Reconstructing of old buildings, we want to use building space as much as possible. Therefore, the basement’s rooms are adapted as a useful area to employ. Since the height of the basement’s rooms often is too small the floors must be pulled down and where it is necessary the foundations must be deepened using best design decisions. Often is used method of undermining under foundations. Using this method it is necessary to solve the following questions: excavation depth and width; the distance between the excavations which are made at the same time; the influence of different settlement on building structures; methods that reduce the effects of different settlements. The excavations change the distribution of the pressure under the foundations. The excavation depth and width must be calculated considering to the pressure increasing under reconstructing foundations. The pressure must not exceed the bearing capacity of the ground. The settlements are inevitable when the foundations are deepened. As the foundations are deepened by separate sections, settlements are different. Reducing the effects of the different settlements, special design decisions must be used."