In construction terms, underpinning is a process whereby the foundation of an existing building or other structure is strengthened and/or stabilised. This process may be necessary for several reasons. For example, in older properties, the original foundation that was laid is not strong or stable enough to withstand the weight of the building. Modern building regulations are typically much stricter and more involved than those used in the past. However, even modern buildings may require underpinning if the original building processes were not up to the required standards.
Other reasons for underpinning a property might include a change in the way the building is used, or a change in the soil underneath the property. The latter can lead to subsidence as soil gradually shifts underneath the property over a long period. This might also be necessary if the design of the property did not accurately identify the nature of the soil in the area where the building was built. Another reason might include the construction of nearby structures that requires the excavation of soil that supports the existing foundations.
One thing all these scenarios have in common is that underpinning is the most economical way of solving whatever issue has arisen with the foundations. Building a new foundation would be very difficult and costly. It is always much more affordable and sensible to work with the structure that is already there. It could be that only a portion of the existing structure requires work, for example, in which case a whole new foundation would not be required anyway.