How many blocks and bricks per m² of cavity wall?

A standard cavity wall uses approximately 10 concrete blocks per m² for the inner leaf (440x215mm blocks at 10mm joints) and 60 bricks per m² for the outer leaf (215x65mm bricks in stretcher bond). Add 5–10% for waste and cuts. These figures assume standard modular sizes — check your specific block/brick dimensions.

What insulation is needed for extension walls to meet Part L?

Extension walls must achieve a U-value of 0.26 W/m²K or better under Part L 2021. For a standard cavity wall, this typically requires 75–100mm partial fill PIR board (e.g. Celotex/Kingspan) or 100mm full fill mineral wool. Timber frame walls need insulation between studs plus a continuous layer to reduce thermal bridging.

How many wall ties do I need?

Wall ties are required at minimum 2.5 per m² in the general wall area (typically 900mm horizontal x 450mm vertical spacing). Additional ties are needed around openings, at vertical movement joints and at wall starts/returns. Stainless steel ties are mandatory for all new build and extension work.

What is a cavity closer and where do I need them?

A cavity closer seals the cavity at the edges of window and door openings. It prevents cold bridging and fire spread through the cavity. You need cavity closers on both vertical sides (jambs) and the head of every opening. They are made from insulated uPVC or mineral wool enclosed in polythene.

Do extension walls need a DPC?

Yes. A damp proof course (DPC) must be installed at least 150mm above finished external ground level in all new walls. The DPC is typically a polythene sheet laid in the mortar joint. It must connect with the DPM (damp proof membrane) in the floor slab to create a continuous damp barrier.

TradeCalculator

Extension Wall Calculator — Block, Brick & Cavity Materials

Calculate wall material quantities for UK extensions including inner block leaf, cavity insulation, outer brick skin, DPC, wall ties, lintels and cavity closers.

Total external wall length

Wall height

Wall type

Insulation type

Number of windows

Number of doors

How We Calculate This

This calculator estimates material quantities for standard UK cavity wall construction based on wall dimensions and openings.

Quantity calculation

Net wall area = gross area minus window and door openings
Blocks: 10 per m² (inner leaf, 440x215mm)
Bricks: 60 per m² (outer leaf, stretcher bond)
Wall ties: based on spacing (typically 2.5 per m²)
Mortar: ~0.04 m³ per m² combined both leaves

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.