Cost and Speed Comparison of Floor, Wall, and Roof Construction Methods for Home Extensions
With UK homeowners increasingly choosing to extend rather than move—driven by rising property prices, stamp duty costs, and space needs—understanding the construction methods for floors, walls, and roofs is critical. Choosing the right approach affects not only the budget and timescale but also the energy performance and long-term value of the extension.
This article explores and compares the most commonly used construction methods for floors, walls, and roofs in UK extensions, with a focus on cost-effectiveness and speed of installation.
1. Floor Construction Methods
A. Solid Concrete Slab
Description:
A poured concrete floor with hardcore, sand blinding, DPM, insulation, and screed.
Cost:
• £120–£160/m² (materials + labour)
Speed:
• Moderate – 2–4 days for average extension floor, plus curing time (at least 1 week before further work).
Pros:
• Durable and load-bearing.
• Good thermal mass when paired with underfloor heating.
• Common and well understood.
Cons:
• Wet process (requires drying/curing).
• Slower to install than suspended systems.
May not be suitable on clay soil with trees nearby
B. Suspended Timber Floor
Description:
Joists suspended over a void with insulation and flooring boards above.
Cost:
• £100–£130/m²
Speed:
• Fast – Typically 1–2 days once the base is in place.
Pros:
• Lightweight, flexible for uneven ground.
• Quick to assemble and adjust on site.
Cons:
• Less thermal mass than concrete.
• Prone to movement or squeaking if not properly installed.
Adequate cross ventilation is required
C. Beam and Block Floor
Description:
Precast concrete beams with infill blocks and insulation above.
Cost:
• £130–£180/m²
Speed:
• Fast to Moderate – 1–3 days depending on size and access.
Pros:
• No need for extensive ground levelling.
• Immediate working platform—no curing time.
Cons:
• Heavier, needs crane or good site access.
• Limited insulation compared to newer methods.
Adequate cross ventilation is required
2. Wall Construction Methods
A. Cavity Wall (Brick + Block)
Description:
Outer brick skin, inner blockwork, and a cavity filled with insulation.
Cost:
• £200–£300/m² (varies by insulation and finish)
Speed:
• Moderate to Slow – Bricklaying is time-intensive (can take 1–3 weeks for a typical extension).
Pros:
• Traditional and widely accepted.
• Excellent longevity and structural integrity.
Cons:
• Labour-intensive and weather-sensitive.
• Slower than modern alternatives.
B. Timber Frame
Description:
Factory-manufactured timber panels assembled on site, often with insulation included.
Cost:
• £150–£250/m²
Speed:
• Fast – Panels can be erected in 2–5 days once on site.
Pros:
• Quick assembly, great for tight deadlines.
• High thermal efficiency.
Thinner buildup yields more internal floorspace
Cons:
• Needs good moisture protection.
• Perceived as less robust by some buyers (though performance is high).
C. Structural Insulated Panels (SIPs)
Description:
Prefabricated panels with an insulating foam core sandwiched between structural boards.
Cost:
• £200–£350/m²
Speed:
• Very Fast – Main shell in place in 2–3 days.
Pros:
• Highly energy-efficient and airtight.
• Minimal site waste, ideal for Passivhaus-style builds.
Thinner buildup yields more internal floorspace
Cons:
• Higher upfront cost.
• Less flexibility for late design changes.
D. Insulated Concrete Formwork (ICF)
Description:
Hollow foam blocks stacked like LEGO®, then filled with concrete.
Cost:
• £200–£300/m²
Speed:
• Moderate – 4–6 days depending on crew experience.
Pros:
• Superb insulation and thermal mass.
• Fast shell erection with fewer trades involved.
Cons:
• Requires experience to avoid poor concrete fill.
• Unfamiliar to some UK builders and inspectors.
3. Roof Construction Methods
A. Cut Roof (Traditional Timber)
Description:
Timber rafters cut and assembled on site.
Cost:
• £80–£130/m²
Speed:
• Slow – 1–2 weeks including support structures.
Pros:
• Flexible for unusual shapes or loft spaces.
• Familiar to most UK trades.
Cons:
• Labour-heavy and weather-dependent.
• Requires skilled carpentry.
B. Trussed Roof (Pre-fabricated Trusses)
Description:
Factory-made trusses craned into place.
Cost:
• £60–£100/m²
Speed:
• Very Fast – 1–2 days installation.
Pros:
• Economical and rapid.
• Less material waste.
Cons:
• Limited attic space.
• Less flexible for complex roof shapes.
C. SIP Roof Panels
Description:
SIP panels used as both structure and insulation.
Cost:
• £120–£200/m²
Speed:
• Very Fast – Installed within 1–3 days.
Pros:
• High insulation performance, minimal cold bridging.
• Reduced roof thickness.
Cons:
• Specialist installation and higher material cost.
• Roof windows or detailing can be trickier.
Key Takeaways
• For speed: Timber frame, SIPs, and trussed roofs are fastest.
• For cost-efficiency: Trussed roofs and timber floors are generally cheapest.
• For energy performance: SIPs and ICF outperform traditional systems.
• For flexibility: Traditional cut roofs and cavity walls allow more on-site changes.
Final Thoughts
Choosing the right construction method for a UK extension hinges on balancing budget, timeline, thermal performance, and aesthetic or structural goals. For speed and sustainability, modern systems like SIPs and timber frame shine. For familiarity and flexibility, traditional brick-and-block and cut roofs remain trusted standards.
When planning an extension, it’s wise to consult both an architectural designer and a structural engineer early in the process, as integrating modern methods may influence foundations, services, and even planning permissions.
Disclaimer: This article is for general informational purposes only and does not constitute legal or professional advice. Always consult relevant professionals and local authorities before undertaking any development or change of use.