Timber Frame Extensions: When to Build in Timber vs Brick and Block, and What Each Costs

**Traditional brick and block (cavity masonry) construction**:

The vast majority of London house extensions built in the 20th century used cavity masonry construction — an outer leaf of brick (or render-on-blockwork), a 50–100mm cavity (now typically filled with mineral wool or rigid insulation), and an inner leaf of dense concrete block. The inner leaf is the structural element; the outer brick leaf is the cladding and weather face.

• *Advantages of traditional cavity masonry*:
• •*Familiar to planning officers*: In Conservation Areas and for extensions to period properties, a brick outer leaf matches the existing house fabric and is the expected solution. A timber-frame structure with an alternative external cladding (fibre cement, render) may be less contextually appropriate in some Conservation Areas
• •*Thermal mass*: Dense masonry (particularly the inner leaf of concrete block) provides thermal mass — the ability to absorb heat during the day and release it during the night, moderating temperature swings. This is a passive thermal quality that timber frame does not replicate (timber itself has low thermal mass, and the insulation between the studs has effectively none)
• •*Durability*: Brick and block construction, properly built, requires minimal maintenance for 100+ years. It is robust to impact and does not require periodic maintenance of external finishes in the way that timber-clad or rendered timber-frame structures do
• •*Subcontractor availability*: Bricklayers are available in good numbers in London and their pricing is relatively competitive; the skills are universal; any contractor can find a reliable bricklaying subcontractor
• •*Planning familiarity*: Where the planning condition or Conservation Area character specifies 'matching brick', a brick outer leaf is the straightforward solution

• *Disadvantages of traditional cavity masonry*:
• •*Slower build*: Cavity masonry is built course by course (each brick laid individually); the structure must be built up in lifts to allow the mortar to achieve sufficient strength; the curing process limits how fast masonry can proceed; a cavity masonry extension typically takes 3–6 weeks to build the structure (depending on size and complexity)
• •*Lower thermal performance for the same wall thickness*: Masonry walls require a thicker section than timber-frame walls to achieve the same U-value; a brick-cavity-block wall meeting current Part L (0.18 W/m²K) typically requires a 100mm outer brick, 100mm mineral wool full-fill cavity, and 100mm dense block inner leaf — total wall thickness 350mm+ (including finishes); a timber-frame wall can achieve the same U-value in 250mm total thickness
• •*Wet trades*: Bricklaying and mortar work are wet trades — they require temperature above 3°C and ideally above 5°C to set properly; in cold winters, masonry work may be slowed or stopped

**Timber frame (stud frame) construction**:

Timber-frame construction uses a structural frame of regularised softwood studwork (typically 89mm × 38mm or 89mm × 63mm CLS timber at 400–600mm centres), with mineral wool or rigid insulation between the studs, a structural sheathing board (11mm OSB3) on the outside of the studs, a breather membrane, a ventilated air gap, and the external cladding.

• *Advantages of timber frame*:
• •*Speed*: A timber-frame structure can be erected in 1–3 days for a typical single-storey extension (vs. 3–6 weeks for the same structure in masonry). The framework arrives pre-cut (system build) or is cut on site from standard sections; panels can be lifted into place and nailed in hours
• •*Precision and buildability*: Timber-frame structures are geometrically precise (right angles, level plates, plumb studwork) — subsequent internal works (plasterboard, joinery, services) fit cleanly; masonry walls can have build-quality variations that create problems for first and second fix
• •*Off-site prefabrication option*: Open or closed panels can be manufactured off-site (SIPs — structurally insulated panels; or pre-fabricated wall cassettes) and craned into position — this is particularly advantageous where site access is restricted and on-site working time is limited
• •*Thermal performance*: Timber-frame walls achieve excellent U-values in a thin wall section; a 140mm stud with mineral wool between studs + 50mm PIR continuous insulation on the inside achieves approximately 0.14 W/m²K — well below the Part L requirement of 0.18 W/m²K and typical London cavity masonry performance
• •*Flexibility for services*: Electrical and plumbing first fix runs easily within the stud cavities — no chasing into masonry is required; services run neatly between studs without structural consequences

• *Disadvantages of timber frame*:
• •*Lower thermal mass*: Timber and insulation have essentially no thermal mass; a timber-frame extension will be more susceptible to temperature swings — it heats up quickly and cools quickly; in an extension used primarily during the evening (kitchen-diner), this can be perceived as a cold feeling even when the heating is adequate
• •*Moisture sensitivity during construction*: Timber is sensitive to moisture — an unroofed, unclad timber-frame structure left wet for an extended period can warp and swell; construction must proceed promptly from frame erection to roof covering to external cladding to protect the frame
• •*Planning and aesthetic considerations*: Some Conservation Area contexts or planning conditions require a brick outer leaf; while a timber-frame structure with a brick slip or brick-effect cladding system is technically possible, it is more complex than a conventional brick outer leaf
• •*Acoustic performance*: Lightweight construction (timber frame) has generally lower sound-reduction performance than heavy masonry construction — relevant if noise from an adjacent road, neighbour, or activity is a concern; heavyweight masonry inner leaf (blockwork) has significantly better acoustic performance than a timber-stud frame without specialist acoustic treatment

**When timber frame is the better choice**:

*1. Speed is critical*: If the programme is constrained — the client needs to be back in the kitchen by a specific date; scaffolding costs are running; or the project is managed around an occupied house — the speed advantage of timber frame is decisive. A structural frame erected in 3 days vs. 4–5 weeks in masonry can compress the programme by 3–4 weeks, which translates directly to lower scaffolding costs, lower contractor prelim costs, and earlier completion.

*2. High thermal performance target*: For extensions aiming for Passivhaus levels of performance or significantly exceeding Part L requirements (e.g., a client adding solar panels and ASHP and wanting the extension to contribute to a near-zero-energy house), timber-frame walls with continuous insulation achieve performance levels that cavity masonry cannot without exceptionally thick wall sections.

*3. Restricted site access*: Where materials and equipment access is very difficult (a narrow terraced property where the extension is at the rear with no rear-lane access, and all materials must pass through the house), the lighter weight and smaller volume of timber-frame materials (compared with the quantity of masonry materials needed for the same structure) can simplify logistics and reduce programme.

*4. Non-standard geometry*: Curved walls, non-orthogonal plans, or complex roof junctions are easier to achieve in timber frame than in masonry — the flexibility of cut timber allows complex geometry without specialist masonry skills.

*5. SIPs (structurally insulated panels)*: For the best combination of speed, structural performance, and thermal performance, SIPs (rigid foam cores bonded between two OSB skins, manufactured to precise dimensions) offer a premium timber-frame option — typically used for high-performance or architect-designed extensions and garden studios where cost per m² is secondary to performance.

**When masonry is the better choice**:

*1. Conservation Area or planning-condition brick-match requirement*: Where planning permission is conditioned on 'matching brick' or the Conservation Area character requires a brick outer leaf, a masonry extension is the straightforward way to meet the condition. A timber-frame structure with brick slip cladding is possible but involves more coordination and detailing.

*2. Thermal mass is a design priority*: For south-facing extensions with significant glazing — kitchen-diners with large bi-fold doors or extensive rooflights — thermal mass in the structure helps moderate the significant solar gains that are likely. A dense masonry inner leaf absorbs solar heat during the day and releases it in the evening, reducing peak temperature and cooling energy.

*3. Acoustic performance is important*: If the extension is close to a busy road, a boundary with a noisy neighbour, or an existing party wall, the heavyweight masonry inner leaf provides significantly better airborne sound reduction than a standard timber-frame wall — typically 45–55dB Rw for 215mm dense blockwork vs. 35–45dB Rw for a standard 140mm timber-frame wall.

*4. Structural integration with the existing masonry house*: Many London Victorian houses have existing masonry rear walls against which the extension will abut. Tying a new masonry extension structure into the existing masonry via wall ties and joist hangers is straightforward; connecting a timber-frame structure to an existing masonry house requires careful design of the junction detail to manage differential movement (masonry and timber move differently with moisture and temperature change).

**Hybrid construction (masonry outer leaf, timber frame inner)**:

• For many London extensions, the best solution is a hybrid — a brick outer leaf (matching the existing house, meeting planning conditions) combined with a timber-frame inner leaf (speed, thermal performance, services flexibility). This approach achieves:
• •Planning compatibility: the brick outer leaf matches the existing and satisfies Conservation Area or condition requirements
• •Thermal performance: the timber-frame inner leaf with full-fill or continuous insulation achieves a significantly better U-value than a conventional full-fill cavity masonry wall
• •Speed: the timber-frame inner leaf is erected quickly; the brick outer leaf is then constructed around it (or the inner leaf can be erected first and the brick outer leaf started once the roof is on)

The detailing of the junction between the timber-frame inner leaf and the brick outer leaf is critical — the wall tie specification must accommodate differential movement, and the VCL and breather membrane must be continuous through the junction.

**Comparative costs for London (2025)**:

Direct comparison between timber frame and masonry is complicated by the fact that they are complete structural systems — the cost comparison must be done on a like-for-like thermal performance basis:

| Construction type | Typical wall element cost (supply + install, exc. external cladding finish) | |---|---| | Traditional cavity masonry (full-fill, 100mm mineral wool) | £85–£130/m² of wall | | Traditional cavity masonry (rigid PIR partial fill, higher performance) | £100–£150/m² of wall | | Timber frame (140mm stud, mineral wool, OSB, breather membrane) | £70–£110/m² of wall | | Timber frame + continuous PIR insulation layer | £90–£135/m² of wall | | SIPs panel (150mm rigid foam core, OSB skins) | £130–£180/m² of wall |

*Notes*: These costs include the structural wall element (frame, insulation, sheathing, membrane) but exclude the external cladding (brick, render, fibre cement — priced separately) and the internal finish (plasterboard, plaster — also priced separately). The timber-frame element cost advantage over masonry is often offset partially by the need for a separate external cladding sub-system; in a straight brick-clad extension, the masonry outer leaf and inner leaf together form both the structure and the cladding.

*Overall programme and prelim cost advantage of timber frame*: The 3–4 week programme saving of a timber-frame structure (vs. masonry of equivalent area) reduces scaffolding costs by £1,500–£3,000 and contractor prelim costs by £2,000–£5,000 — this can offset the marginal element cost premium of timber frame.

**What a structural engineer does for a timber-frame extension**:

• •*Structural frame design*: The engineer designs the stud and plate sections (member sizes), the connections between the frame elements, the loadbearing capacity of the plates and sills, and the robustness of the overall frame
• •*Foundation design*: As for masonry — the engineer designs the foundation for the structural loads from the timber-frame structure
• •*Structural opening design*: Where the new extension connects to the existing house via a new opening, the engineer designs the beam (steel or glulam timber) over the opening and the loadpath down to the foundations
• •*Lateral stability*: Timber-frame structures must be designed for lateral stability (resistance to wind and horizontal loads) — this is typically achieved by the structural sheathing (OSB) acting as a racking panel

**Key specification items for a London timber-frame extension**:

• •*Timber treatment*: All structural softwood should be pre-treated against decay with a boron or similar water-borne preservative (hazard class UC3a for external exposure, UC3b for consistently wet conditions); Thermowood or Accoya for any external timber elements
• •*OSB grade*: OSB3 (third generation — moisture resistant) is the standard sheathing board for structural applications; OSB2 is not adequate for external sheathing
• •*CLS timber grade*: C16 is the standard structural grade (graded per BS EN 14081); C24 is specified for longer spans or higher loads
• •*Pre-cut or system build*: Pre-cutting all structural members off-site from engineered plans (by the structural engineer or the timber-frame manufacturer) minimises on-site cutting, reduces waste, and ensures dimensional accuracy