How much does hardcore cost per square metre?

Hardcore is the unglamorous but critical layer that spreads load and stops a slab or driveway sinking. It is priced per square metre or per tonne, and the depth changes everything. The sections below explain the rate, what depth different jobs need, and what the figure covers.

Hardcore costs by depth and area

Because hardcore is laid to a depth, the per-square-metre cost is meaningless without knowing how thick the layer is. A path needs less than a driveway; a driveway carrying cars needs less than a slab carrying a building. The table shows indicative laid-and-compacted rates at common depths, plus the material price by tonne for reference. As a rough guide, one tonne of MOT Type 1 covers roughly 10m² at 50mm or about 5m² at 100mm once compacted.

Indicative figures for guidance only. Depth and ground conditions drive the rate.

What the hardcore rate includes

A laid-and-compacted hardcore rate covers three things: the material (usually MOT Type 1, a graded crushed stone that locks together when compacted), spreading it to an even level, and compacting it in layers with a vibrating plate or roller so it forms a solid, stable base. Compaction in layers matters — a thick layer dumped and compacted in one go does not consolidate properly and will settle later, so a good crew lays it in 75–100mm lifts.

What the rate usually excludes is the excavation to create the void the hardcore fills, and the disposal of that excavated spoil, both of which are separate groundworks costs. It also typically excludes a geotextile membrane, which is laid between soft ground and the hardcore to stop the stone punching down into the soil and the soil pumping up into the stone — essential on clay or made-up ground but not always needed on firm soil. A blinding layer of sand or fine stone over the hardcore, to protect a damp-proof membrane laid above it, may also be separate. As with any groundworks line, checking whether the rate is material-only, laid, or laid-and-compacted-with-membrane is what makes two quotes comparable.

Estimating how much hardcore you need

Working out the quantity of hardcore is straightforward once you know the area and depth, and it helps when checking a quote. Multiply the area in square metres by the compacted depth in metres to get the volume in cubic metres, then convert to tonnes — MOT Type 1 weighs roughly 2 tonnes per cubic metre once compacted. So a 40m² driveway with a 150mm (0.15m) sub-base needs about 6 cubic metres, or around 12 tonnes of material, before allowing for compaction.

Two adjustments matter in practice. First, hardcore compacts down as it is rolled, so you order more loose material than the finished compacted volume — a common allowance is around 25–30% extra, meaning that 12-tonne finished requirement might need 15 tonnes delivered. Second, on soft ground some of the bottom layer keys into the soil and is effectively lost, so a little more is needed, and a geotextile membrane is worth its cost in stopping that loss. Knowing the rough quantity lets you sense-check a per-square-metre quote: if the figures imply far less material than the area and depth require, the sub-base may be under-specified. Hardcore is one of the few groundworks items a homeowner can estimate themselves, and doing so is a useful guard against a base that is too thin to last, since the temptation to save money by reducing depth is exactly what causes a driveway or slab to fail early.

Getting the depth and ground right

The single most important decision with hardcore is depth, and it should be set by the load and the ground, not guessed. A sub-base under a slab carrying a building, or a driveway carrying cars, needs to be thick enough to spread that load onto the soil below without the surface cracking or sinking. Too thin and the slab or driveway fails early; too thick and you waste material, generate more excavation, and pay for spoil disposal you did not need. Common depths run from around 100mm under a light slab to 200mm or more under a driveway or on soft ground.

Ground conditions change the requirement. On firm, well-draining soil a standard depth works; on soft clay, made-up or wet ground, the hardcore needs to be thicker, and a geotextile membrane below it becomes important to keep the stone and soil separate. Without that membrane, on soft ground the stone migrates down and the soil works up, and the base loses its strength over a few seasons. Compaction in layers is equally critical — properly consolidated hardcore is what gives the base its load-spreading strength, and poor compaction is a common hidden cause of sinking. Because depth, membrane and compaction all affect both cost and performance, it is worth specifying the sub-base properly for the job rather than treating hardcore as a commodity priced only by the tonne. The lowest-cost sub-base is rarely the one that lasts.

For a homeowner checking a quote, the useful sanity check is to tie the per-square-metre figure back to a depth and a ground condition. A driveway sub-base priced at the lower end of the range almost certainly assumes a thinner layer or firm ground, which may be fine — or may be a base that is too shallow for the cars it will carry. Ask the contractor what compacted depth the rate is based on, whether a geotextile membrane is included, and whether the price covers laying in compacted layers rather than a single dumped lift. Those three answers tell you far more about whether a sub-base will last than the headline rate does, because hardcore is one of the few groundworks items where the performance is decided entirely by depth, separation and compaction rather than by the material itself.

Frequently asked questions

Sources & further reading

• Checkatrade — hardcore and sub-base cost guide
• Planning Portal — Building Regulations
• Pavingexpert — sub-bases and MOT Type 1

Figures on this page are typical UK ranges drawn from published sources and depend on your specific site. They are guidance, not a quotation.