Raft Foundation in Ireland — The All-Rounder for Class 3–4 Ground and Subfloor Insulation
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Raft on Class 3–4: €180–€260/m². Spreads the load across the whole footprint. Cheapest path to a high-performing subfloor.
A raft foundation in Ireland is a reinforced concrete slab that spreads the building load across the entire footprint. It is the all-rounder for Class 3–4 ground (firm clay, silt, sand with clay), at €180–€260/m². A 150 m² house: €27,000–€39,000. Raft is the standard for BER A1 builds because the subfloor insulation layer is continuous — underfloor heating integrates easily. Building Regulations 2023 TGD-A (Structure) governs. The site investigation (€1,500–€3,000) decides whether a raft is viable. Raft is faster to build than strip+wall on variable ground. Add €30–€60/m² for tanking if the water table is high. A radon barrier is mandatory on every raft in Ireland.
TL;DR
Raft foundation in Ireland: €180–€260/m² of footprint. The all-rounder for Class 3–4 ground (firm clay, silt, sand with clay).
A 150 m² house on a raft: €27,000–€39,000 for the foundation. Cheapest path to a continuous subfloor insulation layer (underfloor heating integrates easily).
Faster to build than strip+wall on variable ground. Standard for BER A1 builds because of subfloor insulation continuity.
When this matters most
Your site has Class 3–4 ground, or you want a continuous subfloor insulation layer.
When this doesn't apply
Your site has Class 1–2 ground (strip is cheaper) or Class 5+ ground (need piled raft).
Frequently asked questions
How much does a raft foundation cost in Ireland?
€180–€260/m² of footprint. For a 150 m² house: €27,000–€39,000. Add €30–€60/m² for tanking on a high water table. Radon barrier mandatory.
When is a raft foundation the right choice?
Class 3–4 ground (firm clay, silt, sand with clay), variable ground conditions, or when you want continuous subfloor insulation for BER A1. Cheapest path to integrated underfloor heating.
What is the difference between a raft and a strip foundation?
Strip is a continuous concrete strip under each load-bearing wall. Raft is a single concrete slab across the entire footprint. Strip is cheaper on good ground; raft is more forgiving on variable ground and integrates subfloor insulation.
Sources
Building Regulations 2023, TGD-A (Structure). https://www.gov.ie/
Frequently asked questions
How much does a raft foundation cost in Ireland?
€180–€260/m² of footprint. For a 150 m² house: €27,000–€39,000. Add €30–€60/m² for tanking on a high water table. Radon barrier mandatory.
When is a raft foundation the right choice?
Class 3–4 ground (firm clay, silt, sand with clay), variable ground conditions, or when you want continuous subfloor insulation for BER A1. Cheapest path to integrated underfloor heating.
What is the difference between a raft and a strip foundation?
Strip is a continuous concrete strip under each load-bearing wall. Raft is a single concrete slab across the entire footprint. Strip is cheaper on good ground; raft is more forgiving on variable ground and integrates subfloor insulation.
When this matters most
Your site has Class 3–4 ground, or you want a continuous subfloor insulation layer.
When this doesn't apply
Your site has Class 1–2 ground (strip is cheaper) or Class 5+ ground (need piled raft).
Where to go next
Branch into
Adjacent decisions at the same stage.