How to Price a Concrete Slab: Ground Slabs & Foundations

Quick Answer: Ready-mix concrete (C20 / GEN3 strength) delivered costs £130–£175 per m³; mass strip foundation costs £90–£160 per linear metre installed. A 5m × 5m × 150mm garage/shed slab (3.75m³) total installed £900–£1,500 including DPC, mesh, edge formwork, labour. All structural concrete must follow BS 8500-1:2023 mix spec, BS 8110 / Eurocode 2 design, and Building Regulations Part A for any load-bearing application.

Summary

Concrete slabs sit behind every UK building project — foundation strips, ground floor slabs, garage bases, shed pads, driveway sub-bases. Pricing is conceptually simple (volume of concrete × £/m³ + formwork + reinforcement + labour) but practically variable due to access, ground conditions, and reinforcement requirements.

This guide covers the main types of domestic slab (mass strip foundation, ground floor slab, shed/garage base, driveway sub-base), how to specify concrete mix correctly, when reinforcement is needed, and how to itemise the components for a defensible quote. It does not cover engineered foundations (raft, piled) which need structural engineer specification.

The most common pricing mistake: assuming concrete is the biggest cost. For most domestic slabs, concrete itself is 25–35% of total — excavation, hardcore, DPC, formwork, mesh and labour together make up the rest. Quote it line-by-line.

Key Facts

• Ready-mix C20 (GEN3) standard ground floor — £130–£175 per m³ delivered
• Higher strength C25 (RC25/30) — £150–£195 per m³
• C30 (RC32/40) reinforced foundations — £165–£210 per m³
• Foamed concrete (low-strength fill) — £110–£160 per m³
• DIY mixed on site — £80–£120 per m³ of materials, plus labour
• Hardcore (MOT Type 1) — £35–£55 per tonne delivered; 1 tonne ≈ 0.7m³ compacted
• Steel mesh A142 (5mm × 200mm) light reinforcement — £18–£28 per sheet (3.6 × 2.0m), £4.50/m²
• A193 mesh heavier — £26–£40 per sheet, £6.50/m²
• DPM (1200 gauge polythene) — £80–£140 per roll (4 × 25m)
• Edge insulation (50mm PIR) — £8–£15/m linear
• Below-slab insulation (100mm PIR) — £14–£22/m²
• Compaction — Whacker plate hire £40–£60/day; vibrating roller £100–£180/day
• Minimum thickness — Ground floor slab 100–150mm; garage 150–200mm; driveway sub-base 150mm
• Foundation depth — 600–900mm typical for cohesive ground, more for clay (per BS 8000-3)

Quick Reference Table

Detailed Guidance

Concrete mix specification

BS 8500-1:2023 standardises mix designations. Common UK ready-mix grades:

• GEN1 / ST1 — Lean mix, blinding, kerb bedding. Very low strength.
• GEN3 (C20/25) — Standard ground floor slabs, garage bases, paths. Most common.
• GEN4 (C25/30) — Slightly stronger, used for heavier-load slabs.
• RC25/30 (C25/30) — Reinforced concrete grade for foundations
• RC32/40 (C30/40) — Reinforced foundations in aggressive ground
• RC40/50 — High-strength, marine/sulfate exposure
• PAV1 / PAV2 — Paving grade, abrasion-resistant

For most domestic work: GEN3 (or C20/25) is correct for ground floor slab; RC25/30 for reinforced foundations. Specify on delivery ticket.

Slump and aggregate

• Slump 50–75mm — Standard for hand-placed slabs
• Slump 100–150mm — Easier to place but lower strength; used in pumped concrete
• Aggregate 20mm — Standard
• Aggregate 10mm — Use for thin slabs (<75mm) or congested rebar areas

Specify when ordering. Don't request "as supplied" — the result varies by depot.

Volume calculation

Volume (m³) = length × width × thickness.

For ground floor 5m × 6m × 150mm thick:

• 5 × 6 × 0.15 = 4.5m³

Add 5–10% wastage and contingency. Order 5m³ to be safe; cost £680–£875 for ready-mix delivered.

For strip foundation 600mm wide × 250mm deep × 25m linear:

• 0.6 × 0.25 × 25 = 3.75m³

Add 10–15% for over-dig fill and surcharge. Order 4.5m³.

Excavation and prep

Standard sequence:

1. Mark out — Spray paint, string line to engineered or planning drawing
2. Strip topsoil — 200–300mm typical, removed from site or used elsewhere
3. Excavate to formation level — Mini-digger £150–£250/day rate
4. Lay hardcore (MOT Type 1) compacted — 100–150mm layer
5. Compact — Whacker plate, multiple passes until firm
6. Sand blinding (optional) — 25–50mm to protect DPM
7. Lay DPM (1200 gauge polythene) — Continuous, taped at joints, turned up 150mm at edges
8. Lay insulation (if ground floor of habitable building) — 100mm PIR taped at joints
9. Lay reinforcement — A142 or A193 mesh on 25mm bar-chair spacers
10. Formwork at edges — Timber shutters fixed to ground stakes
11. Pour concrete — Distribute, tamp, float
12. Curing — Cover with hessian or plastic 24–72 hours, water if hot weather

For a 5×6m garage slab, this is 2–3 days for 2-fitter team including delivery wait.

Reinforcement — when needed

No reinforcement: Garden paths under 100mm thick, paving sub-base, shed base under 5m².

A142 mesh: Standard ground floor slab, garage base, driveway, small slab (5–20m²).

A193 mesh: Heavier load (commercial driveway, large garage), reinforced foundation strip.

Rebar (B500B): Engineered slabs, beams, retaining walls. Specified by structural engineer.

Always lay reinforcement on bar chairs/spacers — not directly on DPM. Mesh laid on ground = useless (won't resist tension where it's needed).

DPM (damp-proof membrane) — non-negotiable

For habitable building slab:

• 1200 gauge polythene DPM
• Continuous across slab area
• Turned up 150mm minimum at all edges
• Joints taped (75mm overlap minimum)
• Punctures resealed with tape

DPM connects to wall DPC at perimeter. Failure here = rising damp through floor; whole house affected.

Insulation — for habitable building slab

Building Regs Part L 2021 requires ground floor U-value ≤0.18 W/m²K for new dwellings, ≤0.25 for extensions.

Typical achieve:

• 100mm PIR (Kingspan, Celotex, Recticel) = U-value ~0.15
• 75mm PIR + 50mm under-screed = similar
• 50mm PIR ONLY = U-value ~0.22 (fails new build, passes extension if upgrades elsewhere)

Insulation cost: £14–£22/m² for 100mm PIR. Lay between DPM and mesh.

Edge formwork and finish

• Timber shutters (50×100mm or 50×150mm) fixed to stakes 600mm centres
• Diagonal braces to prevent bulging
• Strike formwork 24 hours after pour

Surface finishes:

• Float finish — Standard, slightly rough, suitable for screed or tile bedding
• Trowel finish — Smoother, used for warehouse/garage where stored items rest
• Brushed finish — Slip-resistant for path or driveway
• Tamped finish — Rougher, slip-resistant

Specify in quote.

Curing

Concrete strength develops over 28 days. First 7 days critical:

• Keep moist (cover with hessian, dampen)
• Avoid early loading
• Protect from frost (insulating blankets if temp <5°C)
• No deicing salts until 28 days cured

Premature loading or freezing = cracking and reduced final strength.

Ready-mix delivery considerations

Order from local ready-mix supplier (Hanson, Tarmac, Aggregate Industries, smaller local). Delivery considerations:

• Truck access — Need 4m wide x 4m height clear path. Standard mixer 8m long.
• Distance from truck to pour — Up to 6m chute-distance. Beyond = pump (£200–£500 extra) or wheelbarrows.
• Quantity minimum — Most suppliers minimum 1m³ delivery; smaller = surcharge or DIY mix.
• Delivery slot — Booked the day before. Late delivery = setting delays.

For tight access sites, consider concrete pump or volumetric (on-site mixing) supplier — costs more but solves access.

Worked example — 5m × 6m × 150mm garage slab

• Excavate to 350mm below finished floor level: 6m × 7m × 0.35 = 14.7m³ spoil; hire mini-digger 1 day £200
• Skip for spoil: £280
• Hardcore (MOT Type 1) 150mm: 6 × 7 × 0.15 = 6.3m³ ≈ 9 tonnes × £45 = £405; delivery + spread £150
• Compaction (whacker plate hire 1 day): £55
• DPM (1200 gauge, 30m roll): £100
• Mesh A142 (4 sheets to cover 30m²): 4 × £24 = £96
• Bar chairs/spacers: £25
• Edge insulation 50mm PIR (no under-slab on garage): £45
• Edge formwork (timber + stakes): £75
• Ready-mix C25 4.5m³ @ £155/m³: £698
• Pump or chute (within 6m): included
• 2 fitters, 2 days install + 1 day prep: 3 × 2 × £200 = £1,200
• Curing materials: £40
• Sub-total cost: £3,369
• 28% margin: £943
• Quoted price: £4,312 inc. VAT (5m × 6m = 30m² @ £144/m²)

Frequently Asked Questions

Can I mix concrete on site instead of ready-mix?

For small slabs (<2m³) site mixing is viable but slow. Cost roughly £80–£120/m³ in materials plus significant labour (2 hours+ per m³ mixing). For anything bigger than a shed base, ready-mix is faster, more consistent, and cheaper overall. Site mix is for awkward access or emergency repair only.

How thick should my concrete slab be?

Standard guidance:

• Shed/light storage: 75–100mm
• Garage / driveway light vehicles: 150mm
• Domestic ground floor slab: 100–150mm (over hardcore)
• Driveway with heavy vehicles: 200mm+
• Foundation strips: 250–300mm typical mass concrete

Below 100mm cracks easily. Above 200mm wastes material unless engineered for load.

Do I need to wait 28 days before walking on concrete?

Walking on concrete: 24–48 hours after pour. Driving light vehicle: 7+ days. Full structural loading: 28 days. The 28-day figure is when concrete reaches design strength. For most domestic applications, normal use OK after 7 days; avoid heavy point loads.

Will my driveway crack?

Some hairline cracking is normal in unreinforced concrete (shrinkage cracks during cure). Major cracking results from: poor sub-base preparation, insufficient thickness, no/wrong reinforcement, frost during cure, or excessive load. A properly specified and laid driveway has expansion joints every 4–6m to control crack location.

Do I need Building Regulations for a garden slab?

No for: paths, driveways, shed bases, garden walls under 1m, garage bases for non-habitable buildings. Yes for: foundations of habitable buildings, garage attached to house, conservatory base. Always confirm with Building Control before starting structural work.

Regulations & Standards

• Building Regulations Part A 2013 — Structure

• Building Regulations Part C 2013 — Site preparation, resistance to moisture

• Building Regulations Part L 2021 — Conservation of fuel and power

• BS 8500-1:2023 — Concrete — complementary British standard to BS EN 206

• BS EN 206:2013+A2:2021 — Concrete: specification, performance, production

• BS 8000-3:2020 — Workmanship on construction sites: concrete

• BS 8110 (withdrawn) and Eurocode 2 (BS EN 1992) — Concrete design

• BS 8002:2015 — Earth retaining structures (relevant for foundations)

• BS 8004:2015 — Foundations: code of practice

• British Cement Association / MPA — concrete guidance

• BS 8500-1:2023 — Concrete BSI

• Building Regulations Approved Document A

• Building Regulations Approved Document C

• The Concrete Centre — design guides

• scaffolding pricing guide — adjacent construction trade pricing

• fencing installation pricing guide — concrete-posted fence pricing

• block paving pricing guide — driveway sub-base alternative

• party wall agreement pricing guide — when excavations trigger PW