Worked example: attenuation storage volume for a driveway

Replacing a lawn with a tarmac driveway turns a surface that soaked up most of the rain into one that sends nearly all of it somewhere else. That is why the attenuation volume driveway calculation matters even on a job as small as a front garden: if the new hard surface drains to the public sewer or to the highway, you need permission, and if it drains within your own boundary you need somewhere for the water to go.

The permitted development position comes first. Under Part 1 Class F of the General Permitted Development Order 2015, laying hard surface in the front garden of a house is permitted development only where the surface is porous, or where the run-off is directed to a permeable area within the curtilage. A 30 m² impermeable drive connected to the road drain is not permitted development, and 5 m² is.

Why the impermeable area is the number that counts

Everything in a surface water calculation starts with the contributing area. Measure the new hard surface in square metres, add any roof or path that will drain onto it, and subtract nothing, because an impermeable surface has a run-off coefficient close to 1.

A worked figure makes the scale obvious. Illustrative example: a driveway 6 m by 5 m is 30 m². In a rainfall event delivering 20 mm of depth, the water falling on that area is 30 multiplied by 0.020 cubic metres, which is 0.6 m³, or 600 litres. That is roughly three full water butts, arriving in under an hour, and it has to go somewhere other than the pavement.

The grass it replaced would have absorbed much of that, depending on soil type and how wet the ground already was. The loss of that absorption is the whole problem.

Design rainfall and the return period

Household drainage is not designed for average rain. It is designed for an event of a stated rarity, described as a return period: a 1 in 30 year event for the sizing of most small systems, often with a 1 in 100 year check plus an allowance for climate change where the design is being approved by a drainage authority.

Rainfall depth for a given return period and storm duration comes from published rainfall data for the location, not from a national average, and the depths differ substantially across England. For a householder scheme the sensible route is to use a conservative figure and check it, rather than to pretend to a precision the calculation does not have.

Duration matters as much as rarity. A short intense summer storm and a long winter event produce different volumes, and for a soakaway the critical duration is usually the one that produces the largest stored volume once outflow through the soil is subtracted.

A volume estimate for a small area

Take the same 30 m² drive and a design event depth of 40 mm, which would be in the right territory for a short-duration 1 in 30 year storm in much of England before any climate change uplift.

Illustrative calculation:

• contributing impermeable area: 30 m²
• design rainfall depth: 0.040 m
• inflow volume: 30 multiplied by 0.040, which is 1.2 m³
• climate change allowance at an assumed 40 per cent uplift: 1.2 multiplied by 1.4, which is 1.68 m³

So the storage requirement is in the order of 1.7 m³ before any credit for water soaking away during the storm. A crated soakaway with 95 per cent void ratio would need around 1.8 m³ of crate volume to hold that, which is a pit roughly 1.2 m by 1.5 m by 1.0 m. These figures are illustrative: the real numbers come from local rainfall data and a measured infiltration rate.

Soakaway sizing from an infiltration rate

A soakaway empties through the sides and base of the excavation, and the rate at which it does so is the soil infiltration rate, measured in metres per second by a falling-head test in a trial pit at the proposed depth. The standard approach for small soakaways is set out in BRE Digest 365, which uses the test result to calculate outflow over the storm and sizes the storage for the worst duration.

Infiltration rates vary by orders of magnitude. Sand and gravel drain readily. Firm clay may test so slowly that a soakaway is not viable at all, and in that case the options are a lined attenuation tank with a controlled outflow, or a discharge to a watercourse or sewer with the relevant consent.

Two site constraints bite often. A soakaway should be kept a sensible distance from building foundations, commonly at least 5 m, to avoid saturating the ground beneath them. And the base of the soakaway must sit above the winter groundwater level, because a soakaway in groundwater has nowhere to discharge.

Permeable paving as the simpler answer

For most front gardens, permeable paving is a better answer than a soakaway, and it keeps the works within permitted development.

The system is a sub-base of open-graded aggregate under a permeable block or gravel surface, with the aggregate voids providing the storage and the sub-formation providing infiltration. A 300 mm sub-base at 30 per cent void ratio gives 0.09 m³ of storage per square metre of surface, so 30 m² of permeable paving holds about 2.7 m³, comfortably above the 1.7 m³ in the example. That is why permeable construction usually needs no separate attenuation structure for a domestic drive.

The common failures are practical rather than hydraulic: silt from a gravel path washing into the surface and blinding it, a sub-base contaminated with fines during construction, or the drive laid to fall towards the pavement so that water never reaches the voids at all.

What Class F permits, and what it does not

Class F permits hard surfacing within the curtilage of a house. The restriction that matters applies to land between a wall forming the principal elevation and the highway, where the surface must either be porous or drain to a permeable area within the curtilage.

Several caveats sit around it. An Article 4 direction can remove Class F entirely, which is common in conservation areas. The rights do not apply to flats or maisonettes. If the work involves a new or widened vehicle crossing over the footway, that is a separate highway matter requiring the highway authority's approval under the Highways Act 1980, whatever the planning position. And for a listed building, works affecting the setting or the curtilage may need listed building consent.

Evidence to submit if permission is needed

Where the surfacing falls outside Class F, or an Article 4 direction applies, the application needs modest but specific drainage evidence:

1. A site plan showing the existing and proposed surfaces with areas in m², and arrows showing where water will fall and flow.
2. A construction section through the proposed surface, showing sub-base depth and material.
3. The infiltration test result, if a soakaway or permeable construction relying on infiltration is proposed.
4. A short calculation sheet showing contributing area, design event, climate change allowance and the storage provided, labelled with its assumptions.
5. Confirmation of the discharge destination and any consent needed for it.

A householder scheme does not need a full Drainage Strategy, and submitting one for a driveway invites questions you do not need. A single annotated plan with a one-page calculation does the job. For larger schemes, or where the council has flagged surface water as an issue, the checklist for discharging a drainage condition sets out what a validator is looking for.

If you are unsure which documents your project needs, the 66 report types by category index lists them with difficulty ratings. This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it, and remember that a Lawful Development Certificate is the only formal way to confirm that works are lawful. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.