My house was built around 1900 and had a '70's extension when we bought it. As we wanted to build on top we had to underpin the extension (poor builders through the ages)
If you were planning a single story extension, Building regs would accept a shallower found than if you were building a two story extension. Think about it,,, a two story extension has much more weight sitting on the founds than a single does. That's why you had to have the original extension foundations underpinned.