What is a cavity wall in a house extension?

A cavity wall is two separate skins of masonry with a gap between them: a blockwork inner leaf that carries the load, a facing brick outer leaf that keeps out rain, and insulation in the cavity. Stainless steel wall ties hold the two leaves together at regular intervals.

What blocks are used for extension walls?

Lightweight aerated blocks (brands like Thermalite and Celcon) are standard for the inner leaf above DPC because they are lighter and more thermally efficient. Dense concrete blocks or engineering bricks go below DPC and in load-bearing spots your structural engineer specifies, as they are stronger and more moisture-resistant.

What U-value do extension walls need to meet?

New extension walls must achieve a U-value of 0.18 W/m2K under Part L 2021, a step up from the older 0.28 requirement. In practice this usually means a 100 to 150mm cavity with enough insulation, with the exact build-up specified by your architect and building control.

What are wall ties and why do they matter?

Wall ties are stainless steel connectors built into the mortar joints to stop the inner and outer leaves of a cavity wall separating. They must be bedded in as the courses are laid, not pushed into mortar afterwards, which is a common defect that weakens the wall.

What mortar mix is used for blockwork?

Below DPC a stronger 3:1 sand-to-cement mix is standard, while above DPC with lightweight blocks a weaker 5:1 or 6:1 mix is used. A strong mortar on a weak block causes cracking, so matching the mix to the block matters.

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Phase 4 · Structure · Task 01 of 09

Walls and Blockwork: Building Up from DPC

What happens as your extension walls go up, from DPC to plate height. Covers cavity wall construction, block types, wall ties, cavity closers, scaffolding, weather risks, and the quality checks you need to do at each payment milestone.

11 min readUpdated 2026Free with email

Infographic of a single-storey rear extension wall labelled with four milestones: DPC level and pre-plasterboard as BCO hold points, scaffold-up at 2.4m as a construction lift, and plate height as the single Walls payment milestone, with a weather-risk indicator between construction lifts

Prerequisites

Complete these first

More than a third of all residential construction defects recorded by LABC involve external walls. Not roofs. Not foundations. Walls. The part of your extension that everyone assumes is straightforward is actually where the most things go wrong, from botched DPC installations to wall ties pushed into mortar after the fact rather than set in as courses are laid.

The good news is that cavity wall construction for an extension is well-understood by any competent bricklayer. Your job isn't to lay blocks. It's to understand what's being built, check quality at each payment milestone, and know when to involve your building control officer.

Do this first

Your foundations and DPC must be complete and inspected by building control before blockwork begins. Window and door opening sizes must be finalised (these come from your architect's drawings). If you're having structural steels, they should be ordered or at least specified before walls reach lintel height, because lead times run 2-6 weeks.

What this guide covers

01What You're Actually Building
02Block Types and Mortar
03The Build Sequence: DPC to Plate Height
04Wall Ties: Small Components, Big Consequences
05Cavity Closers: Don't Let These Get Forgotten
06Weather: Your Biggest Enemy at This Stage
07What It Costs
08Quality Checks: What to Look For
09Stage Payments: When to Pay and What to Hold Back
10What Comes Next

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