Irish Soil Classification — Class 1 to Class 5+ and What Each One Costs You in Foundation

The Irish Soil Classification System runs from Class 1 (rock) to Class 5+ (peat, made ground, fill). The class drives foundation choice and cost. Class 1–2 (rock, gravel, dense sand): strip foundation, €120–€180/m² of footprint. Class 3–4 (firm clay, silt, sand with clay): raft foundation, €180–€260/m². Class 5+ (peat, soft clay, made ground, fill): piled raft or driven piles, €280–€450/m². The site investigation (€1,500–€3,000) is non-negotiable; the foundation choice follows. Get the soil test before you commission the foundation design — never assume. Geological Survey Ireland has free soil maps that can narrow down the class before you commission the test, but only a borehole on the actual footprint confirms it. Sloped sites add €40–€80/m² for stepped foundations or retaining; high water table adds €30–€60/m² for tanking; high-radon areas (Carlow, Wexford, parts of Wicklow) add €8–€15/m² for a radon barrier and sump.

TL;DR

• Irish Soil Classification System runs Class 1 (rock) to Class 5+ (peat, made ground). The class drives foundation choice and cost.

• Class 1–2 (rock, gravel, dense sand): strip foundation, €120–€180/m². Class 3–4 (clay, silt, sand with clay): raft, €180–€260/m². Class 5+ (peat, made ground): piled raft, €280–€450/m².

• Get the soil test before the foundation design. €1,500–€3,000 for the test saves €30,000–€60,000 in over- or under-spec'd foundations.

When this matters most

You're choosing a site, you've gone sale-agreed, or your engineer is about to specify the foundation.

When this doesn't apply

The foundation is poured and cured.

Frequently asked questions

What are the Irish soil classes?

Class 1 (rock), Class 2 (gravel, dense sand), Class 3 (firm clay, silt), Class 4 (sand with clay, mixed), Class 5+ (peat, made ground, fill). Each class has a different foundation type and cost.

How much does a soil test cost in Ireland?

€1,500–€3,000 for a site investigation. Includes boreholes, soil samples, and a geotechnical report that the engineer uses to specify the foundation.

Can I build on peat in Ireland?

Yes, with a piled raft foundation (€280–€450/m² of footprint). The piles transfer the building load to a firm bearing stratum below the peat. Standard solution for Class 5+ ground.

Sources

Geological Survey Ireland, Irish Soil Classification System. https://www.gsi.ie/

Building Regulations 2023, Technical Guidance Document A (Structure), Department of Housing. https://www.gov.ie/