Concrete vs Timber Frame Construction for London Extensions

For the vast majority of London domestic extensions, both masonry and timber frame can achieve structurally sound, well-insulated, aesthetically pleasing results. The choice comes down to:

• Programme (speed of construction on site)
• Thermal performance strategy
• Contractor preference and expertise
• Cost (which varies by project)
• Local planning and design considerations

How it works: The external and internal walls are built with concrete blocks (dense or lightweight) or bricks, bonded with mortar. Thermal insulation is integrated into the cavity (cavity fill or partial fill) or applied internally.

Advantages:

• Extremely common in London domestic construction — almost every contractor knows how to build it
• Good airtightness when properly finished
• High thermal mass — regulates temperature swings in summer
• Long durability with minimal maintenance
• No need for specialist subcontractors for the main structure

Disadvantages:

• Slower to build than timber frame (cure times, sequential trades)
• Heavier — may have implications for foundation design
• Cavity insulation alone may not achieve the lowest possible U-values without significant thickness
• Thermal bridging at junctions requires careful detailing

Typical wall build-up for modern masonry extension:

• 102mm brick or render block outer leaf
• 100mm cavity with full-fill EPS or PIR insulation
• 100mm aerated concrete block inner leaf (Celcon, Thermalite)
• 12.5mm plasterboard on dabs (or insulated plasterboard for better performance)

This build-up achieves approximately 0.18–0.22 W/m2K U-value.

How it works: A structural frame of timber studs and rails is erected, sheathed in structural boards (OSB or similar), and then insulated between and/or outside the studs. External cladding or render is applied to a support system or directly to the sheathing.

Advantages:

• Faster to build on site — once the frame is up (days rather than weeks), other trades can follow quickly
• Lighter than masonry — may reduce foundation requirements
• Excellent thermal performance achievable with insulation between, within, and outside the frame
• Better suited to high-performance (Passivhaus-level) targets
• Can accommodate complex geometries more easily than masonry

Disadvantages:

• Lower thermal mass — can overheat in summer if not carefully designed
• Requires vapour control layers and careful airtightness detailing to prevent interstitial condensation
• Timber is vulnerable to moisture during construction — critical to get the structure weathertight quickly
• Not all London domestic contractors are experienced with timber frame; quality of detailing varies
• Fire performance requires careful specification of external cladding (relevant post-Grenfell)

Typical wall build-up for timber frame extension:

• External cladding or render on battens/sheathing
• Breather membrane
• 18mm OSB sheathing
• 140mm timber stud with mineral wool (between studs)
• Service void (additional 50mm mineral wool)
• Vapour control layer
• 12.5mm plasterboard

This build-up can achieve 0.15 W/m2K or better.

SIPs are a variant of timber frame where the structural element and insulation are combined into a factory-manufactured panel. They offer:

• Very fast construction (panels arrive pre-manufactured)
• Excellent airtightness
• High thermal performance
• Higher material cost than site-built timber frame
• Suited to garden rooms, outbuildings, and simple rectangular extensions

SIPs are less common for complex extension shapes in terraced properties but popular for garden offices and outbuildings.

Direct cost comparison is difficult because it depends heavily on the specific project, but as a general guide:

• Masonry single-storey extension (structure only, per m2): £350–£600/m2 for walls and roof structure
• Timber frame single-storey extension (structure only, per m2): £320–£550/m2 for frame, sheathing, and insulation

The cost difference is often minimal for simple rectangular extensions. Timber frame can be more economical on complex shapes. Masonry can be more economical where the contractor has established masonry bricklaying teams.

Choose masonry when:

• Your contractor is experienced in masonry and less so in timber frame
• You want high thermal mass (south-facing extension, warm climate)
• The extension is a relatively simple rectangular shape
• The project has flexible programme and speed is not critical

Choose timber frame when:

• Speed is important (faster to weather-tight stage)
• You are targeting very high thermal performance
• The structure is complex or needs to be lightweight
• You are building a garden room or outbuilding where SIPs or kit systems are available
• Your contractor has strong timber frame expertise

Planning note: The external appearance of the extension is what planners assess — not the structural system. Either system can be clad in brick, render, zinc, timber, or any other approved material. The structural choice has no planning implications.

Whether masonry or timber frame, the quality of the finished extension depends on:

1. The detailing at junctions — where the extension meets the existing house, at windows, and at the eaves and verges
2. The quality of the insulation installation (gaps, compression, and continuity)
3. The airtightness of the finished envelope
4. The quality of the flat or pitched roof membrane
5. The quality of the glazing specification

A well-detailed masonry extension outperforms a poorly detailed timber frame, and vice versa. The structural system is less important than the care and expertise with which it is designed and built.

rcbGroup builds extensions in both masonry and timber frame, selecting the approach that best suits the project, programme, and specification — and always detailing junctions and insulation to the required standard.