Hardstanding and paved area

Frequently asked questions

Why is paving a front garden a regulated act in Britain?

Because of where the water goes, not because of the paving. Replacing a permeable front garden with an impermeable surface sends rainfall that used to soak into the ground into the drainage system instead, and enough of those in a catchment changes how it floods. British permitted development rules therefore allow a hardstanding to be created without planning permission only if the surface is permeable, or if the water is directed to a permeable area within the property rather than to the highway — above a modest threshold area. That makes the DRAINAGE strategy the planning question and the construction a consequence of it, which is the reverse of how most people approach it. A block-paved drive draining to the road is the common case that needs permission and rarely has it.

What actually makes a surface permeable?

The whole construction, not the top layer, and this is where permeable paving schemes most often fail. A permeable pavement works as a system: units with enlarged joints or voids, filled with a clean single-size aggregate that stays open; a laying course of the same; a sub-base of open-graded material that both spreads the load and stores water; and a subgrade able to accept it, or an outlet if it cannot. Using ordinary jointing sand instead of the specified clean grit closes the joints within a season. Using a conventional well-graded sub-base — which is designed to compact to a dense, interlocked layer with few voids — leaves nowhere for the water to go. So a permeable surface laid on a conventional base is an impermeable pavement with a permeable top, and it behaves like one in the first heavy rain.

What thickness does it need, and what decides it?

The subgrade and the traffic, in that order, and neither is the paving. The subgrade's strength — how much the ground beneath will deform under load — sets how much construction is needed above it to spread a wheel load to something it can take, and that is why the same driveway needs far more sub-base over soft clay than over gravel. Traffic then sets how much: a domestic car is a small fraction of the load a delivery lorry applies, and a hardstanding that will take a skip lorry or a motorhome is a different pavement from one taking a hatchback. The commonest domestic failure is neither: it is omitting the edge restraint, which lets the whole field creep sideways under braking and turning until the joints open and the surface loses its interlock.