A worked example
A single-storey extension with walls of 6 m, 4 m and 4 m, all 2.5 m high, in a cavity wall with facing brick outside and block inside: 35 m² of wall. Take off two windows of 1.2 × 1.2 m and a pair of patio doors 2.4 m wide and 2.1 m high, and 27.1 m² of wall is left. With 5% for cuts, that’s about 1,690 bricks and 282 blocks, a box of wall ties, cement and sand, two 1.5 m lintels for the windows and a 2.7 m lintel over the doors. Put your own brick price in and the cost comes with it.
How it works
Each wall is its length times its height. The openings come off, and what’s left is the area you lay. A half-brick skin takes 59.3 bricks per m² (215 × 65 mm bricks with 10 mm joints). A 100 mm block skin takes 9.9 blocks per m² (440 × 215 mm). A cavity wall gets 2.5 ties per m². Mortar is worked out from what’s laid, because brickwork takes about three times the mortar of blockwork. Each opening gets a lintel at least 150 mm longer than the opening at each end, rounded up to the next stock length. Over 4.5 m, you need a beam designed by an engineer. These are the same figures Bricken’s quoting uses for a cavity wall. Prices are typical UK merchant prices before VAT, checked in September 2026: put your own in for bricks and blocks.
It’s a materials estimate. It doesn’t cover the foundations, insulation, cavity closers, DPC, scaffold or labour, which Bricken’s full quote does.
Questions builders ask
How many bricks per m²?
About 60 standard bricks per square metre of half-brick wall with 10 mm joints. A one-brick (double skin) wall takes about 120. Add 5 to 10% for cuts and breakages.
How many blocks per m²?
About 10 standard 440 × 215 mm blocks per square metre. A cavity wall with a block inner skin takes about 10 blocks for every square metre of wall, plus waste.
How many wall ties per m²?
Commonly about 2.5 per square metre across the cavity, with extra ties around openings. Follow the tie spacing your designer or Building Control asks for.
What size lintel do I need?
At least the clear width of the opening plus 150 mm bearing at each end, so a 1.2 m window takes a 1.5 m lintel. The load decides the type and strength, so check the manufacturer’s tables, and for wide openings or heavy loads get an engineer.
How much waste should I allow for bricks?
5% is typical on a straightforward wall. Allow nearer 10% with lots of cuts, corners and openings, or with soft or hand-made bricks.
Written by Tomasz Kurpanik, a builder and the founder of Bricken.