Concrete vs. Timber Ground Floors in Extensions: Which Is Right for You?

A concrete ground-bearing slab is the default floor construction for most single-storey extensions in the UK. The standard construction sequence (working from ground upwards) is:

• **1. Excavation to formation level**:
• •Remove topsoil and any vegetable matter — organic material must not be left below a concrete floor as it degrades and causes the floor to settle
• •Excavation depth depends on the required insulation and slab thickness plus the formation depth below finished floor level

• **2. Sub-base**:
• •150mm compacted Type 1 granular sub-base (crushed concrete or limestone aggregate) — provides a stable, level platform for subsequent layers and distributes the slab load over the natural ground

• **3. Blinding layer**:
• •50mm sand or weak (1:12) concrete blinding — fills the voids in the sub-base surface and provides a smooth surface for the DPM without puncturing it

• **4. Damp Proof Membrane (DPM)**:
• •1200 gauge polythene sheet, laid continuously with sealed laps, to prevent ground moisture rising through the slab
• •The DPM must be continuous and lapped up the walls to connect with the wall DPC (damp proof course) to form a continuous vapour barrier

• **5. Insulation**:
• •Part L1B requires: U-value ≤ 0.22 W/m²K for a new floor in an extension
• •To achieve 0.22 W/m²K, typical insulation thicknesses:
• – PIR (rigid foam, e.g., Kingspan TP10 / Celotex GA4000): 90–100mm to achieve 0.22 W/m²K
• – EPS (expanded polystyrene, e.g., Jablite): 140–150mm to achieve 0.22 W/m²K
• – PIR is thermally superior (lower lambda value) so requires less thickness
• •The insulation must be rated for below-slab installation (compressive strength suitable to resist concrete load and point loads above)

• **6. Concrete slab**:
• •Typically 100mm structural C25 concrete reinforced with A193 mesh
• •For a 5m × 4m extension, the slab typically requires approximately 2m³ of concrete — a ready-mix delivery (transit mixer) is the most reliable option; site-mixed concrete is rarely adequate quality for a structural slab

• **7. Finished floor level**:
• •The total build-up (sub-base 150 + blinding 50 + DPM 1 + insulation 100 + slab 100 = 401mm minimum) must be accommodated below the finished floor level of the extension
• •The finished floor level of the extension should ideally match the finished floor level of the existing house — which means excavating to the appropriate depth to accommodate the full build-up below
• •Where the existing house has a high finished floor level relative to external ground level, the build-up must still be accommodated — which either means raising the extension floor or raising the garden level

• **Advantages of concrete ground-bearing slab**:
• •Compatible with underfloor heating (UFH pipes embedded directly in the screed or laid on top of the structural slab under a screed)
• •No void below — no risk of sub-floor ventilation issues, rodent access, or pipe condensation in the void
• •Good thermal mass when left uninsulated — but for extensions, insulation above the structural slab is standard to comply with Part L
• •Simple maintenance: unlike timber, concrete does not rot, decay, or require structural inspection over time

A suspended timber ground floor consists of timber joists spanning between masonry walls (or new blockwork sleeper walls) with the floor surface boarded above and a void below that is ventilated to prevent moisture accumulation.

In the context of a new house extension, suspended timber is used in specific circumstances:

• **1. Where the existing house has a suspended timber ground floor**:
• •In many Victorian and Edwardian properties, the ground floor is suspended timber — the floor sits on joists supported by brick sleeper walls, with a ventilated void below
• •If the extension connects directly to the existing ground floor and the levels must match, it may be simpler to continue the suspended timber construction rather than create a level transition between a new concrete slab and the existing timber floor
• •Requires adequate ventilation of the void — the void of the new extension must connect to the existing ventilation network, or new airbricks must be provided

• **2. Where beam-and-block construction is specified**:
• •Beam-and-block (concrete T-beams spanning between walls with concrete infill blocks) is a variant of suspended floor that avoids the ventilation issues of timber but provides a similar structural function — used where ground conditions are poor (made ground, high water table, tree roots) and a ground-bearing slab is not appropriate
• •More expensive than a ground-bearing slab but can span poor ground conditions that would require piling or significant sub-base treatment for a concrete slab

• **3. Tree proximity (root zones)**:
• •Where the extension is within the influence zone of a large tree on London Clay, a ground-bearing concrete slab is at risk of heave or subsidence as the clay moves with moisture content changes
• •In this case, beam-and-block (or piled foundation with suspended slab) allows the floor to span over the problem ground without bearing on it — the floor structure is supported on the foundation walls (which are themselves piled to stable strata) rather than on the clay directly

• **Disadvantages of suspended timber in new extensions**:
• •UFH compatibility: warm water UFH is technically possible with suspended timber (specialist systems exist) but requires specific joist configuration and is more complex and expensive than UFH in a concrete slab
• •Thermal performance: achieving Part L U-value of 0.22 W/m²K with suspended timber requires 125–150mm of mineral wool between joists plus a continuous rigid insulation layer — the total insulation thickness is greater than with PIR in a concrete build-up
• •Void maintenance: the void below a suspended timber floor is potentially at risk from moisture, rodents, and service pipe failures — requires damp-proof ground cover and maintained ventilation
• •No load-bearing capability for heavy items: suspended timber floors have a lower load capacity than concrete slabs — less suitable for heavy kitchen islands, stone flooring, or large-format tiles (though most domestic loads are within typical timber floor capacity)

• **Choose concrete ground-bearing slab if**:
• •The extension is on stable, normal ground without significant tree proximity issues
• •UFH is planned or may be planned in the future
• •The finished floor level allows 400–500mm of construction depth below it
• •Stone, large-format tile, or polished concrete finishes are planned
• •You want the lowest maintenance floor option

• **Consider suspended floor (beam-and-block or timber) if**:
• •The extension is within the influence zone of significant trees on London Clay
• •The existing house has a suspended timber ground floor and matching the level is a priority
• •Ground conditions are poor and a full concrete sub-base to normal depth is not practical
• •The structural engineer specifically recommends it after ground investigation

**Key practical point: finished floor levels**: The most common coordination problem in extension ground floors is the relationship between the finished floor level of the extension and the finished floor level of the existing house. A concrete slab with 100mm insulation requires 400–500mm of build-up depth below the finished floor. If the existing house floor is 150–200mm above external ground level (common in a Victorian property with suspended timber), the extension floor build-up must fit within a tighter vertical envelope — or the garden must be lowered. Resolve this coordination at structural design stage, not during construction.

• **Cost comparison (London 2025)**:
• •Concrete ground-bearing slab (100mm insulation, reinforced slab): approximately £80–£120/m² supply and install including excavation and sub-base
• •Beam-and-block (spanning over poor ground): approximately £120–£180/m² supply and install
• •Suspended timber (new construction, including sleeper walls): approximately £90–£130/m² supply and install

For a 20m² extension, the cost difference between concrete slab and beam-and-block is typically £800–£1,200 — a meaningful but not decisive factor in the choice, which should be based on ground conditions and engineering recommendation rather than purely on cost.