Timber Frame vs Masonry for London Extensions: Which Should You Choose?

When planning a rear extension in London, one of the first technical decisions is the construction method for the walls. Masonry (brick and block) has been the default for generations of London building. Timber frame has grown significantly in popularity. This guide gives an honest comparison.

Masonry (Cavity Wall Construction)

The traditional approach for London extensions. A masonry extension uses:

• Outer leaf: Brick or render block, usually 102mm thick
• Cavity: 100mm insulation-filled space (full-fill or partial-fill)
• Inner leaf: 100mm thermalite or concrete blockwork
• Internal plasterboard: Dot-and-dab or friction-fixed on battens

The whole wall assembly is typically around 300–350mm from inside face to outside face.

Advantages of masonry:

• Familiar to all London contractors and Building Control inspectors
• High thermal mass — absorbs heat during the day, releases it at night (beneficial in spring and autumn)
• Acoustic performance — denser structure attenuates sound well
• Robustness and durability — resists impact, moisture ingress, and vermin
• Matches existing Victorian and Edwardian brick construction aesthetically
• No specialist skills required — every RCB trade team works in masonry

Disadvantages of masonry:

• Slower to build — bricklayers are the critical path
• Higher dead load — requires heavier foundations
• Less flexible for late design changes once walls are up
• Limited supply of matching brick in some London stock types

Timber Frame

A timber frame extension uses a structural timber stud frame (typically 140mm C24 studs at 400mm centres) with:

• External cladding or render on battens over breather membrane
• Insulation between and/or outside the studs
• Internal vapour barrier and plasterboard finish

The wall assembly can be significantly thinner than masonry — giving more internal floor area from the same external footprint.

Advantages of timber frame:

• Speed on site — panelised systems can be erected in days
• Excellent insulation values — easier to achieve very low U-values
• Lower dead load — may reduce foundation requirements on weaker London clay soils
• More flexible for changes during design
• Greener credentials — lower embodied carbon than masonry if sustainably sourced

Disadvantages of timber frame:

• Lower thermal mass — more susceptible to overheating in summer if not designed carefully
• Acoustic performance needs careful detailing to match masonry
• Moisture risk if vapour control is incorrectly designed or installed
• External render on timber frame has historically had higher maintenance requirements than brick
• Less familiar to some inspectors and insurers for long-term domestic use
• Does not aesthetically match brick rear walls on Victorian terraces

Which Is Better for a London Rear Extension?

For most London rear extensions to Victorian, Edwardian, or inter-war properties, RCB's recommendation is masonry for these reasons:

1. Match to existing structure: A masonry extension bonds to or reads consistently with the existing brick construction. Planning officers and conservation officers in London often prefer brick to match character — even for modern designs.

2. Thermal mass: London's climate means spring and autumn overheating is a real risk. Masonry's thermal mass moderates temperature swings naturally.

3. Local contractor base: Every bricklayer in London is trained in masonry. Timber frame requires different supply chains and skills.

4. Durability record: Brick-built London extensions from the 1980s and 1990s are still performing well. The long-term performance of timber frame in London's damp, clay-heavy environment has a shorter track record.

Timber frame makes more sense when:

• Speed is paramount
• The external finish is planned as cladding, not brick
• Insulation targets exceed what standard masonry can easily achieve
• Soil bearing capacity is a concern
• The project is in a location where material delivery logistics favour lighter components

Structural Insulated Panels (SIPs)

A third option worth mentioning: SIPs use a rigid insulation core bonded between structural boards. They offer excellent insulation and speed, but come with higher material costs and require specialist installation. SIPs extensions in London are less common than either masonry or standard timber frame, though they suit flat-roof contemporary designs well.

Cost Comparison

For a typical 4m × 4m single-storey rear extension in London, labour and materials (structure only — no foundations, roof, electrics, or fit-out):

Method	Approximate Structural Cost
Masonry (brick/block cavity)	£12,000–£20,000
Timber frame (panelised)	£10,000–£17,000
SIPs	£14,000–£22,000

The differences narrow or reverse when you factor in foundation design, external cladding, and insulation strategies. Total extension cost does not vary dramatically by structural method.

RCB Design & Build

RCB builds extensions in both masonry and timber frame, and will advise on the right approach for your property, plot, and planning context. Contact us to discuss your project.