This picture tells the story. The bricks of the foundations to the old house are 1.2 metres deep, with the bricks resting on concrete which is 60mm thick at its thickest, but generally about 30mm thick, and about 30mm wider than the bricks themselves This is resting on the white clay soil.
On the other hand, the foundations of the new building, and of this extension, are 1.8 metres deep. The bricks rest on a footing of reinforced concrete (containing ten longitudinal reinforcing rods and regular lateral rods) that is 400 mm thick and 1,000mm wide, resting on a layer of gravel placed over the clay soil: massive by comparison.
In another place, the old foundations do descend 1.8 metres like our new construction, but again they rest on very thin concrete, and in this place that concrete footing is no wider than the bricks themselves.
The old building has stood up very well over the twenty-five years since its erection: we can expect at least as good a performance from this new one.