Floor Build-Up and Oversite Estimator

An extension floor is built up in layers on top of the foundations: compacted hardcore, a sand blinding, a damp-proof membrane, rigid PIR insulation, and a concrete slab. This estimator turns a floor area and a few layer depths into the quantities to order for each layer, plus an all-in cost. For a typical 30m² ground-bearing floor the whole oversite build-up runs £1,500 – £3,000 in materials and labour, and the bill is dominated by the insulation and the slab, not the stone underneath.

Enter your floor area, the depth of the hardcore sub-base, the insulation thickness, and the slab thickness. The result lists the hardcore in tonnes and in bulk bags, the sand blinding, the membrane rolls, the insulation boards, and the concrete volume, so you can price the order line by line and check it against a groundworker's quote.

How the quantities are worked out

Hardcore is the bulk item. The calculator takes your area and sub-base depth as a compacted volume, then converts it to weight because merchants sell aggregate by the tonne. Compacted hardcore is close to two tonnes a cubic metre, so a 30m² floor at 150mm comes out around nine tonnes. Buying that loose at £35 – £45 is cheaper per tonne than bulk bags at £52 – £81 each, and the result shows both so you can see where the loose load overtakes bagged delivery.

Above the stone sits a 50mm sand blinding, then the membrane, which is priced as 4m by 25m rolls with an allowance for the laps and the perimeter upstands. The insulation is counted in standard boards, each covering 2.88 m², with a margin for cutting waste. The concrete is simply your area multiplied by the slab thickness to give a volume in cubic metres, which is what a ready-mix supplier needs to load a truck.

Ground-bearing or suspended

Most extension floors are a ground-bearing slab: the concrete sits directly on the compacted hardcore and passes floor loads straight down into the ground. That is the cheapest floor and what this estimator prices. The deciding factor is how much fill sits under the slab. Once that depth passes 600mm, the fill can settle and crack the slab, so a suspended beam-and-block floor is used instead. That is a different construction with its own cost, and your structural engineer specifies it where the ground is poor or the levels drop away.

The insulation thickness is not a free choice either. A new extension floor in England has to meet the 0.18 W/m²K floor limit under Part L, which in practice means 100mm of PIR for a comfortable margin on the U-value. Thinner boards heat the ground instead of the room and will be picked up before the slab is covered.

A worked example

Take a 30m² ground-bearing floor with a 150mm hardcore sub-base, 100mm of PIR insulation, and a 100mm slab. The hardcore lands near nine tonnes: as a loose tipper load at £35 – £45 that is the sensible buy, whereas the same tonnage in bulk bags at £52 – £81 each costs noticeably more per tonne and only wins on a tight-access site. The insulation divides into boards of 2.88 m² each, and the 100mm slab needs a few cubic metres of C25/30 ready-mix concrete poured over the membrane once it is lapped and taped to the wall DPC.

Add it together and a 30m² floor build-up sits in the £1,500 – £3,000 band shown above, scaling with your own area. The one line most people forget is the small stuff: the membrane jointing tape and the perimeter edge-insulation strips. They cost little, but the pour stalls without them, so buy them with the membrane and the boards.