What is the standard ground floor slab buildup for an extension?

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

How thick should the concrete slab be?

100mm is standard for domestic extensions. The concrete should be a minimum C25 mix 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 non-standard slab thickness is required.

What insulation thickness do I need for Part L floor compliance?

Part L 2021 requires extension floors to achieve a U-value of 0.22 W/m²K or better. With PIR insulation (lambda 0.022 W/mK), this typically requires 100mm thickness. Check the insulation manufacturer's U-value calculator for the exact thickness needed with your specific buildup and floor perimeter ratio.

Why is there a DPM in the slab buildup?

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.

How much hardcore compaction should I allow for?

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.

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Extension Slab Calculator — Ground Floor Buildup Quantities

Calculate extension ground floor slab material quantities: hardcore, sand blinding, DPM, PIR insulation, concrete and mesh reinforcement to Part L standards.

Slab length

Slab width

Insulation thickness

Concrete thickness

How We Calculate This

This calculator determines material quantities for a standard ground floor slab buildup based on plan dimensions.

Layer calculation

Hardcore: area x depth x compaction factor
Sand blinding: area x 25mm depth
DPM: area x 1.15 (for overlaps and turn-ups)
Insulation: area x 1.05 (for waste)
Concrete: area x thickness x overpour allowance
Mesh: area / sheet size x 1.1 (for overlap)

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.