The Two Main Construction Systems
Almost all single-storey rear extensions in London are built using one of two structural systems:
- Masonry cavity wall: Two leaves of masonry (brick outer, block inner) with an insulated cavity between them
- Timber frame: A structural timber frame with an external cladding and internal lining
Both systems are widely used and comply with Building Regulations when correctly detailed and insulated. The choice depends on several factors.
Masonry Cavity Wall Construction
The traditional London extension is built with brick and block cavity wall construction:
- Outer leaf: Facing brick, matching the existing house where visible, or stock brick for rear elevations
- Cavity: 100–150mm, filled with rigid insulation boards or partial-fill insulation
- Inner leaf: Thermal block (typically Toplite or similar) or standard concrete block
Advantages of Masonry
- Familiar to all London builders — low risk of subcontractor issues
- Good thermal mass: absorbs and slowly releases heat, reducing temperature swings
- Durable and low-maintenance external skin
- Easy to match the existing house brickwork
- Robust: less susceptible to minor impact damage
Disadvantages of Masonry
- Slower to build — mortar needs curing time
- Heavier — requires more substantial foundations
- Harder to achieve very high insulation levels within a thin wall
Timber Frame Construction
Timber frame uses structural softwood studwork as the primary load-bearing element. The external cladding can be brick (hung on a separate structure or on metal ties), timber, or render.
Advantages of Timber Frame
- Faster to erect — the frame can go up in a day or two, watertight quickly
- Lighter — may allow shallower foundations on some sites
- Easier to achieve high insulation levels (thick insulation between studs, minimal thermal bridging)
- Better suited to contemporary designs with non-brick cladding
- Easier integration of large spans and glazing within the structural system
Disadvantages of Timber Frame
- Requires accurate moisture management — timber must be protected during construction
- Less familiar to some London subcontractors
- Needs good airtightness detailing for best energy performance
- External brick cladding on timber frame requires careful detailing at damp-proof course level
Structural Insulated Panels (SIPs)
A third option increasingly used for smaller extensions and outbuildings is SIPs — composite panels with an insulating foam core bonded between two structural boards (usually OSB). SIPs provide structure and insulation in one element.
Advantages: Very fast, excellent airtightness possible, good thermal performance Disadvantages: Higher material cost, requires specialist installation, limited to certain structural spans
Which Is Right for a London Extension?
| Factor | Masonry | Timber Frame |
|---|---|---|
| Build speed | Slower | Faster |
| Material cost | Lower | Higher |
| Labour availability in London | High | Medium |
| Thermal performance (comparable specifications) | Similar | Similar |
| External brick finish | Direct | Via separate brick or cladding |
| Contemporary non-brick finish | Via render | Direct or via cladding |
For most standard rear extensions in London, masonry cavity wall is the default — it is well understood, competitively priced, and gives a result the client can recognise. Timber frame becomes attractive where build speed is critical, where the design calls for contemporary cladding, or where the lightweight structure is advantageous.
Building Regulations Compliance
Both systems can comply with Building Regulations Part L (Conservation of Fuel and Power). The key metric is the U-value of the wall assembly. Modern Part L requirements for extensions require a wall U-value of 0.18–0.30 W/m²K depending on the route chosen. Both masonry and timber frame can achieve this with appropriate insulation specification.
RCB Design & Build
RCB specifies the appropriate construction system for each project based on design intent, budget, programme, and site conditions — and coordinates the structural, thermal, and specification details with the architect and Building Control from the outset.