Steel Frame vs Timber Frame vs Masonry for House Extensions: Which Is Right for Your Project?

**What traditional masonry means in a London extension context**:

• For most London house extensions, 'masonry' means brick-and-block cavity wall construction:
• •Outer leaf: facing brick (typically 102.5mm standard brick) matching or complementing the existing house
• •Cavity: 100–150mm (standard 100mm, or wider for enhanced Part L thermal compliance)
• •Inner leaf: 100mm lightweight blockwork (Celcon, Thermalite, or similar)
• •Cavity insulation: Rigid foam boards (PIR — Celotex, Kingspan) partially filling the cavity, or full-fill mineral wool where moisture is managed

*The flat roof or pitched roof above is typically: timber joists (roof or floor); with flat roof — built-up felt system, GRP, or EPDM; with pitched roof — traditional rafters and tiles.

**Advantages of traditional masonry for London extensions**:

• •*Aesthetically compatible*: Facing brick is the dominant external material in London's Victorian and Edwardian housing stock. Planning authorities generally prefer brick extensions on brick houses — 'similar appearance' is one of the PD conditions, and Conservation Area consents frequently specify brick as a material requirement.
• •*Widely understood*: Every main contractor and most subcontractors in London can price, manage, and build masonry — there is no specialist knowledge or system dependency.
• •*Durable*: Brick and block construction has a design life of 100+ years; it is robust, fire-resistant (Part B compliant without specialist treatment), and acoustically efficient.
• •*Adaptable during construction*: Where site conditions change (drain position, foundation depth, neighbour constraint), masonry is relatively easy to adapt without specialist system input.
• •*Planning compliance*: The external appearance of the extension looks like the existing house — important for Conservation Area consent, PD 'similar appearance' conditions, and neighbour acceptability.

**Disadvantages of traditional masonry for London extensions**:

• •*Slower programme*: Masonry is slower to erect than frame construction — bricks are laid one course at a time; mortar must cure before each load-bearing element can be loaded above it. A standard single-storey rear extension takes 4–8 weeks to get to roof structure, depending on size.
• •*Trade sequencing constraints*: Masonry requires bricklayer, followed by blocklayer, followed by plasterer — with drying time between each. In London's trade market (where programmes are often compressed), this sequencing adds programme.
• •*Thermal performance*: Standard 100mm cavity construction with 50mm PIR board achieves approximately U-value 0.25–0.30 W/m²K (Part L compliant). To achieve higher thermal performance (U-value 0.18 or below) with masonry requires a wider cavity or additional insulation strategies that add cost and complexity.

**When structural steel frame is used in extensions**:

Structural steel frame refers to primary structural columns and beams forming the load-bearing skeleton of the extension — the steel structure supports the floor and roof loads, and the external cladding is applied separately (brick, render, rain screen, or timber). This is different from standard masonry where the brick and block wall carries its own load.

**Where steel frame is appropriate for a London extension**:

• •*Large spans*: Where the extension design requires large open spans (6m+ wide rear elevations, large roof lights, minimal internal columns) that masonry cannot span without intermediate support, steel provides the structural solution
• •*Basement or lower ground floor structures*: Where the extension is part of a basement conversion scheme with a retaining structure, steel elements (universal columns, universal beams as retaining wall soldiers) are standard
• •*Heavily glazed or open-frame designs*: Extensions where the brief is maximum glass and minimal solid wall — a structural steel post-and-beam grid allows large glass panels between columns; masonry cannot replicate this
• •*Addition of rooftop structures*: Where a new structure is added above the existing house (a mansard loft or roof extension over existing flat roof), steel frames are used because of their efficiency at carrying the loads to the existing structure at specific points

**Structural steel in standard masonry extensions**:

• It's important to distinguish full steel-frame construction from structural steel elements within a masonry extension. Most masonry extensions in London include structural steel elements — particularly:
• •*RSJ steels as lintels* over large openings (bi-fold door openings, structural openings between existing house and extension)
• •*Flat-roof beam steel* where the flat roof spans require structural steel rather than timber joists
• •*Padstone and column plates* where point loads from above are transferred to the masonry

These steel elements are standard engineering within masonry construction — not 'steel frame' construction.

**Full steel frame for an extension — cost and programme**:

• Full steel frame construction for an extension costs more than masonry in most cases — the steel fabrication and erection costs exceed the saving in brickwork. However:
• •*Programme*: A steel frame can be erected in 1–3 days once the frame arrives on site; versus 4–8 weeks for equivalent masonry construction
• •*Design freedom*: Column positions can be minimised (6–9m spans standard), maximising internal open plan
• •*Cladding options*: Any external cladding can be applied to the steel frame — brick (brick slip on steel stud liner), zinc, timber, render, composite panel

*Cost premium over masonry*: Approximately 15–30% for the structure itself; total extension cost may be similar once programming savings and cladding costs are included.

**Timber frame for London extensions — the SIPs and light gauge steel options**:

Timber frame construction for extensions covers several systems:

**SIPs — Structural Insulated Panels**:

SIPs are factory-manufactured panels comprising OSB (oriented strand board) outer facings bonded to a rigid PIR insulation core. A typical SIP panel is 142mm thick (full structural wall thickness including insulation — 15mm OSB + 112mm PIR core + 15mm OSB) or 172mm for enhanced thermal performance.

• *Advantages*:
• •U-value 0.18 W/m²K or better achieved within the panel thickness — no additional insulation needed
• •Very fast erection: a single-storey extension shell in SIPs can be erected in 1–2 days by a specialist team
• •High air tightness: the panel joints are sealed with tape and mastic, achieving air tightness of 3–5 m³/hr/m² (better than standard masonry)
• •Low construction moisture: no wet trades (concrete, mortar, plaster backing coat) until fit-out stage

• *Disadvantages*:
• •External facing must be applied separately — SIPs walls typically receive a render system, timber cladding, or brick slip; matching brick-work appearance requires brick slip systems (cost £80–£120/m² vs standard brickwork £60–£100/m²)
• •Cannot be easily modified on site — cuts and penetrations through SIPs panels require specialist repair; design must be frozen before manufacture
• •Cost: SIPs supply and erect for a single-storey extension shell: approximately £200–£350/m² of floor area for the structure (similar to masonry but faster)
• •Fire: SIPs panels meet Part B requirements but require fire-rated board (plasterboard) to the internal face

**Light gauge steel frame (LGSF)**:

Similar to SIPs in concept — factory-manufactured panels built from cold-formed steel 'C' sections, typically at 400–600mm centres, infilled with mineral wool insulation and sheathed with OSB or plasterboard internally. Achieves similar U-values to SIPs with excellent acoustic performance.

*Common in London extensions where*: The structural loading requirements are complex; a non-combustible structure is required (Part B in a multi-storey extension); or where the speed and factory quality of LGSF suits the programme.

**Practical recommendation for most London extensions**:

• For the majority of standard London rear extensions (single-storey, brick-finish, up to 5m × 5m), traditional masonry construction remains the default for good reason: it is familiar, it matches the existing building, and it is generally the most cost-effective and commercially understood option in London's trade market. Steel frame or SIPs become appropriate where:
• •The design requires large spans or glazing that masonry cannot achieve
• •The programme is critically constrained (SIPs can save 4–6 weeks on structure erection)
• •The thermal performance target is above what standard cavity masonry achieves cost-effectively (SIPs achieves U-value 0.18 with no additional insulation)
• •The cladding is not brick (render, zinc, or timber cladding is as easily applied to a frame as to masonry)