Drainage for a single plot: soakaways and the regulations

Single plot soakaway drainage starts with a hole in the ground and a stopwatch, not with a calculation. Until you have tested the infiltration rate in the position where the soakaway will actually go, every number in the drainage note is a guess, and a planning officer who has seen a few of these can tell the difference. The order of work is: test the ground, size the storage from the tested rate, then check the distances and the groundwater, then write it up.

For one house the whole exercise is a day of site work and a two page note. It is the sequence that trips people, not the difficulty.

The hierarchy that decides the answer

National policy and the non-statutory technical standards for sustainable drainage set a preference order for surface water, and your drainage note has to work down it in writing.

1. Discharge to the ground by infiltration, which for a single plot means a soakaway or infiltration trench.
2. Discharge to a surface water body, such as a watercourse or a pond.
3. Discharge to a surface water sewer or a highway drain, with the consent of the owner.
4. Discharge to a combined sewer, which is the last resort.

The point of writing it down is that the authority wants to see each option rejected for a reason, not skipped. "Infiltration is not feasible due to clay soils" with no test result is the single most common weak line in a householder or single plot drainage note. If the ground will not take water, the test proves it in an afternoon and the rejection becomes evidence.

Discharging into a combined sewer where infiltration is possible is the arrangement water undertakers now resist hardest, and it is worth knowing that before you design around it.

Testing the ground before sizing anything

The method is set out in the BRE digest on soakaway design, and the principle is simple. Dig a trial pit at the proposed soakaway location, to the proposed depth, fill it with water, and time the fall of the water level through the middle portion of the effective depth. Repeat it three times in the same pit, because the first fill wets the soil and gives an optimistic answer, and use the slowest of the three.

Three details matter more than the arithmetic.

Test where the soakaway will go, not at the most convenient corner of the plot. Ground conditions change across a site, and a result from the only accessible spot is worth little if the soakaway ends up thirty metres away.

Test at the right depth. Infiltration rates in made ground near the surface bear no relation to rates in the natural stratum below.

Record the groundwater level you encounter, with the date. A pit that fills from below has told you something important, and a test carried out in August tells you nothing about February.

Sizing a soakaway for one roof and a drive

Storage volume is the difference between the inflow from the contributing area in a design event and what the soil can absorb during that event. The inputs are the impermeable area, the design rainfall, the tested infiltration rate and the area of soakaway surface available to infiltrate.

Decide the design event before you start. Building control works to the standard in Approved Document H. Many planning conditions and local requirements ask for the one in one hundred year event with a climate change allowance applied to the rainfall. Where both apply, design to the more onerous of the two and say in the note which you have used, because a soakaway sized to the lesser standard will be queried.

Two practical points from experience. Include the driveway and any patio draining towards the house, because the roof alone almost never represents the real impermeable area. And resist the temptation to apply an allowance for future urban creep on a single plot, which belongs on larger schemes. The choice of rainfall data and storm profile matters more than most people expect, and our note on rainfall data and the design storm profile explains the inputs.

Half-emptying time is the check people forget. A soakaway that drains too slowly between storms is full when the next one arrives, and the standard sizing method tests for that. If the half-drain time fails, the answer is a larger infiltration area rather than more storage.

Where the soakaway can and cannot go

Position is often what kills a design that worked on paper.

On a tight infill plot those constraints routinely leave no viable location, which is a legitimate reason to move down the hierarchy. Show the exercise on a plan with the exclusion zones drawn, because that single drawing answers the question faster than any amount of text. Where the ground is affected by contamination the position is more involved, and our note on sustainable drainage on contaminated ground deals with it.

When infiltration does not work and a connection is the answer

If the ground fails the test or the plot has nowhere to put a soakaway, you move to a watercourse or a sewer.

A watercourse connection needs the riparian owner's agreement and, in most cases, land drainage consent from the lead local flood authority for any new outfall or structure in an ordinary watercourse. A connection to a highway drain needs the highway authority's consent, and highway authorities are not obliged to accept private flows.

A public sewer connection engages the right to communicate under the Water Industry Act 1991, exercised through the undertaker's application process. The undertaker can require surface water to be dealt with otherwise where its network has no capacity, and it will usually ask what you did about infiltration first. Expect a restricted discharge rate and therefore on-site attenuation, which is where the difference between a greenfield and a brownfield plot shows up. Our note on discharge rates and betterment explains how that rate is set.

Building control and planning asking different questions

The two regimes overlap and are not the same. Building control is concerned with whether the drainage works and complies with Part H. Planning is concerned with flood risk, with whether the arrangement is deliverable, and with the effect on others.

For a single dwelling the lead local flood authority is not a statutory consultee, since that role attaches to major development. So the planning authority either deals with drainage itself or imposes a condition requiring details before construction. A condition is common and perfectly workable, as long as you understand you are deferring the problem rather than solving it. If infiltration later fails and the only option is a sewer the undertaker will not accept, you have a scheme that cannot be built out.

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 to deal with. Check the position when you submit, because this has been under discussion for years.

What to put in the application

Keep it short and complete. A single plot drainage note that satisfies most authorities runs to two or three pages plus a plan, and contains: the contributing impermeable areas with a plan, the infiltration test results including the date, pit location, depth and the three readings, the design event used and its source, the soakaway calculation with the half-drain time, a plan showing the soakaway position with the distance to buildings and boundaries dimensioned, the exceedance route for an event beyond the design standard, and the maintenance arrangement.

Where the site is in Flood Zone 2 or 3, or exceeds one hectare, the drainage note sits inside a Flood Risk Assessment rather than standing alone. For anything larger than a single plot a full Drainage Strategy is the document the authority expects, and it is worth drafting the structure before the test results come back so you know which gaps the site work has to fill.

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.