The invention relates to a one-piece hairspring (21, 21′) including, a balance spring (25, 25′) coaxially mounted on a collet (27, 27′), made in the same layer of silicon-based material. According to the invention, the hairspring includes an elevation device (2, 2′) for the outer coil of the balance spring above the layer of silicon-based material so as to improve the concentric development of the hairspring. The invention also relates to a timepiece including a hairspring of this type and the method of manufacturing the same. The invention concerns the field of timepiece movements.