Conservatory Base Calculator — Foundation & Slab Quantities

Conservatory foundations are typically 450mm deep strip footings, which is shallower than house extension foundations because conservatories are lighter structures. On clay soils or near trees, deeper foundations may be needed. Building Control will specify the required depth based on ground conditions at inspection.

A typical conservatory slab is: excavation to formation level, 150mm compacted hardcore (MOT Type 1), sand blinding (25mm minimum, 50mm is common over coarse or open hardcore to protect the membrane), 1200-gauge DPM (damp proof membrane), 100mm concrete slab in a GEN3 (C20/25) designated mix to BS 8500-1 with A142 mesh reinforcement. Edge insulation prevents cold bridging at the perimeter. This calculator uses the 25mm minimum sand blinding — order 50mm if your hardcore is coarse.

Typically 2–4 courses of brickwork above DPC level, giving a dwarf wall height of 150–300mm. The dwarf wall supports the conservatory frame and raises it above ground level. Some modern conservatories sit directly on the slab without a dwarf wall. The height is usually specified by the conservatory manufacturer.

The brick count is for a SINGLE brick leaf only, calculated at 60 bricks/m² (stretcher bond, with 75mm per course — 65mm brick plus a 10mm joint, three courses to 225mm). A single skin is only stable up to roughly 450mm high. Most conservatory dwarf walls are built as a cavity wall — an outer brick leaf and an inner blockwork leaf with a 50–75mm insulated cavity — to match the house and meet Part L. For a cavity wall, double the figure for the outer leaf and add a separate blockwork allowance (typically 10 blocks/m²) for the inner leaf.

The rectangular and Edwardian (square-front) shapes give exact plan quantities. The Victorian (3-facet bay) and P-shape figures are APPROXIMATE geometric estimates — faceted footprints vary widely with the wing depth and facet angles, so the perimeter and area are modelled, not measured. The P-shape allowance is deliberately conservative (it errs towards more material to avoid undersupply). For an exact order on a faceted plan, measure the actual footprint and use the rectangular mode with your measured dimensions.

Yes. A damp proof membrane (DPM) is essential under the concrete slab to prevent moisture rising through the base. Use minimum 1200-gauge (300mu) polythene with 150mm overlaps at joints and turn up at edges to connect with the DPC. The DPM sits on the sand blinding to prevent puncture.

If the conservatory is at ground level, under 30m², separated from the house by external-quality doors, and has an independent heating system, it is exempt from most Building Regulations. However, the base and any electrical work still need to comply. If in doubt, apply for Building Regulations approval to protect your position.