Permeable Paving Calculator — Sub-base Depth & Storage

For a domestic concrete block permeable driveway, the minimum STRUCTURAL sub-base thickness is 250mm of 4/20mm coarse graded aggregate, regardless of how little storage your soil needs (Interpave/Brett 'Concrete Block Permeable Paving in Domestic Applications', per BS 7533-103:2026, which superseded the withdrawn BS 7533-13:2009 in March 2026). Increase to 350mm for driveways carrying vans or heavy 4×4s. On top of the structural minimum, the sub-base also acts as a temporary storage reservoir: where soil infiltration is poor (e.g. clay), the hydraulic depth needed (storage volume ÷ void ratio, void typically 30%) can exceed 250mm, and the deeper of the two figures governs. Lay the sub-base in maximum 150mm compacted layers.

The void ratio for clean angular aggregate (Type 3 sub-base or 4/20mm single-size stone) is typically 30%. This means 30% of the sub-base volume is empty space that can store water. Do not use standard Type 1 sub-base — it compacts with very few voids and won't provide storage.

Yes, permeable paving is a recognised SuDS (Sustainable Drainage System) technique and supports good surface-water management. Under permitted development rules in England, driveways using permeable surfaces or draining to a permeable area don't need planning permission. Note that mandatory SuDS under Schedule 3 of the Flood and Water Management Act 2010 has not been commenced in England — in June 2025 the Government decided to pursue a planning-policy and national-standards approach instead, so SuDS requirements are currently driven by national planning policy (the NPPF) and Defra's National Standards rather than Schedule 3. For non-permeable surfaces, you may need a soakaway to handle surface water.

Yes, a geotextile membrane is essential. Use a non-woven geotextile on the base and sides of the excavation to prevent fine soil particles migrating into the aggregate and clogging the voids. Do NOT use impermeable membrane (polythene) — the system needs water to infiltrate into the ground below.

Permeable block paving typically costs £40-60/m² for materials, plus £20-35/m² for sub-base aggregate. Installation labour adds £25-40/m². A typical 50m² driveway might cost £4,000-7,000 installed. While more expensive than standard paving, you avoid the cost of separate drainage and may not need planning permission.