue, it is dealt with separately through a Foul Sewage Assessment rather than inside the surface water strategy, and utility capacity questions belong in a Utilities Assessment.

One final point on the statutory background. The sustainable drainage approval provisions in Schedule 3 to the Flood and Water Management Act 2010 are not in force in England, so there is no separate approving body and everything runs through the planning process. Check the position when you submit, because it has been under discussion for years. Where a variation to an approved scheme later reopens drainage, the document list in our note on section 73 application documents shows what travels with it.

Ask the lead local flood authority for its pre-application response before you commission the modelling. The rate and the allowances it gives you determine most of the cost.

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