Draining a New Driveway So It Stays Permitted Development

Five square metres, and which side of the wall they are on

Almost nobody builds a front drive. They replace one. The crazy paving comes out, or the tarmac that has been patched twice, or the pair of concrete wheel strips somebody poured in the seventies with grass down the middle, and thirty-odd square metres of new blocks go back in the same rectangle. Because nothing about the footprint has changed, the job feels like maintenance, and the drainage question never gets asked until a letter arrives. The wording of the rule does not care that it is a replacement. It counts the area of hard surface replaced exactly as it counts new ground covered.

What governs it in England is the Town and Country Planning (General Permitted Development) (England) Order 2015, in the class dealing with hard surfaces incidental to the enjoyment of a dwellinghouse. That class grants the permission; attached to it is a condition, and the condition is the whole of this guide. Where the surface sits on land between a wall forming the principal elevation and a highway, and the area covered or replaced exceeds five square metres, either the surface has to be made of porous materials or provision has to be made to direct run-off from it to a permeable or porous area within the curtilage. Two limbs, and satisfying either is enough. Nothing in there about colour, thickness, kerb detail or who lays it.

Measure the caught area before pricing anything, because it is rarely the whole drive. The condition bites on ground between the principal elevation and a highway, so a drive wrapping round the flank of a semi has one part inside the rule and one part outside it — and the wrap-round part can be the receiving area for the front part. Highway includes the footway and the verge, and adoption is not the test: a lane the authority does not maintain is still a highway if the public has a right of passage over it, and only the maintenance liability differs. The list of streets records what is maintainable at public expense rather than what is a highway, so a frontage onto a genuinely private drive is a question for the title plan rather than for that record. Then look for what switches the permission off entirely: an Article 4 direction, which conservation-area front gardens attract more than any other land; a condition on a newer estate removing householder rights; a flat, which is not a dwellinghouse for these purposes; and a listed building, where the curtilage works are their own conversation.

What arrives at the edge, and what has to be held

A drive that drains to a border asks a small piece of ground to deal with a large one's rainfall. The ratio is the first number to write down: forty square metres of blocks onto a four square metre gravel trench is ten to one, and every assumption downstream is being multiplied by ten. Published run-off coefficients put a well-laid impermeable surface close to nine tenths of the rain that lands on it, gravel appreciably lower but nowhere near zero, and mown grass on clay far higher than anyone expects once February has saturated it. Take the values from CIRIA C753, The SuDS Manual, and take the wet-season one.

Two sums come out of that catchment and they are habitually confused. Peak flow — litres per second at the worst moment of the storm — sizes anything the water has to pass through: the throat of a channel drain across the threshold, a gully outlet, the gap left in an edging, the pipe to the border. Volume sizes the hole. The rational method gives the first, from area, a run-off coefficient and a rainfall intensity, quickly enough to be worth doing before the excavation is set out. It does not give the second, because volume needs a storm duration, and the duration that fills a small system fullest is very often not the short intense one everybody pictures. That is why BRE Digest 365, Soakaway Design, works through a range of durations and takes the worst answer, from FEH or FSR rainfall data for the postcode.

What matters on site is that the volume sum is done at all, and done for the whole contributing area — including the front roof slope if its downpipe is being connected, which on a bay-fronted terrace can be more area than the drive itself.

Build the receiving area as a filter trench rather than a flowerbed. A metre-wide strip of gravel, four hundred or so millimetres deep over a geotextile with an open-graded fill, running the length of the frontage against the wall or the boundary, is a genuine store with a genuine infiltrating face. A planted bed with topsoil and bark over it is not: the fines wash to the low end within three winters, the surface seals, and the run-off that used to soak in starts sheeting past it towards the road. If the client wants planting, plant into the gravel with pockets of soil rather than turning the whole trench into a border.

When the surface is the answer, the tank is underneath it

Permeable blocks store nothing. Neither does resin-bound gravel, porous asphalt, or a cellular grid. Every litre a permeable drive holds sits between the stones of the sub-base beneath, and on a UK domestic job that means clean angular aggregate to BS EN 13242, typically a 4/20 mm grading, holding near three tenths of its own volume as air. Interpave's guidance for concrete block permeable pavements is the trade reference for the build-up; the structural design side now sits in the restructured BS 7533 series, where the older parts have been superseded and renumbered, so check which part your specifier is calling up rather than quoting the one you learned.

The failure this section exists to prevent is a sub-base of MOT Type 1. Type 1 is dense graded, with fines engineered into it precisely so it locks up and sheds water, and permeable blocks laid over it produce a bath: water goes through the joints, reaches the Type 1, spreads sideways and comes out at the lowest point of the drive, which on nine frontages out of ten is the entrance. The open-graded alternative in UK practice is the reduced-fines Type 3 of the Series 800 material specifications in the Specification for Highway Works, or a clean single-graded quarry aggregate ordered by designation and checked on delivery. Worth a phone call to the merchant, because a yard that mostly sells to groundworkers sends Type 1 by default.

Depth then stops being a structural question. A car on a residential frontage is a light vehicular load, satisfied by a modest sub-base; the hydraulic depth is usually the deeper of the two, set by how much has to be stored and how fast the ground will let it go. Two constraints cap it. BRE Digest 365 keeps infiltration systems clear of building foundations by a stated separation — five metres is the figure it works to — and CIRIA C753 wants unsaturated ground beneath the base of the system, conventionally a metre above the seasonal high water table. Over a sensitive aquifer, the Environment Agency's published approach to groundwater protection decides whether unlined infiltration is acceptable at all; in an inner source protection zone the answer is often no.

Half empty in a day, or the depth is wrong

Storage is only storage if it is empty when the next storm arrives. BRE Digest 365 states the performance criterion as the time to half-empty, and takes twenty-four hours as the benchmark for a domestic system; local authority drainage teams will quote you their own maximum, sometimes as a full drain-down within a day or two. Either way it is a time, not a volume, and it is the check that most often sends a proposed section back to be redrawn deeper, wider or with an outlet added.

The input it turns on is the infiltration rate of the subgrade at the level the excavation will actually reach, measured rather than assumed: a hole in the front garden and three timed fills, which this site covers properly elsewhere. What is worth carrying across is the arithmetic afterwards. The measured rate is divided by a safety factor before anything is designed to it, and under a typical Victorian front garden — made ground, brick rubble, clay beneath — the honest answer is often a rate that will not empty anything in a day.

That result is not the end of the job, it is the branch point. A raised underdrain converts the section to partial infiltration: stone below the pipe invert still soaks away and counts as storage, everything above it is detention released slowly through the pipe. A lined attenuation cell with a throttled outlet stores and releases, but the outlet has to go somewhere lawful, which brings in the water company or the highway authority. Or the surface stays impermeable, the run-off goes to a border sized for slow release, and if none of that works the application route is what is left.

The version that gets built and then fails is a permeable drive on clay with no underdrain and no overflow. It performs for two winters while the sub-base is dry and the joints are clean, so everyone concludes it works. Then a wet November arrives with the stone already full, the water has nowhere to go but back up through the joints, and it runs to the lowest point — the entrance, then the footway, then the channel. That is the outcome the condition was written to prevent, achieved with a permeable specification and a compliant invoice. Every frontage system needs a defined exceedance route, and it must not be the road.

1. Dig the trial hole to the proposed formation level, not to spade depth, and log what the arisings actually are.
2. Run the timed fills and take the slowest of them as the design rate.
3. Divide by the safety factor before the rate goes anywhere near a storage calculation.
4. Check the drawdown against the authority's stated maximum, then against the twenty-four hour half-empty benchmark.
5. If it fails, decide between a deeper section, a raised underdrain, a lined store with a throttled outlet, or the application route — before the blocks are ordered.
6. Fix the exceedance route on the drawing and make sure it discharges inside the curtilage.

The joints are the inlet, and a front garden is a hostile place for them

Everything a permeable drive does, it does through a small percentage of its own plan area. The joints and apertures between the units are the only way in, and the material filling them is a working part of the drainage system rather than a finish. On concrete block permeable paving the jointing aggregate is a small angular grading — Interpave's guidance sets out the 2/6.3 mm range for the joints and laying course alongside the coarser sub-base — and it is neither kiln-dried nor polymeric sand. Either of those converts a reservoir into an ordinary drive that weeps, and cannot be vacuumed back out.

A domestic frontage treats those joints worse than a supermarket car park does. There is usually a street tree dropping leaf litter onto forty square metres of open joint; a neighbour's building sand delivered onto the pavement and tracked across; a bag of grit spread by hand in January; and, reliably, somebody with a pressure washer in April who blasts the jointing stone out of the top thirty millimetres and then sweeps it up. Say all four out loud at handover and write them down, because the owner is the maintenance regime and nobody has told them so.

Order the stone knowing the joints take more than their nominal volume. They swallow material on the first fill, more after the first compaction pass, more again after the second, and a drive that finishes level on the day looks visibly hungry after a fortnight of traffic. Leaving a bulk bag on site is cheap; going back for a quarter of a tonne of the correct grading eight months later is not, and that is what leads to whatever is in the shed going into the joints instead.

Gravel: porous on day one, on the footway by March

Loose gravel is the cheapest way through the first limb of the condition and the one most likely to fail a different test. Nobody argues about whether it is porous. The complaint that arrives is about stone on the pavement and in the channel, and it comes from the highway inspector rather than the planning officer, which means satisfying the drainage condition perfectly is no defence at all. Depositing material on a highway is dealt with by its own provisions in the Highways Act 1980, and the practical remedy is always the same: the owner sweeps it up, repeatedly.

What keeps gravel where it was laid is angular stone, an edge that restrains it and a threshold that steps down rather than out. Rounded pea shingle rolls under a tyre; a 10 to 20 mm angular crushed grading knits and stays. The wearing layer wants enough depth to cover the base and no more, because a deep loose layer ruts and throws. Restrain the perimeter with an edging on a concrete haunch, and set the entrance metre or two in a cellular gravel grid, so the wheels turning off the crossing are turning on something that cannot travel.

Beneath it, the same rule as the blocks applies and gets broken just as often. Gravel over MOT Type 1 is a mulch on a sealed surface — the water reaches the Type 1, runs along the top of it, and takes the stone with it to the entrance. Gravel is porous only if what is underneath it is too, which means an open-graded sub-base and a subgrade that will take the water, or a filter trench along the low edge to catch what the ground will not.