How cavity walls work on extensions
Most modern house extensions in England are built using cavity wall construction. This means two separate walls — an outer leaf (usually brick) and an inner leaf (usually block) — with a gap between them. The cavity is filled with rigid insulation to meet current Building Regulations thermal performance requirements.
The two leaves are connected by wall ties — small metal clips or wire ties embedded in the mortar at regular intervals. Wall ties are critical to structural stability and must be positioned correctly (typically every 450mm vertically, 900mm horizontally). Poor tie placement is one of the most common quality failures on extension brickwork.
Matching the existing brickwork
Matching existing brickwork is one of the most important — and most challenging — aspects of an extension. The closer the match, the more the extension feels like part of the original house rather than an add-on.
A good bricklayer will try to source:
- A brick that closely matches the colour, texture and size of the original
- A mortar mix that matches the joint colour of the original walls
- The same joint profile (e.g. bucket-handle, flush, weathered)
- The same bond pattern (usually stretcher bond on modern houses)
Even with the best match, new brickwork will look slightly different to old weathered brickwork. This typically fades over 2–5 years as the new bricks weather.
Tip: Ask your bricklayer to lay a small sample panel and let it dry before committing to the full job. Brick and mortar colour always look different wet versus dry, and in different lighting conditions.
Lintels and openings
Any opening in the brickwork — for windows, doors, or bifold doors — requires a structural lintel above it to carry the load of the wall above. Lintels must be correctly sized for the span and load. They are usually steel (galvanised to resist rust) or reinforced concrete.
The brickwork above the lintel is often the first place to crack if the lintel is undersized or incorrectly installed. Always confirm lintel specifications with your structural engineer or building inspector.
What do building inspectors check?
Building Regulations approval is required for most extensions. A building inspector will visit at various stages. For brickwork, they typically check:
- Wall tie type, number and placement
- Cavity width and insulation coverage
- Lintel size, bearing and installation
- Damp proof course (DPC) installation at threshold and above ground
- Cavity closers at window and door reveals
- Weep holes at the base of the cavity above the DPC
A good bricklayer will build to these standards as a matter of course, not just when the inspector visits.
How long does extension brickwork take?
| Extension type | Brickwork duration (approx.) |
|---|---|
| Single-storey rear (6m x 4m) | 2–3 weeks |
| Single-storey side return | 1–2 weeks |
| Double-storey rear | 3–5 weeks |
| Garage conversion (new brickwork openings only) | 3–5 days |
Brickwork is weather-dependent. Frost, rain and strong wind can all cause delays. Allow some buffer in your programme for this.
Do I need planning permission for an extension?
Many extensions fall within Permitted Development rights and do not need planning permission. The key limits for a single-storey rear extension are:
- Maximum 4 metres deep for a detached house
- Maximum 3 metres deep for a semi-detached or terraced house
- Must not exceed the original roof height
- Must not cover more than 50% of the original garden area
These limits can be extended through the Prior Approval process. Always check with your local planning authority before starting work — rules in conservation areas and for listed buildings are stricter.
Frequently asked questions
- Can the bricklayer start before foundations are poured?
- No. Brickwork must wait until the concrete foundation has been poured and has cured sufficiently — usually at least 5–7 days, though this depends on conditions and the mix used. Starting too early risks the brickwork settling unevenly as the concrete cures.
- What is a cavity closer and do I need one?
- A cavity closer is a pre-formed section that seals the cavity at window and door openings. It provides both thermal insulation and a fixing for window frames. They are required by Building Regulations on all new extensions.
- What causes cracks in extension brickwork?
- Minor hairline cracks at mortar joints are normal as a new building settles. Larger cracks, diagonal cracks (particularly at corners of windows), or cracks that widen over time can indicate foundation movement, inadequate lintels, or failed wall ties. If in doubt, have a structural engineer or surveyor assess the cracks.