It depends on a variety of factors. Underpinning and rebuilding are both methods of strengthening or stabilizing a foundation, but they differ in cost and effectiveness depending on the specific circumstances of the building and its foundation.
Underpinning is generally a less expensive option than rebuilding, as it involves reinforcing an existing foundation rather than completely tearing it down and starting over. However, underpinning may not always be the most effective solution if the existing foundation is severely damaged or if the building has undergone significant structural changes over time.
Rebuilding, on the other hand, may be a more expensive option but can offer a more permanent solution to foundation problems. It allows for a completely new foundation to be constructed, which can be designed to meet the specific needs and requirements of the building.
Ultimately, the decision to underpin or rebuild will depend on a range of factors, including the severity of the foundation problem, the age and condition of the building, and the budget and timeline for the project. It is recommended to consult with a qualified structural engineer or contractor to assess the situation and determine the best course of action.