Concrete Floors for London Extensions: Slab Types, Insulation, and What They Cost

**The two main floor construction types for single-storey extensions**:

The choice between a solid concrete ground-bearing slab and a beam and block suspended floor is one of the most frequently misunderstood aspects of extension design. The decision depends on ground conditions, the presence of trees, proximity to the boundary, the intended ground-floor level, and whether underfloor heating is planned.

**Solid concrete ground-bearing slab**:

The most common floor type for London single-storey rear extensions. A typical build-up (from bottom to top):

1. *Sub-base*: 150mm compacted Type 1 granular sub-base (crushed stone) on undisturbed natural ground or compacted imported fill 2. *Radon membrane* (if required in radon-affected areas): 1200-gauge polythene sheet, lapped and taped at joints, to prevent radon gas ingress from sub-base 3. *Blinding layer*: 50mm lean-mix concrete (1:20 mix) laid on sub-base to provide a clean, level surface for the DPM 4. *DPM*: 1200-gauge polythene sheet damp-proof membrane, lapped at joints and at the edge of the slab (turned up to connect with the cavity tray DPC in the external walls) 5. *PIR insulation*: 100mm–150mm rigid polyisocyanurate (PIR) insulation boards (typically Kingspan Kooltherm K103 or Celotex GA4000) — below the slab for a 'warm floor' (insulation below slab = no thermal bridging at slab edges; slab above insulation = thermal mass effect for UFH) 6. *Concrete slab*: 100mm–125mm structural concrete, typically C20 (house extension, no significant loading), reinforced with A142 steel mesh fabric 7. *Floor finish*: Cement-sand screed (65mm bonded or 75mm unbonded) for UFH pipes; or floor tiles/engineered timber direct to structural slab

• *Advantages of solid ground-bearing slab*:
• •Structurally simple and well-understood
• •Compatible with underfloor heating (UFH pipes embedded in a sand-cement or liquid screed over the structural slab)
• •Thermal mass — useful with UFH (the slab/screed stores heat and releases it slowly, smoothing temperature fluctuations)
• •Lower cost than beam and block for typical extension sizes

• *Disadvantages*:
• •Not suitable where made ground requires significant excavation and replacement fill
• •Not suitable near trees where tree roots may be below the slab (roots will disturb the slab over time; and slab disrupts tree root moisture extraction, triggering heave)
• •Not suitable where existing drainage runs beneath the proposed slab (drainage must be rerouted or the slab must bridge it)
• •More susceptible to moisture if DPM or waterproofing is not correctly lapped and sealed

**Beam and block suspended floor**:

A beam and block floor consists of pre-stressed concrete beams spanning between the external walls of the extension (on dwarf walls or bearings), with concrete infill blocks sitting between the beams. An oversite concrete mix (typically a 'grout' or a structural screed) is poured over the top to tie the system together.

*Typical build-up (from bottom to top)*: 1. *Underfloor void*: A ventilated void (minimum 150mm clear height between underside of beams and ground level) — typically with air bricks in the external walls of the extension to provide cross-ventilation (prevents moisture and gases from accumulating below the floor) 2. *Pre-stressed concrete beams*: Typically 155mm–225mm deep, spanned between load-bearing walls or dedicated dwarf wall beams; span direction chosen to minimise beam depth and number 3. *Concrete infill blocks*: 440×215×100mm aggregate concrete blocks seated on the beam flanges between beams 4. *Reinforcement (if required)*: A142 mesh laid on top of blocks for structural screed bond 5. *PIR insulation*: 100mm–150mm rigid PIR between the top of the blocks and the floor finish (above-slab insulation) — less thermally effective than below-block insulation but simpler to construct 6. *Sand-cement screed or liquid screed*: 65–75mm over insulation (or UFH pipes laid in insulation groove, then screed)

• *Advantages of beam and block*:
• •Structurally suitable where ground is not reliable for a ground-bearing slab (made ground of uncertain depth, near trees, variable soil conditions)
• •Provides ventilated void below the floor — moisture and gases are ventilated rather than sealed in
• •Not in contact with the ground — eliminates some groundwater/moisture risk
• •Can be constructed without deep excavation (advantage where excavation is constrained)

• *Disadvantages*:
• •More expensive than solid slab (beams are manufactured offsite, crane or forklift may be needed for delivery, additional dwarf walls may be needed for beam bearing)
• •The ventilated void must be maintained and kept clear of debris
• •Air bricks in external walls must never be blocked
• •Slightly reduced thermal performance compared to well-insulated solid slab if insulation is placed above rather than below beams

**When to use each type**:

| Condition | Recommended floor type | |---|---| | Normal London Clay or gravel ground, no trees nearby | Solid ground-bearing slab | | Made ground of uncertain composition or depth | Beam and block (avoid loading variable fill) | | Trees within 5m of slab edge, London Clay | Beam and block (avoids slab bearing on shrink-swell clay in tree root zone) | | Existing drainage running beneath proposed slab | Beam and block (drainage can remain — void provides access) or reroute drain | | Low external ground level (slab would be too close to ground level for adequate DPM) | Beam and block or raised slab with perimeter dwarf wall | | UFH required | Both are suitable (UFH pipes in screed over structural slab or over beam-and-block) |

**Part L (Conservation of Fuel and Power) — floor insulation requirements**:

For a new extension, the ground floor must achieve a maximum U-value of 0.25 W/m²K (the current Part L Approved Document target for new dwellings as of the 2021 revision of Part L). For an extension specifically, the Part L Approved Document L1B 'Conservation of fuel and power in existing dwellings' applies (as opposed to L1A for new dwellings), and the target U-value for a new ground floor within an extension is typically 0.25 W/m²K.

To achieve 0.25 W/m²K in a solid ground-bearing slab with typical PIR insulation:

| PIR insulation thickness | Approximate floor U-value (W/m²K) | Complies with Part L? | |---|---| | 75mm (Kingspan K103 or equivalent) | ~0.35 | No | | 100mm | ~0.27 | Marginal/borderline | | 125mm | ~0.21 | Yes | | 150mm | ~0.17 | Yes (better than minimum) |

Note: U-value also depends on floor dimensions (the edge loss fraction) — larger floors have lower edge loss per unit area. For typical London extension floors (15–30m²), 100mm PIR is borderline; 125mm is the standard specification to provide comfortable compliance margin.

• *Position of insulation*:
• •*Below slab* ('warm floor'): PIR insulation below the structural concrete slab — the slab sits on top of the insulation. This is the best configuration thermally (no cold bridging at the slab edges, better Part L calculation for floor perimeter). Requires the slab to be stable on the insulation during pour (boards are laid in a staggered pattern, and concrete is poured carefully to avoid displacing boards).
• •*Above slab* ('cold floor'): Insulation laid on top of the structural slab, with screed on top. Slightly less effective (cold bridging at slab edges) but common in practice.

**Underfloor heating (UFH) integration**:

Water underfloor heating (wet UFH) is the most popular heating choice for London extension kitchen-living spaces — it provides even heat distribution, is compatible with contemporary interior design (no radiators), and is very efficient when coupled with a modern condensing boiler or (better) a heat pump.

*UFH build-up over a concrete floor*:

1. *Structural slab* (100–125mm concrete, on DPM and insulation as above) 2. *UFH insulation boards*: 25–50mm rigid insulation with UFH pipe channels (Kingspan Thermafloor or similar) — these boards serve dual purpose: acoustic separation between slab and screed, and UFH pipe location 3. *UFH pipes*: Barrier polyethylene pipe (typically 16mm or 20mm diameter) at 150mm–200mm centres for heating zones; laid on or clipped into the insulation channels 4. *Manifold*: UFH manifold typically located in a plant cupboard — controls flow to each room or zone independently; each zone typically 100–150m² maximum 5. *Screed*: Sand-cement screed (minimum 65mm over top of pipe) or proprietary liquid anhydrite screed (minimum 35mm over top of pipe) — liquid screeds are faster to lay, self-levelling, and transmit heat more quickly but require sealing before any moisture-sensitive finishes

*UFH pipe commissioning*: UFH must be pressure-tested before screeding (typically 4–6 bar hydrostatic test for 24 hours) and must be running during the initial screed curing period (controlled warm-up sequence to avoid cracking).

*UFH system cost*: Supply and installation of a wet UFH system for a 25m² extension zone: typically £2,500–£5,000 (depending on manifold location, boiler compatibility, and installer).

**Level threshold between extension and garden (Part M)**:

Part M (Access to and use of dwellings) requires the extension to have an accessible threshold — specifically, the threshold at external doors should ideally be level (flush) or have a maximum 15mm upstand. Achieving a truly flush threshold between the internal floor and the external patio requires careful design:

• *The coordination problem*:
• •Internal floor finished level = top of screed = typically (from outside in): DPM + insulation + slab + screed = approximately 300–400mm above subgrade
• •External patio finished level = must match internal floor level within 10–15mm for a truly flush threshold
• •The patio must drain away from the house (minimum 1:80 fall, ideally 1:50) and must not direct water towards the extension

*Design solution*: The extension floor level and external patio level must be coordinated at the design stage — setting the floor level too high relative to the external ground level makes the threshold step excessive; setting it too low relative to the external ground level can cause water ingress at the threshold. This coordination between the floor build-up specification (insulation thickness, screed depth) and the external drainage design must be resolved in the pre-construction stage, not on site.

**Compatible floor finishes over a concrete extension floor**:

• *Ceramic and porcelain tiles*:
• •Most compatible with concrete floors
• •Suitable over either screed or direct to concrete slab (with flexible tile adhesive)
• •Suitable for UFH (good heat conductor)
• •Cost: £30–£100/m² supply and lay (format and tile quality dependent)

• *Engineered timber*:
• •Must be laid over a fully cured screed (moisture content <75% RH for engineered timber — typically 6–8 weeks for sand-cement screed, 3–5 weeks for anhydrite liquid screed)
• •Compatible with UFH (must be specified by the manufacturer as UFH-compatible — maximum surface temperature typically 27°C)
• •Floating (over underlay) or glued-down to screed
• •Cost: £40–£100/m² supply and lay

• *Natural stone (limestone, slate, travertine)*:
• •Suitable over screeded concrete floors
• •Very good heat conductor — excellent for UFH
• •Requires higher-strength flexible adhesive (C2TE classification)
• •Sealing required (particularly for limestone and travertine)
• •Cost: £60–£200+/m² supply and lay

• *Polished concrete*:
• •For a polished concrete finish, the structural slab itself is ground and polished — requires a very high-quality slab pour (low water-cement ratio, good vibration and trowelling), a power-float finish, and then diamond grinding and polishing with progressively finer grits after curing
• •Not compatible with screed (the screed would be polished, not the structural slab)
• •Cost: £60–£120/m² for grinding and polishing an existing slab to a polished finish

• *Luxury Vinyl Tile (LVT)*:
• •Floating or glued-down over screed
• •Compatible with UFH (check manufacturer's maximum surface temperature — typically 27°C)
• •Requires very flat substrate (±3mm in 2m maximum)
• •Cost: £25–£60/m² supply and lay

**Typical costs for extension floor construction in London (2025)**:

| Floor element | Typical cost per m² | Notes | |---|---| | Excavation and remove spoil (300mm depth) | £30–£60/m² | Access and skip cost dependent | | Type 1 sub-base (150mm compacted) | £15–£25/m² | | | DPM and blinding | £5–£10/m² | | | PIR insulation (125mm, Part L compliant) | £25–£45/m² | PIR board cost | | Concrete slab (125mm C20 reinforced) | £40–£70/m² | | | Sand-cement screed (75mm) | £25–£45/m² | | | UFH pipe and insulation boards (if included) | £30–£60/m² | Supply and lay | | Floor tiles laid (standard porcelain) | £40–£80/m² | Including adhesive and grout |

*Typical total extension floor cost (solid slab + insulation + screed, excludes floor finish)*: £100–£180/m² as a subcontract package, or approximately £3,000–£5,500 for a 25–30m² extension floor.

**Common mistakes in extension floor construction**:

*1. Insufficient insulation thickness*: Specifying 75mm PIR to save cost — this results in a floor that fails Part L compliance at Building Control inspection, requiring remedial work before the slab is poured.

*2. DPM not lapped to wall DPC*: The DPM in the floor must be lapped to connect with the cavity tray DPC in the walls at the perimeter. If this connection is not made, moisture can track from the ground into the wall base — causing damp internally at the skirting board.

*3. Floor level not coordinated with external ground level*: As discussed above — the finished floor level must be coordinated with the external patio level at design stage.

*4. Screed laid too thin over UFH pipes*: Less than 65mm of sand-cement screed over UFH pipes results in pipe print-through (visible pipe indentations in the floor surface) and possible pipe damage. Minimum 65mm cover for sand-cement screed; minimum 35mm for anhydrite liquid screed.

*5. Screeds laid on inadequately cured concrete*: Sand-cement screed bonded directly to green (uncured) concrete can delaminate. The structural slab must cure for minimum 3 days before a bonded screed is applied; for an unbonded screed, the slab must cure for minimum 7 days and a slip membrane must be laid between slab and screed.