How deep should conservatory foundations be?

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.

What is the slab buildup for a conservatory base?

A typical conservatory slab is: excavation to formation level, 150mm compacted hardcore (MOT Type 1), 25mm sand blinding, 1200-gauge DPM (damp proof membrane), 100mm concrete slab (C25 mix) with A142 mesh reinforcement. Edge insulation prevents cold bridging at the perimeter.

How many courses of dwarf wall does a conservatory need?

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.

Do conservatory bases need DPM?

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.

Does a conservatory base need Building Regulations approval?

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.

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Conservatory Base Calculator — Foundation & Slab Quantities

Calculate conservatory foundation concrete, slab thickness, DPM, hardcore, edge insulation and dwarf wall brickwork for UK conservatory bases.

Conservatory shape

Length

Width (projection)

Dwarf wall courses

How We Calculate This

This calculator estimates material quantities for a conservatory base construction based on plan dimensions and shape.

Foundation

Strip footing at 450mm wide x foundation depth around the perimeter. Concrete volume includes overpour allowance.

Slab buildup

Hardcore (150mm) + sand blinding (25mm) + DPM + concrete slab (100mm) + edge insulation around perimeter.

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.