Drainage strategy for a minor residential scheme
On schemes under ten dwellings the lead local flood authority is not a statutory consultee, so the drainage strategy has to convince a case officer on its own.
A drainage strategy for minor development, meaning fewer than ten dwellings, has to satisfy the council's validation list without the benefit of a statutory consultee looking at it. The lead local flood authority is a statutory consultee on major development, not on minors, so on a four-plot scheme the case officer is often the only person who reads your calculations. That cuts both ways. Nobody will help you improve the strategy, and nobody will defend it either if a neighbour objects about flooding.
Write it so a non-specialist officer can follow the logic from survey to discharge rate in ten minutes.
Existing drainage and the site survey
Begin with what is there, evidenced rather than assumed. A topographical survey with levels, the position and invert levels of any existing manholes, a sewer records enquiry from the undertaker, the position of any ditch, culvert or watercourse within and adjoining the site, and photographs after heavy rain if you have them.
Record the existing impermeable area as well. On a redevelopment site that figure sets the baseline against which betterment is measured, and on a greenfield plot its absence is the point.
Note also whether the site drains to a combined sewer. Many older urban streets do, and a combined system changes the argument about discharge rates considerably.
Working down the hierarchy in order
The drainage hierarchy is not a menu. Each option has to be discounted with evidence before moving to the next.
- Infiltration to ground, through soakaways, permeable paving with an attenuating sub-base, swales or infiltration basins.
- Discharge to a surface water body, meaning a watercourse, ditch or lake.
- Discharge to a surface water sewer.
- Discharge to a combined sewer, as a last resort and normally only with the undertaker's agreement.
The commonest failing in a minor scheme submission is jumping to option four with a sentence saying the ground is clay. That is an assertion, not evidence. Where infiltration is discounted, the strategy needs test results or a documented reason, and our worked example of a drainage hierarchy decision shows how the reasoning is set out.
Infiltration testing and where it fails
Soakaway design testing follows the BRE digest on soakaway design. The method involves trial pits at the proposed soakaway locations and depth, filled and emptied three times, with the time recorded for the water level to fall from 75 per cent to 25 per cent of the effective storage depth. The slowest of the three results is used.
Two constraints then govern whether infiltration is usable at all:
- The base of the soakaway should sit clear of the highest expected groundwater level, with a margin of about a metre, so a winter groundwater monitoring period may be needed on marginal sites.
- Soakaways are normally kept at least five metres from buildings and from the highway, which on a tight plot can leave nowhere to put them.
Contamination adds a third. Infiltrating surface water through made ground on a former industrial site can mobilise contaminants, and the Environment Agency position on groundwater protection may rule it out regardless of permeability.
Test in the right season and at the right depth. A test pit dug at 1.2 metres in August tells you very little about a soakaway at 2.5 metres in February.
Sizing attenuation and setting the discharge rate
Where infiltration is not viable, the scheme attenuates and discharges at a controlled rate. Two numbers define the design.
The discharge rate should be the greenfield runoff rate for a greenfield site, and for previously developed land it should be reduced towards greenfield as far as reasonably practicable. Many authorities set a minimum practical discharge rate below which flow controls block, which is why very small sites often end up with a stated floor rather than a calculated figure.
The storage volume comes from the difference between inflow and outflow over the critical duration, for events up to and including the 1 in 100 year return period, with an allowance for climate change taken from the current Environment Agency peak rainfall allowances for the relevant river basin district and epoch. Those allowances are revised, so quote the version and date you used. An allowance for urban creep is usually added for residential development.
Illustrative example only. On a small infill plot, the designer establishes the existing and proposed impermeable areas from the site layout, applies the current climate change allowance for the relevant river basin district to the rainfall inputs, adds an urban creep allowance to the proposed impermeable area, tests a range of storm durations to find the critical one, and sizes the storage so that outflow never exceeds the agreed discharge rate. The output is a single table of storage volume, discharge rate and exceedance route. Site-specific figures must come from your own calculations and the current published allowances.
Always describe the exceedance route. Where storage is overwhelmed, water has to go somewhere, and showing overland flow running to an area away from buildings and off the site boundary is often what persuades an officer.
Foul drainage and the connection agreement
Foul and surface water are separate questions and should be separate sections. The right to connect foul drainage to a public sewer comes from the Water Industry Act 1991, and the undertaker's agreement covers the point of connection and any capacity constraint.
Where no public sewer is available, a package treatment plant or septic tank with a drainage field needs its own justification, percolation testing to the relevant standard and, depending on the discharge, a permit or registered exemption. A Foul Sewage Assessment covers that ground where the council asks for one.
Maintenance, adoption and who owns the pipe
Officers ask who maintains the system for the next 60 years, and on minor schemes the honest answer is often the householders. Say so, and set out the arrangement: a management company, a deed of covenant, or individual responsibility for plot-level features with a shared arrangement for anything communal.
Adoption of sewers by the undertaker runs under the agreement provisions of the Water Industry Act 1991 and has its own design requirements, which are stricter than planning. The separate statutory approval regime for sustainable drainage in Schedule 3 to the Flood and Water Management Act 2010 remains uncommenced as at September 2026, so verify its status before relying on either position.
What the authority expects at validation
A workable Drainage Strategy for a minor scheme runs to a dozen pages and contains:
- A site description with levels, existing drainage and a sewer records extract.
- The flood risk context, cross-referring to any Flood Risk Assessment where one is required.
- Infiltration test results or the evidenced reason infiltration is discounted.
- The chosen discharge point, the agreed or proposed rate, and evidence of contact with the undertaker or riparian owner.
- Calculations with the rainfall data source, climate change allowance and urban creep allowance stated.
- A drainage layout drawing with pipe sizes, invert levels and the location of storage.
- The exceedance route shown on a plan.
- Maintenance responsibilities and an outline schedule.
One honest uncertainty. Requirements vary by authority and some local lists ask for far more on a small site than others. Read the validation checklist before you commission testing, because a winter groundwater monitoring period cannot be retrofitted into a programme.
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.
Frequently asked questions
Is the lead local flood authority consulted on a small scheme?
Not as a statutory consultee. That role applies to major development, which for housing means ten dwellings or more. Councils may still seek informal drainage advice, and some have internal drainage engineers, but on a minor application the case officer is often the only reader of the strategy.
How do I prove infiltration is not possible?
With test results rather than an assertion about clay. Carry out trial pit testing at the proposed soakaway location and depth following the recognised soakaway design method, record all three fills, and report the slowest result. Where groundwater, contamination or proximity to buildings rules infiltration out, evidence that constraint instead.
What discharge rate will the council accept?
For a greenfield site, the greenfield runoff rate. For previously developed land, a rate reduced towards greenfield as far as reasonably practicable, which usually means demonstrating betterment against the existing impermeable area. Many authorities apply a minimum practical rate because very small flow controls block in service.
Which climate change allowance should I use?
The current Environment Agency peak rainfall allowance for the relevant river basin district and the design lifetime of the development. The allowances are revised periodically, so state the version and date used in the calculations. Using a superseded figure is a common reason for a drainage condition being refused at discharge stage.
Who maintains the drainage after the houses are sold?
Whoever the strategy says, and the council will want that set out. On small sites it is commonly the individual plot owners for their own features, with a management company or deed of covenant covering shared elements. Adoption by the sewerage undertaker requires an agreement and stricter design standards.
Sources and further reading
This article is general guidance about the planning system in England and, where stated, the rest of the UK. It is not legal or professional advice. Check your local planning authority’s validation requirements and take professional advice for your specific site.