Extension Wall Calculator — Block, Brick & Cavity Materials

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.

Extension walls must achieve a U-value of 0.18 W/m²K or better. This is the limiting value in Approved Document L 2021 Volume 1, Table 4.2 (new thermal elements in an existing dwelling) — do not confuse it with the 0.26 W/m²K figure in Table 4.1, which applies only to new-build dwellings. To reach 0.18 W/m²K in a masonry cavity wall you typically need a wider cavity than the traditional 100mm: roughly 90–100mm of high-performance full-fill PIR/phenolic board (λ ≈ 0.021 W/mK) in a 100–150mm cavity, or an equivalent full-fill mineral wool build-up. Timber frame walls need insulation between studs plus a continuous layer to reduce thermal bridging. Always confirm the exact thickness with the insulation manufacturer's U-value calculation for your specific build-up.

Wall ties are required at a minimum of 2.5 per m² in the general wall area (typically 900mm horizontal × 450mm vertical spacing), per PD 6697:2019 / BS EN 1996-1-1 (Eurocode 6) UK National Annex. This calculator floors the density at that 2.5 per m² minimum. Additional ties are needed around openings, at unbonded edges and at vertical movement joints — one tie per 300mm of reveal height, located within 225mm of the edge — and the calculator adds these automatically. Stainless steel ties are standard for all new-build and extension work.

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.

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.

Multiply the net facing area of the new walls (gross area minus window and door openings) by 60 bricks per m² — the standard rate for the outer brick skin of a cavity wall in stretcher bond using UK-standard 215×65mm face bricks with 10mm joints. For example, a 4m × 3m rear extension with 2.4m-high walls has three new walls totalling about 24 m² gross; deducting 5 m² of openings leaves 19 m², which is 1,140 bricks — around 1,250 after a 10% waste allowance. Enter your own dimensions and openings above and the calculator does this deduction automatically, alongside the inner block leaf (10 blocks per m²), wall ties and cavity closers.

Measure the length of each new external wall and add them together — that combined figure is the total external wall length the calculator needs — along with the wall height and the width and height of the window and door openings in those walls. The calculator works from net wall area (gross wall area minus openings), and only the new masonry counts: the existing house wall the extension abuts is excluded. Measure openings accurately rather than guessing, because as well as deducting their area the calculator adds extra wall ties around each opening and three cavity closer lengths per opening. When setting out heights on site, remember the damp proof course must sit at least 150mm above finished external ground level.