Abstract: A method for constructing underground garage parking units comprises a step of excavating two parallel trenches into which are subsequently sunk preformed reinforced concrete frames each composed of two vertical pillars with an "H"-shaped cross-section and upper and lower horizontal connecting beams disposed between them. The vertical grooves forming the "H" sections being opposed and constituting a guide for the insertion of a plurality of sheet piles so as to form continuous retaining walls in said trenches. The lower of the plurality of sheet piles being subsequently removable with a sideways movement, after excavation of a corridor by removal of the earth between said two walls, for insertion of prefabricated garages at an oblique or right angle into the vertical portions of earth thus uncovered.