Extension Floor Types in London: Concrete Slab, Beam and Block, or Suspended Timber

The ground floor structure of a London home extension is designed by the structural engineer based on the ground conditions, the spans involved, and the loading requirements. The main options are a ground-bearing concrete slab, a beam-and-block suspended concrete floor, or a suspended timber floor. Each has different cost, performance, and suitability characteristics.

A ground-bearing concrete slab is the simplest and most common floor type for single-storey rear extensions in London where ground conditions are suitable. It consists of a layer of compacted hardcore, a sand blinding, a damp-proof membrane, a layer of rigid insulation board, and a reinforced concrete slab. The slab bears directly on the ground beneath it. This is the lowest cost option and provides a strong, durable base for almost any floor finish. The limitation is that it requires the ground beneath to be suitable — it cannot span across soft spots, voids, or areas with poor bearing capacity without additional ground treatment.

A beam-and-block suspended concrete floor is used where ground conditions make a ground-bearing slab unsuitable — for example, where made ground, soft fill, or tree root influence requires the floor to span across rather than bear on the ground. It consists of pre-stressed concrete beams spanning between foundation walls, with concrete infill blocks laid between the beams. The whole assembly is then covered with a screed incorporating insulation. A beam-and-block floor provides a true suspended floor — it does not depend on the ground beneath for support. This is important in London where tree roots and variable made ground conditions are common.

A suspended timber floor is historically the ground floor type in Victorian and Edwardian London houses — timber joists spanning between sleeper walls on a sub-floor ventilated void. For new extensions, a suspended timber floor is less common than concrete options because it is more complex to detail for thermal performance and moisture management. However, for extensions where the floor level must match an existing suspended timber floor in the original house, a continued suspended timber floor may be specified to maintain the same structural format and avoid differential movement at the junction.

The choice between these options depends primarily on the structural engineer's assessment of the ground conditions. A ground investigation (trial pit to confirm soil conditions and check for made ground, tree roots, or existing drainage) before foundation and floor design is strongly recommended for London sites where ground conditions are uncertain.

All floor types must comply with current Building Regulations thermal performance requirements (U-value maximum 0.13 W/m²K for a ground floor). The insulation specification differs for each floor type — rigid PIR board beneath a slab, rigid EPS or PIR above the beam-and-block system, and mineral wool between joists for a suspended timber floor.

RCB designs and builds extension ground floors across London in coordination with experienced structural engineers. +44 7359 872594 | connect@rcbGroup.co.uk | www.rcbGroup.co.uk