System for fixing to the ground walls of buildings

System for fixing walls of a building to a ground, comprising a plurality of supporting elements interposed between a concrete foundation of the building and said walls, further comprising connecting elements for connecting at least two supporting elements that are aligned with each other, adjusting elements for adjusting a distance of said supporting elements from said foundation and for correcting a possible tilt of said supporting elements with respect to a horizontal reference plane, anchoring elements for anchoring said supporting elements to said foundation, said supporting elements being made of plastics, each of said supporting elements comprising a pair of sides, interconnected by a plurality of crosspieces, wherein through ventilation holes are made in said crosspieces, wherein a lower part of each side has a length in a direction parallel to a longitudinal axis of the supporting element that is less than a length of a rest of the side in the same direction, such that a step is defined between said lower part and the rest of the side, at opposite ends of the side that are perpendicular to said longitudinal axis, wherein said connecting element comprises a body having a height that is substantially the same as a height of said step and a length that is substantially the same as a width of said supporting elements in a direction that is perpendicular to said longitudinal axis, at opposite ends of the body respective uprights being provided that are adapted to be fixed to the sides of two adjacent supporting elements to be connected, said uprights extending perpendicularly to the body.