Extension Slab Calculator — Ground Floor Buildup Quantities

The standard buildup is: 150mm compacted hardcore (MOT Type 1), 25mm sand blinding, 1200-gauge DPM (damp proof membrane), 100mm PIR insulation (to achieve the Part L floor U-value), 100mm C25/30 concrete slab with A142 mesh reinforcement. The total buildup is approximately 375mm from formation level to finished floor.

100mm is standard for domestic extensions. The concrete should be a minimum C25/30 mix (the current BS 8500 designation, equivalent to the older C25) with A142 mesh reinforcement. For garages or areas with heavy point loads, 150mm with A193 or A252 mesh may be needed. Your structural engineer will specify if a non-standard slab thickness is required.

Approved Document L 2021 (Table 4.2) sets the limiting U-value for a new floor in an extension to an existing dwelling at 0.18 W/m²K (tightened from the old Part L 2013 figure of 0.22). With PIR insulation (lambda 0.022 W/mK), around 100mm typically achieves this, but the exact compliant thickness depends on your floor's perimeter-to-area ratio under the BS EN ISO 13370 method — a slim, long extension needs more insulation than a square one. Check the insulation manufacturer's U-value calculator for your specific buildup and perimeter/area ratio.

The DPM (damp proof membrane) prevents ground moisture rising through the slab into the habitable space. It must be minimum 1200 gauge (300 micron) polythene, lapped 150mm at joints and turned up at edges to connect with the wall DPC. The DPM goes above the sand blinding (to prevent puncture) and below the insulation.

MOT Type 1 hardcore compacts by approximately 15–25% when properly compacted with a plate compactor or roller. A typical 20% compaction factor means you need to order 20% more hardcore than the finished volume. Hardcore should be compacted in layers of no more than 150mm for proper compaction.