How to Price a Concrete Slab: Ground Slabs, Foundations and Paths

Quick Answer: A 100 mm reinforced ground-bearing slab costs £85–£140 per m² supply-and-lay in 2026, rising to £140–£220 per m² for a 150 mm structural slab with mesh, DPM, insulation and power float finish. Concrete itself is £125–£170 per m³ delivered (C25 to C30, ready-mix from a UK supplier), and the price is dominated by formwork, sub-base, reinforcement and finishing labour rather than the concrete itself. Specifications must comply with BS 8500-1:2023 for concrete and BS 8204-2:2003 for industrial floors.

Summary

Concrete slabs are everywhere on UK building sites — extension floors, garage bases, shed pads, garden room foundations, driveway slabs, footpath strips. The pricing range is wide because the word "slab" hides huge variation in specification: a 100 mm shed base with no reinforcement is cheap and quick; a 150 mm power-floated industrial floor with insulation, DPM and A393 mesh is six times the price per m². Quotes that don't break the slab spec down get into trouble both ways — under-priced jobs lose money, over-priced jobs lose work.

The cost drivers are sub-base preparation (excavation, hardcore, compaction), formwork (timber edging, expansion joints, levels), reinforcement (mesh, fibres, structural rebar), the concrete mix itself (strength, workability, additives), and finishing labour (tamped, brushed, power-floated, polished). On a typical extension slab, concrete is only 20–30% of the cost. Get the spec right at quote stage or you'll absorb the difference on site.

The single biggest cost-control decision is access for the concrete pump versus barrowing in. A 6 m³ pour barrowed across a domestic site by hand takes 3–4 men a long morning and risks cold joints. The same pour with a line pump from the kerb is a clean 1.5–2 hour exercise. Pump hire is £400–£800 per pour but often pays back through speed alone — and for any pour over 5 m³, pump access should be assumed in the quote unless the wagon can reach the formwork directly.

Key Facts

Quick Reference Table

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Slab type Spec Cost per m² Programme (40 m²) Notes
Shed / garden building base 100 mm C25, no mesh £55–£90 1–2 days Light loadings only
Garden room / studio base 100 mm C30 + A142 mesh + DPM £85–£140 2–3 days 25 mm insulation optional
Extension floor (Building Regs) 100–150 mm C30 + A193 mesh + DPM + 100 mm PIR £140–£220 3–4 days Part L compliant
Garage floor (single) 150 mm C30 + A193 mesh + DPM £100–£160 2–3 days Tamped finish typical
Driveway slab 150 mm C30 fibre-reinforced £85–£130 2–3 days Brushed finish, pattern joints
Power-float industrial floor 150–200 mm C40 + A393 mesh £180–£280 3–5 days Workshop, light commercial
Patio sub-slab (under stone/porcelain) 75 mm C20 lean mix £55–£85 1–2 days Bedding base only
Pile cap / foundation pad 200–300 mm C30 + structural rebar £200–£350 2–4 days SE-designed mixes

Detailed Guidance

Ground-Bearing vs Suspended Slabs

Ground-bearing slabs sit on a compacted sub-base and transfer load directly to the ground. Standard for extensions, garages, sheds and patios. Cheaper, simpler, and fine for any reasonable ground bearing capacity (most UK ground supports 50 kN/m² which is well above domestic floor loadings).

Suspended slabs span between supports — ground beams, walls, or piles. Required where ground is unreliable (made-up ground, heave-prone clay, contaminated land, mining areas). Suspended in-situ slabs are commonly 175–225 mm thick with structural rebar; suspended pre-cast slabs (beam and block, hollowcore) are quicker but need cranes. Roughly 1.5–2× the cost per m² of ground-bearing.

Ground-bearing slabs over heave-prone clay (London clay, Oxford clay, Gault clay) sometimes need a void former — a compressible card or polystyrene layer that absorbs upward soil movement. £8–£14 per m² added. Specification depends on a soil report.

The Sub-Base: Where Most Slab Failures Start

A slab is only as good as what's under it. The standard build-up:

  1. Excavation to formation level — typically 350–500 mm below final slab top depending on insulation and sub-base depth
  2. Geotextile membrane (optional but worthwhile) — separates sub-base from soft ground, prevents fines pumping up. £1.50–£3 per m².
  3. Sub-base — MOT Type 1 limestone or granite — 100–200 mm depth depending on slab loading. Laid in two 75–100 mm lifts, each compacted with a vibrating plate or roller.
  4. Sand blinding — 25–50 mm of sharp sand on top of MOT Type 1, levelled to receive DPM
  5. DPM (Damp Proof Membrane) — 1200-gauge polythene, lapped 150 mm minimum at joints, taped, dressed up walls 150 mm above slab top
  6. Insulation (where required) — typically 50–150 mm PIR boards, butt-jointed, taped at joints
  7. Slip layer (optional) — 500-gauge polythene over insulation to prevent concrete bonding to insulation
  8. Mesh on chairs — A142 to A393 mesh on plastic chairs to give 25–40 mm bottom cover, lapped 1 sheet width at joints

Cutting corners on the sub-base shows up as differential settlement, cracking, surface dishing, and damp problems — usually 1–3 years post-completion when the contractor has long since gone home. The Building Regulations Approved Documents make compliance with BS 8204 part of the standard for any habitable floor.

Concrete Mix Selection

The mix code (e.g. C25/30) names two strengths — the cylinder strength and cube strength in N/mm². For UK practice the cube strength is the working number. Common selections:

Workability is the second axis. S2 slump (50 mm) is standard for ground slabs; S3 (100 mm) for pumped concrete; S4 (160 mm) for self-levelling areas. Higher slump = wetter mix = easier to place but lower strength gain unless plasticised.

Fibre reinforcement (polypropylene macro fibres) is increasingly used as an alternative to mesh on driveway and ground-bearing slabs. Cheaper and faster than mesh, eliminates the 'chair lifting' labour, but doesn't replace structural mesh on heavily loaded slabs.

Finishes: Tamped, Floated, Polished

Tamped finish — straight-edged with a tamping board. Grippy, ridged finish suitable for drives, garage floors, external slabs. Cheapest finish, included in any standard slab quote.

Brushed finish — tamped then dragged with a stiff brush across the surface. Better grip than smooth, used for paths and ramps. Adds £3–£8 per m² of labour.

Power-float finish — tamped, allowed to firm up, then power-floated to a smooth, dense surface. Used for warehouse floors, workshop floors, garage floors that will be screeded over. Adds £15–£30 per m² to cost (float hire, labour, late finish on pour day). Requires the gang to be on site for 4–8 hours after the pour to time the floating window correctly.

Polished concrete — power-floated then ground, sealed and polished as a finished floor surface. £80–£180 per m² above slab cost. Specialist trade.

Programme and Pour Logistics

A 40 m² ground-bearing slab pour at 100 mm thickness needs 4 m³ of concrete — one wagon. A 60 m² slab at 150 mm is 9 m³ — one wagon plus a top-up, or a truck with extra capacity. Coordinate:

Reinforcement Detailing

Mesh laps — 300 mm or 1 sheet width minimum, tied with steel wire. Cover — 25 mm to top, 40 mm to bottom on a ground-bearing slab. Bend up at edges where the slab meets a wall. Provide expansion gaps and crack inducers (tooled joints at 1:1 to 1:1.5 ratios on length:width) to control where the inevitable shrinkage cracks form.

Structural slabs (basement floors, suspended ground floors) need a structural engineer's design for rebar, not just mesh. Quote should include the SE design fee separately if not already commissioned.

Frequently Asked Questions

How much concrete do I order for a slab?

Volume in m³ = length × width × thickness, all in metres. 40 m² × 0.10 m = 4 m³. Order 5–10% over to allow for sub-base irregularities and edge spillage. Suppliers charge for partial loads back to the plant — better to slightly over-order than to send the wagon back twice.

Do I need building control for an extension slab?

Yes — any habitable extension floor needs to comply with Approved Document C (resistance to moisture) and Approved Document L (thermal performance). Building control wants to see DPM dressing, insulation thickness, and the slab thickness. Inspection happens at sub-base and at slab-prep stages.

Can I lay a slab in winter?

Yes, with care. Below 5°C ambient, concrete won't gain strength normally. Options: hot water in the mix, insulating blankets over the slab, accelerator admixtures (calcium chloride for unreinforced, non-chloride for reinforced), or simply waiting for warmer weather. A frosted slab is a ruined slab — protection is non-negotiable below 3°C.

What's the actual difference between A193 and A393 mesh?

The number is the cross-sectional area of steel per metre width, in mm². A193 has 193 mm² per metre (light/medium); A393 has 393 mm² (structural). For domestic ground-bearing slabs A142 or A193 is fine. A393 is for heavily loaded floors, industrial slabs, or where the SE has specified it.

Do I need a structural engineer for a slab?

For a habitable extension over good ground: usually no — the Approved Document C standard build-up is acceptable and building control signs it off. For basements, suspended slabs, slabs over made-up ground, or anything outside standard guidance: yes, an SE design is required.

Regulations & Standards