A cavity wall has separate inner and outer leaves with a space between them. The assembly may include insulation and connecting components. A cavity’s presence alone does not establish the insulation level or condition of the wall.
How this term relates to your project
When an extension meets an existing elevation, establish the actual wall build-up. Old drawings may help without removing the need for investigation. Ask how insulation, moisture management and frame edges come together in the proposed detail. A new wall using matching facing bricks may still differ technically from the original construction.
Compare more than the masonry visible after completion. Identify newly built and retained layers. Where moisture or existing insulation is uncertain, assess the situation before adding material. Keep the agreed detail and relevant product information for later changes to windows or the elevation, when the internal arrangement may no longer be visible.