A proportionate drainage strategy for one dwelling

For a single dwelling, a drainage strategy is a two-page document and a soakage test, not a modelled catchment study. The lead local flood authority is a statutory consultee on major development, which a single house is not, so the person reading your submission is the case officer or the council's own drainage engineer, and what they need is evidence that rainwater will soak away or discharge somewhere lawful, and that foul drainage has a destination. Get those two answers right and the condition writes itself.

Consultants quote four figures for this work because the brief is never defined. Define it.

What a council actually needs for a single plot

The validation requirement for a minor application is usually a surface water drainage statement, and the local validation list will say so. Read it before commissioning anything, because the list sets the scope.

What satisfies it, in almost every case:

• The site area and the existing and proposed impermeable area, in square metres.
• The proposed method of surface water disposal, chosen in the order national guidance sets out.
• Evidence supporting that choice, which for infiltration means a soakage test result.
• A drainage layout drawing showing the roof and hardstanding areas, the pipe runs, the soakaway or attenuation feature and its position relative to buildings and boundaries.
• The foul drainage destination, with the sewer undertaker's position where a connection is proposed.
• A note on exceedance: where the water goes when the design event is exceeded.

That is it. If a quotation includes hydraulic modelling of the downstream network for a single house, ask what the council asked for.

The hierarchy, and the simple calculation

Surface water disposal follows an order of preference that both Building Regulations and planning guidance use. Infiltration to the ground first. If that is not feasible, discharge to a watercourse. If that is not feasible, discharge to a surface water sewer. A combined sewer is the last resort and water companies increasingly refuse it.

Work down the list and record why each option was rejected. A statement that says "infiltration not feasible, connecting to surface water sewer" with no test result behind it will come back.

The arithmetic for a single plot is straightforward. Measure the roof plan area and the hardstanding, add them, and apply the design rainfall for the relevant return period with the current climate change allowance. For a soakaway the volume required follows from the inflow and the measured infiltration rate, using the standard soakaway design method. For attenuation with a restricted discharge, the storage is the difference between inflow and the permitted outflow over the critical duration.

Which rainfall data you use matters and is a live question, covered in which rainfall dataset to cite in a drainage calculation. State the source and the return period in the strategy so the reviewer can check it.

Infiltration first, and the test that supports it

A soakage test costs little relative to what it settles, and it is the single most useful thing you can do before designing.

Dig the trial pit at the location and depth of the proposed soakaway, not in the most convenient corner of the site. Fill and empty it three times, record the water level against time, and calculate the infiltration rate from the recorded fall. The resulting rate drives the soakaway size, and a poor rate is as useful a result as a good one because it tells you to design something else.

Clay sites often fail the test, and that is a result, not a problem. It moves you to the next step in the hierarchy with evidence rather than assertion. On sloping ground the runoff question is different again and is dealt with in managing runoff on a steeply sloping site.

Water butts, borders and permeable surfacing

For one house, the sustainable drainage components that work are small and inexpensive, and councils like seeing them because they show the hierarchy was applied rather than ignored.

Water butts on each downpipe give a small storage volume and, more usefully, reduce the frequency of discharge. They are not attenuation in a calculable sense, so do not claim storage credit for them unless the local guidance allows it.

Permeable paving on a drive, laid over a sub-base that acts as storage, removes the drive from the impermeable area calculation entirely where the ground accepts infiltration. Front garden paving has its own permitted development constraints, and the rules on surfacing are stricter than most homeowners expect.

Rain gardens and planted borders fed from a downpipe handle roof water from a single elevation and look like landscaping rather than drainage, which helps on a sensitive site. Keep them away from the building line by the same margin as a soakaway.

Soakaway crates, filled trenches and filled borders are all variants of the same idea, sized by the same method. Choose the one that fits the plot.

Foul drainage where there is no mains sewer

This is where single-dwelling schemes most often stall, because the answer takes longer to assemble than people expect.

Where a public foul sewer is available, apply for the connection and include the undertaker's correspondence. Where it is not, the options are a septic tank discharging to a drainage field, or a package treatment plant discharging to a drainage field or to a watercourse. Discharges to ground from a septic tank or treatment plant are regulated by the Environment Agency, and small discharges can operate under general binding rules where the conditions are met, or need an environmental permit where they are not. Discharges to a surface watercourse from a septic tank are not permitted, so a treatment plant is required.

A drainage field needs its own percolation test, which is a different test from a soakage test and uses a different method. It also needs enough land at the right separation distances from buildings, boundaries, watercourses and any water supply. On a small plot that is often the binding constraint, and it should be checked before the house is positioned.

A Foul Sewage Assessment is the usual vehicle where the position is anything other than a straightforward mains connection, and the authority will condition the details either way.

Exceedance, overland flow and the finished levels

Every drainage system is exceeded eventually. For one house the question is simply where the water goes when it is, and the answer is set by levels rather than pipes.

Set the finished floor level above the surrounding ground, typically by 150 mm, and fall the external surfaces away from the building. Make sure the overland flow route from the drive and the garden leads to the highway or to open ground rather than to the front door or to the neighbour's garden. Discharging water onto a neighbour's land creates a liability regardless of planning permission.

Show the levels and the arrows on the drainage layout. Two arrows and three spot levels answer the exceedance question for a single plot, and their absence is a common reason for a drainage condition that takes three months to discharge.

One thing to keep an eye on. The statutory sustainable drainage approval regime in the Flood and Water Management Act 2010 has been legislated for but not brought into force for England, and commencement has been announced more than once. Check the current position before assuming the planning route is the only route, because when it is brought into force it will add a separate approval step for most development.

A Drainage Strategy drafted to this scope gives the council what it asked for without the fee that comes with a major-scheme methodology. Enter the postcode on PlanScope to see the flood and drainage constraints recorded against the plot.

This is general guidance, not legal advice. For specialist technical reports, qualified professional input may be required. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.