How to Price a Raft Foundation: Labour, Materials and Margin Guide

Summary

Raft foundations get mispriced in two directions. Some tradespeople quote them like an oversized strip foundation and lose money on the extra concrete, reinforcement and formwork a raft actually needs. Others quote defensively high because rafts have a reputation for being "expensive" without pricing the actual cost drivers — thickness, reinforcement density, and whether the raft also has to perform as the insulated ground floor slab.

Rafts are specified when strip foundations won't work: low bearing-capacity soil, variable or made ground, shrinkable clay where strip depths become impractical, or where a structural engineer determines uniform load spreading is needed to avoid differential settlement. Because Approved Document A provides no prescriptive table for rafts, every raft is a bespoke structural engineer's design — this fundamentally changes both the cost structure and the quoting process compared with a strip foundation, where a groundworker can often price directly from Table A1.

This guide covers pricing the design fee, the concrete and reinforcement package, the groundworks that sit around a raft (blinding, DPM, insulation if the raft doubles as the floor slab), and a worked example for a typical single-storey extension.

Key Facts

• No prescriptive table — raft design always requires a structural engineer's calculation; this is a mandatory quoted cost, not a discretionary upgrade — see raft foundation guide
• Structural engineer's design fee (domestic extension raft) — typically £600-£1,800 depending on complexity and whether a full ground investigation (trial pits/borehole) is also needed
• Ground investigation — trial pits £300-£700; a borehole survey for more complex or contaminated sites £800-£2,500+
• Concrete volume — a raft uses significantly more concrete than an equivalent-footprint strip foundation because it covers the full building footprint, not just the wall lines — typically 225-300mm thick across the full slab area
• Reinforcement — all rafts are reinforced (mesh and/or bar to engineer's spec); reinforcement is priced by weight (£/tonne fixed in place) and is a substantial share of raft cost, unlike most strip foundations which are unreinforced mass concrete
• Concrete grade — typically RC35/37 or RC40 per structural engineer's specification, higher grade than the ST2/GEN3 commonly used for strip foundations
• Formwork/edge shuttering — needed around the full raft perimeter, and around any edge-thickening/upstand beams
• Blinding layer — minimum 50mm weak mix (ST2) laid on prepared/compacted ground before reinforcement fixing
• Hardcore/sub-base — typically 150-300mm compacted hardcore below blinding, priced per m³ including compaction plant
• DPM (damp-proof membrane) — 1200 gauge polythene minimum, lapped and sealed, required where the raft also forms the ground floor slab
• Insulation — if the raft doubles as the insulated ground floor slab (common on single-storey extensions), rigid insulation (typically 100-150mm PIR) is added to meet Part L U-value requirements — this is a substantial extra cost not present in a strip foundation quote
• Building control inspection — foundation stage inspection required before pouring, same as strip foundations, but building control will also check the reinforcement fixing against the engineer's drawings before concrete is poured
• Concrete pump — often used on raft pours given the volume and continuous-pour requirement; £350-£600/day depending on reach and access
• Margin — 15-25%; rafts carry more design and sequencing risk than strip foundations (continuous pour, reinforcement inspection, more exposure to ground condition surprises) so sit at the higher end unless the ground has already been fully investigated

Quick Reference Table

Figures assume normal access for a concrete pump or barrow run, no contaminated ground, and a raft that also forms the insulated ground floor slab (insulation included). Deduct roughly 10-15% where the raft is foundation-only with a separate suspended floor above.

Detailed Guidance

Why a Raft Costs More Per Square Metre Than It Looks

The instinctive comparison — "it's just a bigger slab" — undersells what's different about raft pricing. A strip foundation only needs concrete under the wall lines; a raft needs full-depth concrete across the entire footprint, so the volume comparison is not linear with a strip job of the same size. Add mandatory reinforcement (rafts are never mass concrete, unlike most domestic strip foundations), edge thickening or downstand beams at the perimeter for stiffness, and — if the raft is also the floor slab — insulation and a screed or floor finish build-up, and the true cost gap between strip and raft becomes clear. Quote the raft as its own structural package, not a "foundation line" borrowed from a strip foundation estimate.

Pricing the Structural Design — Not Optional

Because there is no Approved Document A equivalent for rafts, a structural engineer must produce the raft thickness, reinforcement layout (bar diameter, spacing, cover), and any edge beam detail before groundworks can be priced accurately or building control will approve the foundation stage. Get this quoted and instructed before finalising the raft price to the customer — pricing a raft on an assumed thickness and reinforcement spec, only to find the engineer specifies something heavier once ground investigation results come back, is one of the most common causes of raft foundations running over budget. Where ground conditions are already known to be poor (made ground, former quarry/pit, known clay issues), build the ground investigation cost into the initial quote rather than treating it as a possible extra.

Groundworks Build-Up Before the Pour

A raft sits on a prepared formation: excavate to the engineer's specified level, compact hardcore to the specified thickness, blind with a weak concrete layer to give reinforcement fixers a clean, level working surface, then fix reinforcement to cover and spacing tolerances before the DPM and main pour. Price hardcore and blinding as their own line items — they're often bundled vaguely into "groundworks" on inexperienced quotes, but a raft's hardcore depth and compaction standard (checked by building control before blinding) is a discrete, inspectable stage with its own labour and material cost.

When the Raft Also Forms the Floor Slab

On single-storey extensions it's common for the raft to serve as both foundation and insulated ground floor slab in one pour, avoiding a separate suspended floor. This saves programme time but adds insulation cost (rigid PIR board sized to meet the extension's Part L U-value target, typically 0.13-0.18 W/m²K depending on whether it's a new dwelling or an extension to an existing one) and requires the DPM to be correctly positioned relative to the insulation (usually below the insulation, above the blinding) per the engineer's detail. Confirm with the engineer and building control which floor build-up is being used before pricing insulation thickness — get this wrong and the U-value calculation fails at building control sign-off.

Concrete Placement — Pump vs Barrow

Raft pours are typically continuous (no cold joints across the slab unless specifically detailed with a construction joint), which makes a concrete pump the practical choice on anything beyond a small footprint — barrow-run placement on a raft of 30m²+ struggles to keep pace with the specified pour rate and risks cold joints forming. Price pump hire (including a standby/wasted-load allowance if the pour has to pause) rather than assuming barrow labour will cover it, and confirm site access for the pump boom or line before quoting.

What to Include in the Quote

A complete raft foundation quote should show: the structural engineer's design fee and any ground investigation as discrete lines, groundworks (excavation, hardcore, compaction, blinding) broken out from the concrete/reinforcement/formwork package, insulation if the raft forms the floor slab, concrete pump hire if used, building control fees, and VAT at the standard rate (20% on a private domestic extension in almost all cases — reduced/zero rating only applies in narrow new-dwelling-qualifying circumstances, which needs an accountant's confirmation, not an assumption).

Frequently Asked Questions

Can I avoid the structural engineer's fee by copying a raft design from a similar job?

No. Every raft is specific to the ground conditions and loads on that site — building control will require calculations specific to the project, and a raft copied from elsewhere without a site-specific ground investigation risks under- or over-design. The design fee is unavoidable and should always be quoted as a discrete line.

Is a raft always more expensive than a strip foundation for the same building?

Usually yes, on a direct cost basis — but the comparison is often moot because a raft is specified precisely in situations where a strip foundation isn't viable (poor bearing capacity, made ground, extreme clay shrinkage depths). Where ground conditions genuinely allow either option, get the structural engineer to cost-compare both before committing the customer to the raft.

Does building control need to inspect the reinforcement before the pour?

Yes. In addition to the standard foundation-stage inspection of the excavated and prepared formation, building control will typically want to inspect the fixed reinforcement (checking bar size, spacing and cover against the engineer's drawings) before the concrete pour — factor this inspection into programme, as pouring before sign-off risks having to break out non-compliant work.

How does a raft affect groundworks compared with a strip foundation on the same job?

A raft generally needs deeper and wider excavation across the whole footprint rather than trenches along wall lines, more spoil to remove, and a compacted hardcore/blinding build-up across the full area rather than just under the walls — all of which should be priced as their own groundworks package rather than assumed to scale directly from a strip foundation estimate.

Regulations & Standards

• BS 8004:2015 — Code of practice for foundations; covers raft design principles alongside strip and pile foundations

• Building Regulations 2010 Approved Document A — Structure; confirms rafts require engineer's design (no prescriptive table, unlike strip foundations)

• BS 8500-1:2015+A2:2019 — Concrete specification and guidance, including exposure classes relevant to raft concrete

• Building Regulations Approved Document L — energy efficiency requirements affecting insulation specification where the raft forms the ground floor slab

• CDM Regulations 2015 — safety planning for groundworks and concrete pour operations

• Approved Document A - GOV.UK — confirms scope and limits of prescriptive foundation tables

• BSI - BS 8004:2015 — Code of practice for foundations

• gov.uk - Approved Document L — energy efficiency standards for floor insulation

• raft foundation guide — technical design guidance: when a raft is specified, thickness, reinforcement and detailing

• strip foundation pricing guide — cost comparison for the alternative foundation type

• foundation depths and types — overview of foundation types and when each applies

• structural calculations guide — where raft design fits within the wider structural calculations process for an extension