Drainage conditions: what a discharge submission needs

A surface water drainage condition is discharged by an application for approval of details under article 27 of the Development Management Procedure Order 2015, and the submission fails far more often on completeness than on engineering. Authorities and lead local flood authorities return packages because the calculations do not match the drawings, because no climate change allowance is stated, or because nobody has said who will maintain the attenuation tank in 2050. Design is the easy half.

What follows is the package that gets signed off first time.

Read the condition before you design

Conditions are drafted differently and the wording decides the scope. Some require "details of surface water drainage" before any development. Some require a scheme "based on the principles set out in the approved Flood Risk Assessment". Some require the works to be completed before first occupation, which is a different trigger again.

Three things to extract before you start:

1. What exactly has to be approved, and by whom.
2. When the trigger bites, whether that is before any work starts, before works above slab level, or before occupation.
3. Whether the condition refers to an approved document, because any departure from that document has to be explained.

Where the condition refers back to the approved Drainage Strategy, the detailed design has to be recognisably the same scheme. A tank that has moved across the site and halved in volume is a different scheme, and the officer will say so.

The calculations an authority expects

Four sets of numbers carry the submission.

Discharge rates come first. The restricted rate is usually the greenfield equivalent, estimated by an accepted hydrological method, with the calculation sheet included rather than a bare figure. On brownfield sites the starting point is the existing positively drained area and the betterment the policy requires.

Storage volume follows, sized for the critical duration event with the current climate change allowance applied to rainfall intensity. State which allowance you used, which management catchment and epoch it came from, and the date you took it, because the published allowances are revised. The usual design standards are no flooding in the 1 in 30 year event and no flooding of property in the 1 in 100 year plus climate change event.

Then infiltration. If the scheme relies on soakaways or permeable paving discharging to ground, include infiltration testing at the depth and location of the proposed features, carried out to a recognised method, with groundwater levels recorded. A single trial pit in the corner of the site will not support a scheme of twenty soakaways.

Finally half drain time and the behaviour of the system between storms, which the lead local flood authority will look for on anything using attenuation.

Drawings and the level of detail

The drainage layout has to be buildable. That means pipe diameters, gradients, materials, cover and invert levels, manhole references and a schedule, the attenuation structure in section, flow control details with the orifice or vortex size stated, and the outfall arrangement including headwall and scour protection.

Add two drawings that are often missed. An exceedance plan showing where water goes in events above the design standard, with finished floor levels and overland flow routes marked. And a construction detail sheet for every SuDS component, since a swale drawn as a line on a layout tells a contractor nothing.

Show the catchment areas you used in the calculations on a plan, coloured and labelled, so a reviewer can check the areas in the model against the areas on the drawing. That single sheet removes most review queries.

Water quality

Surface water discharges carry pollution, and authorities increasingly expect the simple index approach from the industry SuDS guidance: a pollution hazard index for the land use, a mitigation index for the proposed treatment train, and a comparison showing the mitigation is sufficient for each pollutant.

Car parks, service yards and roads need treatment before discharge. A single underground tank with no treatment stage will be queried, and on a site discharging to a watercourse or a source protection zone it will be refused.

Maintenance and who owns it in year 20

This is the section that most often triggers a return. The authority needs to know that the system will still work in thirty years.

Where the outfall crosses third party land or discharges to an ordinary watercourse, the consent position belongs here too. Works to an ordinary watercourse need separate consent, which we cover in land drainage consent for works to an ordinary watercourse, and a drainage approval does not grant it.

Who approves and how long it takes

The local planning authority is the decision maker. On major development the lead local flood authority is a statutory consultee for surface water and will usually be the body that actually reviews the calculations. The water undertaker deals separately with the connection and with any adoption agreement.

The determination period for an application for approval of details is eight weeks. A deemed discharge route exists under section 74A of the Town and Country Planning Act 1990 for some conditions, with a list of exclusions, so check whether your condition qualifies before relying on it rather than assuming it does.

Build the lead local flood authority's own turnaround into the programme. Many run a queue measured in weeks, and a request for further information restarts the clock in practice even where it does not in law.

Why submissions come back

The recurring reasons are dull and avoidable:

• the discharge rate is stated but the derivation is missing
• the climate change allowance is not identified by catchment, epoch and date
• modelled impermeable areas do not match the areas shown on the layout
• no exceedance plan and no finished floor levels
• infiltration assumed with no testing, or testing at the wrong depth
• a maintenance schedule with activities but no frequencies and no responsible party
• the flow control is specified at an orifice diameter below what the manufacturer or the authority accepts as non-blocking
• the scheme differs from the approved strategy with no explanation

Where the site sits in a flood zone, cross-check the detailed design against the approved Flood Risk Assessment as well, including any resilience measures. Our note on residual flood risk and property flood resilience covers what those measures look like in practice.

Phasing a discharge across a large site

On a multi-phase site, ask for the condition to be worded by phase at the decision stage, before you need it. Discharging "in respect of phase 1 only" is possible where the wording allows and awkward where it does not.

An illustrative case: a 140 unit site with a single pre-start drainage condition covering the whole development. The developer wanted to start the spine road and phase 1 while the phase 3 attenuation basin design waited on ground investigation. Because the condition was not phased, the authority could only approve details for the entire site, and the start slipped a full season. A variation of the condition under section 73 fixed it, at the cost of another application and another eight weeks.

Submit the strategic design with the application and the detailed design in phases, and say so in the covering statement.

Browse the report types PlanScope drafts if you want a drainage strategy drafted and then reviewed by a qualified engineer before it goes in.

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.