Underpinning Cost Calculator

About this calculator

Underpinning is the process of extending a building's foundation deeper or wider to correct subsidence, increase load capacity, or allow excavation beneath an existing structure, and estimating its cost starts with the excavation volume: wall length multiplied by underpinning depth multiplied by underpinning width, which gives the cubic volume of soil that must be dug out in sections and replaced with new concrete foundation, then multiplied by a local cost-per-cubic-metre rate that reflects labor, materials, and the specialist engineering involved.

Structural engineers, builders and homeowners calculate this before committing to underpinning work — often triggered by cracking walls, subsidence from tree roots or clay soil shrinkage, or plans to dig a basement — because underpinning is one of the most expensive categories of structural work per linear metre, done in short sections (typically 1-1.5m at a time) to avoid destabilizing the existing structure, which is part of why the cost per metre runs so much higher than ordinary foundation work.

This calculator takes your wall length, underpinning depth, width and local rate per cubic metre and returns the total excavation volume, total cost, and cost per metre of wall underpinned.