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Underpinning Cost Calculator

Enter the wall length, underpinning depth and rate to see total cost, volume and cost per metre.

How it is calculated

Length, depth and width are in metres and the rate is the price per cubic metre of underpinned ground. The engine works out the excavation volume as volume = length × depth × width (m³), the cost as cost = volume × rate, the cost per metre of wall as cost ÷ length, and the number of bays as ceil(length ÷ 1.2), which assumes traditional mass-concrete bays of about 1.2 m each. A rate of 0 is accepted and gives a zero cost; length, depth and width must be above zero. Amounts are in US dollars. The rate you enter should already cover labour, materials, spoil removal and engineering, because the calculation adds nothing else. It is an order-of-magnitude estimate, not a quotation.

Worked example

A wall is 12 m long and is to be underpinned 1.5 m deep and 0.6 m wide, at a rate of 450 per m³. The volume is 12 × 1.5 × 0.6 = 10.80 m³. The cost is 10.80 × 450 = $4,860.00. Spread over the wall, that is 4,860 ÷ 12 = $405.00 per metre. With bays of about 1.2 m, 12 ÷ 1.2 gives 10 bays, which would be dug and filled one at a time.

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Common mistakes

  • Entering the depth of the whole foundation instead of the extra depth being added. The volume only counts the ground you intend to dig out and replace.
  • Using the width of the wall above rather than the width of the new underpinning base, which is often wider.
  • Entering a rate per square metre or per linear metre when the field asks for a price per cubic metre; the cost will be wrong by a large factor.
  • Entering lengths in feet or centimetres. All three dimensions are metres, so 60 cm is 0.6.
  • Reading the bay count as the number of workdays or as a number of separate quotes. It is only the length divided by 1.2 m and rounded up, and real bay sizes depend on the engineer's sequence.

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.

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