Sustainable drainage on a small infill plot

On a suds small infill site the council is not asking for the attenuation design a major application needs. It wants three things: where the water will go, proof that the route works, and confirmation that the new house does not make surface water worse for the neighbours. A single plot can usually answer all three on two sheets of paper and a percolation test. What fails validation is a drainage note that says water will discharge to a soakaway without saying how big, how deep, how far from the building or what the soil does with it.

What a minor site actually has to demonstrate

The lead local flood authority is a statutory consultee on major development, which starts at ten dwellings or the floorspace and site area equivalents. A single plot or a scheme of a few houses is minor development, so the lead local flood authority will often not be consulted at all. The drainage still has to satisfy the local plan policy and the case officer.

What the officer is checking is short.

1. That the surface water drainage hierarchy has been followed, with infiltration considered before a watercourse and a watercourse before a sewer.
2. That the discharge route physically exists and someone has a right to use it.
3. That the storage or infiltration provided is sized for a stated design event with an allowance for climate change.
4. That the finished levels do not push water onto adjoining land.

Approved Document H of the Building Regulations sets the same hierarchy for surface water, so a drainage note that follows it is consistent with what the building control body will want later. Getting the two to agree at the start avoids redesigning the driveway after the foundations are in.

Roof and driveway areas and the run-off they generate

Start with the impermeable area, measured rather than estimated, because everything else scales from it.

Take the roof area on plan, not the sloping area. Add the driveway, the paths, any patio and the footprint of a garage or outbuilding. A modest four-bedroom house with a two-car drive commonly reaches 150 to 200 m² of impermeable surface, and on an infill plot carved from a garden that is 150 to 200 m² of previously permeable ground.

Then compare it with the existing situation honestly. An infill plot in a garden is close to greenfield, so the council is entitled to expect the discharge rate to be held near the greenfield rate rather than simply matched to whatever the plot does now. A plot replacing a hardstanding or a demolished building is a betterment case, and saying so plainly, with both areas stated, is the fastest way through.

Design events and climate change allowances change, so state the event you used and check the Environment Agency's current peak rainfall allowances for your river basin district rather than copying a figure from an older report.

Permeable surfacing, tanks and rain gardens

Three options do almost all the work on small sites, and they combine well.

Permeable paving over a sub-base reservoir handles the driveway and doubles as storage. It is the most reliable small-site measure because it deals with the water where it falls. It needs a permeable sub-base of stated depth and porosity, geotextile in the right place and an outlet or an infiltration test to justify it.

Soakaways and infiltration trenches handle roof water. Size them from a percolation test rather than a soil description. BRE Digest 365 is the method most councils expect, and it also gives the practical constraint that a soakaway should sit a reasonable distance from the building, commonly taken as at least 5 m, to avoid affecting foundations.

Rain gardens and shallow basins handle the overflow and look like landscape rather than drainage. A rain garden taking roof water from one downpipe, with a stone-filled reservoir below and an overflow to the soakaway, is within the ability of a self-builder to construct and of a householder to maintain.

On the front drive, remember that Class F of Part 1 of Schedule 2 to the General Permitted Development Order treats a hard surface of more than 5 m² between the house and the highway as permitted development only where it is porous or where run-off is directed to a permeable area within the curtilage. So a sealed block drive to the front can need a planning application in its own right.

Rainwater harvesting is worth doing for water use, and it is usually not counted as attenuation storage unless the tank is guaranteed to be empty before the storm, which a domestic butt or tank cannot be. Treat it as a water efficiency measure and claim the storage separately, a point explored in whether rainwater harvesting counts as attenuation.

Discharge to a garden and its limits

"It will soak away into the garden" is the most common drainage statement on minor applications and the one most often rejected.

It fails for three reasons. There is no evidence the ground takes water, which a percolation test would settle in a day. There is no defined route, so in a storm the water finds the lowest point, which is often the neighbour's patio. And there is no allowance for the ground being saturated already, which is exactly when the storm matters.

A garden can absolutely be the destination. It has to be a designed destination: a soakaway or trench of stated dimensions in a stated position, with a stated infiltration rate from a test, an overflow route shown on the drawing, and levels that keep water on your land. Discharging surface water onto a neighbour's property can also give the neighbour a claim in civil law, quite apart from the planning position.

Where infiltration will not work, because the test fails or groundwater sits close to the surface, say so and move down the hierarchy. A clay site is a normal finding, not a failure of the application.

Why a tiny site still needs a discharge route

Every scheme needs a route that will still exist in twenty years and that someone has the right to use. That means checking three things before the drawings are finished.

• Whether there is a public surface water sewer, and whether the connection is a right to communicate under section 106 of the Water Industry Act 1991, which is a different section 106 from the planning obligation.
• Whether any watercourse is an ordinary watercourse, in which case works in, over or affecting it need consent from the lead local flood authority under the Land Drainage Act 1991.
• Whether the route crosses third-party land, and if so whether an easement exists in the title.

Combined sewers are the last resort and the water company may refuse or require attenuation before accepting the flow. Discovering that after permission is granted is a common and avoidable delay. The evidence needed for a one-off house is set out further in drainage evidence for a single new dwelling.

Maintenance a householder can realistically do

Design for the person who will own the house, not for a maintenance contractor. This is where small-site drainage quietly fails.

A householder will sweep a permeable drive, clear a gully and pull leaves off a grating. They will not jet a perforated pipe, lift a proprietary filter cartridge or carry out a silt survey. So prefer measures that are visible and simple, keep access hatches where a person can reach them, and avoid anything that needs a service contract.

Write the maintenance down in a short schedule and hand it over with the house. Two sides of A4 listing each item, what to do and how often is enough for a single plot, and councils increasingly ask for it as a condition. Note where the overflow goes, because the first owner who blocks it will not know it exists.

A proportionate drainage note

For a single plot, the document the council wants is short and specific. A Drainage Strategy for an infill plot should run to a few pages, not fifty, and contain the following.

The existing and proposed impermeable areas in m². The results of the percolation test with the date, location and method. The chosen measures with dimensions and storage volumes. The design event and the climate change allowance used, with the source. The discharge route with any consent needed. The finished floor level and site levels relative to the neighbours. A simple maintenance schedule. And a drawing showing all of it.

Where the site is in Flood Zone 2 or 3, or is one hectare or more, a Flood Risk Assessment is needed as well and the drainage work feeds into it. Below those thresholds, a proportionate drainage note is usually the whole requirement.

PlanScope drafts a Drainage Strategy from your site details and your own test results, with "TO BE CONFIRMED" flags wherever a figure is missing. 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, and infiltration testing and soakaway sizing should be checked by a suitably qualified engineer.

This is general guidance, not legal advice. Check the position with your local planning authority and the lead local flood authority before relying on it. Enter your postcode on PlanScope to see which reports your plot is likely to need.