Concrete vs Timber Floor for an Extension: Which Is Right?

The ground floor of an extension can be either a concrete slab (beam and block, or solid concrete on insulation) or a timber suspended floor (joists over a ventilated void). Each has advantages — the right choice depends on the site conditions, the intended floor finish, and whether underfloor heating is planned.

Concrete Slab (Solid Ground Floor)

The standard ground floor construction for UK extensions is a concrete slab. The typical build-up from the ground up is:

• Compacted subgrade and hardcore fill
• Sand blinding layer
• DPM (damp-proof membrane)
• 100mm Rigid insulation (typically PIR board, 70–100mm)
• 100mm Concrete slab (or 65mm oversite slab with beam and block)
• Optional: underfloor heating pipes embedded in screed
• 65–75mm sand-and-cement screed or liquid anhydrite screed
• Final floor finish

Advantages of concrete slab:

• Thermal mass — the slab stores heat, beneficial with underfloor heating
• Excellent base for underfloor heating embedded in screed
• Very stable — no springiness, no squeaks
• Good for heavy floor finishes (stone, porcelain)
• Drainage can be connected directly

Disadvantages:

• Slower to construct — concrete needs to cure
• More expensive than basic timber suspended floor in some configurations
• Thermal inertia means slow response time if heating is intermittent

Timber Suspended Floor

A timber suspended floor uses structural timber joists spanning between the external walls over a ventilated void beneath. The original Victorian terrace ground floor was almost universally a suspended timber floor.

For extensions, a timber suspended floor is viable where:

• The ground conditions make deep excavation for a slab impractical
• There are existing drainage runs under the extension footprint that cannot be moved
• The extension connects to an existing suspended timber floor for level consistency

Advantages of timber suspended floor:

• Faster to build than concrete slab
• Flexible for drainage routing (pipes can run between joists)
• Warmer underfoot (perceived) though insulated slabs are equally warm in practice

Disadvantages:

• Less suitable for underfloor heating (UFH works best in thermal mass systems with concrete and screed)
• Risk of squeaks over time
• Void beneath requires ventilation (air bricks maintained and clear)
• Not ideal for heavy stone flooring

Cost Comparison

A concrete slab with insulation and screed is typically £50–£80/m² for labour and materials (excluding floor finish). A timber suspended floor with joist insulation is typically £40–£65/m² but excludes screed — the boarding itself is the floor level, so a separate screed or boarding layer is still needed.

The most cost-effective ground floor for a standard extension is a concrete slab — it is the industry standard for good reason.

RCB Design & Build

RCB Design & Build specifies and delivers the appropriate ground floor system for each project — advising clients on the implications for underfloor heating, drainage, and floor finish specification.