Assessment Scope Sets the Floor
A major cost driver in a foundation retrofit is how much of the foundation needs investigation before a fix can even be designed: a soil report and a handful of test pits cost far less than the underpinning or piling work a bad soil condition can trigger. Budgets built before the geotechnical assessment is complete are budgets built on a guess, and that guess is usually optimistic.
Access and Sequencing Costs More Than Materials
Underpinning or piling beneath an occupied or structurally sensitive building costs more for access and sequencing than for the concrete and steel themselves — cramped working zones, staged excavation to avoid destabilising the existing footing, and extended timelines to keep the structure safe throughout. A retrofit costed only against material quantities, without pricing the access constraint, will come in under budget on paper and over budget on site.
Where Contingency Actually Belongs
Foundation work carries more uncertainty than almost any other structural retrofit category, because the actual soil and existing-footing condition is only fully known once excavation starts. A realistic budget puts a larger, explicit contingency against the foundation line item specifically — not spread thinly across the whole retrofit — so a genuine surprise below grade doesn't force a scope cut somewhere else in the project.