A self build plot where drainage decided the layout

On this plot, drainage decided site layout before anything else did. The house moved four metres, the driveway changed shape, and the rear garden lost a third of its usable area, all because three trial pits in February came back with a permeability figure too low for a soakaway to work. The plot and the sequence below are illustrative, drawn from the pattern that recurs on clay infill sites, and the lesson is the inexpensive one: test before you draw.

The site is a 0.12 hectare garden plot with an existing vehicle access, a road at the front with a combined sewer under it, and no watercourse within any practical distance.

The plot and the first layout

The first layout was the obvious one. House set back to match the building line, parking to one side of a straight drive, garden to the rear, and a soakaway in the corner of the garden as far from the foundations as the plot allowed.

The design team had assumed infiltration would work because the neighbouring house had a soakaway. That assumption is the one that costs money.

Infiltration testing and what it showed

Testing followed the BRE Digest 365 method: trial pits at the proposed soakaway location, filled and allowed to drain, with the fall in water level timed, repeated three times at each pit.

The results were consistent and bad. The pits took a long time to empty on the first fill and considerably longer on the second and third, which is the signature of a low permeability soil rather than a one-off obstruction. The calculated infiltration rate was an order of magnitude below what the required storage volume needed to discharge within the usual design period.

Two other findings came out of the same exercise. Groundwater was standing at around 1.4 m below ground level in the wettest pit, which would have left too little unsaturated depth beneath a soakaway base even if the permeability had been adequate. And the made ground at the front of the plot was deeper than expected, which mattered later for the foundation design.

The neighbour's soakaway, it turned out, discharged into a gravel lens that did not extend across this plot. Our note on when BRE 365 infiltration testing is worth doing sets out how to scope this properly and when a desk study is enough.

Why the discharge hierarchy forced a rethink

National policy and the technical standards for sustainable drainage set a hierarchy for surface water: infiltrate to the ground where you can, discharge to a surface water body where you cannot, then to a surface water sewer, and to a combined sewer only as a last resort.

With infiltration ruled out by testing and no watercourse available, the plot fell to the sewer. There was no separate surface water sewer in the road, so the only option was the combined sewer, which is the bottom of the hierarchy and the one water companies resist.

The water company's position was straightforward. A connection would be permitted, at a restricted discharge rate, with on-site attenuation storing the difference between the rate of rainfall and the rate of discharge for the design event. In practice that turned a soakaway in the garden into a storage tank somewhere the levels worked.

That is the point at which drainage stops being a discipline that follows the layout and starts being one that sets it.

Attenuation storage and where it went

The storage had to satisfy two constraints at once: enough volume for the design event including the climate change allowance, and a level arrangement that lets water leave by gravity into a sewer at a fixed invert.

The garden corner failed on levels. It sat lower than the road and a gravity outfall from a tank there would have needed a pumped system, which adds a mechanical failure point, a maintenance obligation and an electrical supply, and which the water company and the authority both prefer to avoid on a single dwelling.

The front of the plot worked. A cellular storage tank under the driveway, with permeable paving above it acting as the first stage of treatment, a flow control chamber on the outlet and a gravity connection to the sewer, met both constraints. Roof water went into the same system through a filter, and the driveway surface was designed to drain into the tank rather than to the road, which also answered the Class F condition for front hardstanding.

The consequence for the layout was the four metre shift. The tank needed a footprint clear of foundations, with cover depth for vehicle loading and access for maintenance, and the house had to move back to create it.

The last line is worth dwelling on. Rainwater harvesting is a good thing and it is not attenuation, because the design event arrives when the butt is already full.

The lead local flood authority response

For a single dwelling the lead local flood authority is not a statutory consultee in the way it is on major development, and authorities vary in whether they consult it at all. Here the case officer did, informally, and the response shaped the condition.

The comments asked for three things: the infiltration test results in full rather than a summary conclusion, confirmation of the discharge rate agreed with the water company, and a maintenance schedule for the flow control and the permeable surface. All three were reasonable, and the first one is the reason to keep the raw test data rather than only the calculation.

The condition that followed required the drainage details to be implemented as approved and the maintenance arrangements to be in place before occupation. That is lighter than a pre-commencement condition, and it was lighter because the submitted Drainage Strategy had already answered the questions.

Foul drainage was separate and simpler, with a connection to the same combined sewer under the right to connect, and the calculation set out in a short Foul Sewage Assessment.

The approved layout and what changed

Against the first sketch: the house moved back four metres, the drive became wider and shallower to accommodate the tank footprint, the rear garden gained depth but lost the corner that had been earmarked for a garden room, one ground floor window position changed, and the finished floor level rose by 150 mm to keep the gravity fall to the outlet.

Nothing about the house itself changed in area or appearance. The site plan changed considerably.

The programme cost was about seven weeks, most of it redesign and re-coordination rather than the testing. The financial cost was the cellular tank and its flow control against a soakaway, plus the additional design fees, offset slightly by not needing a pump.

What earlier testing would have saved

Three things, in order of value.

The seven weeks. Testing at the feasibility stage, before the architect drew anything, would have produced the same result at a point when the layout was a sketch.

The garden room. Knowing the storage had to sit at the front would have let the rear garden be planned around it from the start, and the garden room could have been included in the application rather than left as a future application.

And the argument about the neighbour's soakaway, which took two meetings to put to rest.

For a site with any flood risk in addition to the drainage question, a Flood Risk Assessment covers a different set of tests and is triggered by flood zone or site area rather than by drainage difficulty. One point of genuine uncertainty for self-builders: the sustainable drainage approval provisions in Schedule 3 to the Flood and Water Management Act 2010 have not been brought into force in England, and if they are, the approval route for drainage on small sites changes. Check the current position before relying on the existing arrangements.

Enter your postcode on PlanScope to see which reports your plot is likely to need.

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.