When a building is build it applies a load to the soil. The compression makes the soil stronger. When the soil has been compressed sufficiently, it becomes strong enough to support the building. During this process the building settles down until it reaches equilibrium. Some cracks will occur during this process. When a extension is added to an existing building it also undergoes an initial settling process.
This can cause movement to occur between the original part and the new extension.After initial settlement the movement should stop. Cracking is usually minor. They are non progressive but if re- pointed they will recur seasonably