Drainage strategy for a single infill plot

For a single infill plot, a drainage strategy is a two or three page note plus a soakaway calculation and a plan, not the document a 200 home site needs. The council wants to see three things: where the rain that falls on your roof and drive will go, why that destination is the highest one available in the drainage hierarchy, and evidence that the ground or the receiving system can take it. Everything else is detail you can leave to a condition.

The reason small sites still get drainage refusals or long condition lists is that applicants skip the hierarchy and write "connect to existing drain" on the plan. That answer fails at the first question an officer asks, which is whether infiltration was tested.

What a single plot actually has to show

Two regimes apply at once. Approved Document H of the Building Regulations sets the surface water hierarchy: discharge to an adequate soakaway or other infiltration system where reasonably practicable, then to a watercourse, and only then to a sewer. Planning policy asks the same question in different words, through the expectation that sustainable drainage is used where it is appropriate and that development does not increase flood risk elsewhere.

Lead local flood authorities are statutory consultees on major development, which a single dwelling is not. So your drainage note will usually be read by the case officer, sometimes passed to a drainage engineer inside the council, and judged against the local validation list rather than a formal standard. That cuts both ways: less scrutiny, but also less chance that a technically strong scheme is understood without being spelled out.

Where the site is in Flood Zone 2 or 3, or is one hectare or more, a Flood Risk Assessment is also needed and the drainage note sits inside it rather than standing alone.

Proportionate investigation and testing

Percolation testing on one plot is a morning's work. BRE Digest 365 sets out the method: dig trial pits at the proposed soakaway location and depth, fill and empty them three times, and record the time for the water level to fall from 75 per cent to 25 per cent of the effective depth. From that you get an infiltration rate and you size the soakaway for the design storm.

Three things make the test credible to a reader who was not there.

• Pit locations plotted on the site plan, at the soakaway position rather than wherever the digger could reach.
• The date and recent weather, because a test in a saturated February gives a different answer from one in August, and the conservative result is the one to design on.
• The depth of the pit relative to any water strike, with groundwater level recorded if found.

If infiltration does not work, say so with the figures rather than asserting it. A recorded rate that cannot take the roof area is a much stronger justification for a sewer connection than a sentence saying the ground is clay. Where the British Geological Survey mapping shows made ground or the site sits in a groundwater source protection zone, infiltration may be constrained for water quality reasons too, and that belongs in the note.

Source controls that fit one plot

You are not designing a basin and a swale for one house. The measures that work at this scale are small, visible and easy to condition.

Water butts are the one people over claim. They fill in the first storm of the autumn and stay full, so the attenuation credit is modest, which we look at in more detail in our note on rainwater harvesting and whether it counts as attenuation.

Permeable paving is the opposite: undramatic, quick to install, and it usually does more for the numbers than anything else available on a plot this size. Specify the sub-base depth and the geotextile on the drawing, because a permeable surface laid over compacted clay with no storage void is decoration.

Connecting to an existing drain

Infill plots often come with a drain already crossing the garden, and the assumption is that the new house can use it. Check three things before you rely on that.

1. Whether the drain is a public sewer. Many private drains and lateral drains transferred to the sewerage undertaker in 2011, so a pipe your neighbour calls private may now be the undertaker's asset.
2. Whether you have a right to connect. Section 106 of the Water Industry Act 1991 gives a right to connect foul drainage to a public sewer, subject to the undertaker's conditions, and the position on surface water is different: an undertaker can refuse a surface water connection to a combined sewer where infiltration is practicable.
3. Whether the existing drain has capacity and where it goes. A 100mm clay pipe of unknown condition serving one bungalow is not evidence of capacity for a second house.

Get the undertaker's position in writing early. On a single plot the correspondence is short, and it converts an officer's question into an answered point.

Existing surface water paths

The least expensive drainage evidence available is a walk round the site in heavy rain. Where does water pond now, where does it leave the plot, and what is downhill of the new building. Add the surface water risk mapping from the long term flood risk service, and you have the beginning of an exceedance story.

Two mistakes recur on infill plots. Building across an overland flow route that has always run between two houses, so the water now stops at your wall and then at the neighbour's back door. And raising the ground level for a level threshold, which redirects flow sideways. Both are avoidable at sketch stage and expensive afterwards.

Finished floor level, external level changes and the position of any threshold drain belong on the proposed levels plan, not in prose.

What the council will condition

Expect a condition requiring the detailed drainage design to be submitted and approved before development begins above ground, with a further condition requiring the permeable surfacing to be retained and maintained. Many councils add a maintenance statement requirement, which for one house is a short paragraph naming the owner as responsible and listing the inspection intervals.

Discharge of conditions carries a fee per request, so it is worth submitting the drainage detail with the calculation, the pit results and the plan in one go rather than in instalments. A Drainage Strategy drafted to the section headings the council uses makes that submission quicker to check.

Where a full strategy really is unnecessary, and on a single plot in Flood Zone 1 with a proven infiltration rate it often is, say what you have done and stop. A short note that answers the hierarchy, shows the test and sizes the soakaway is more useful to a case officer than 30 pages of generic SuDS text. The same instinct applies to the broader evidence set for one house, which we cover in drainage evidence for a single new dwelling.

One honest caveat about design storms. Climate change allowances are updated periodically, and the allowance to apply depends on the river basin district and the development lifetime, so take the current figures from the Environment Agency guidance at the time you design rather than copying a percentage from an older report.

Enter your postcode on PlanScope to see which drainage and flood reports your plot 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. This is general guidance rather than legal advice, so confirm drainage requirements with your local planning authority and the sewerage undertaker before you design.