The mixing unit for a mixer tap has a generally cylindrical shape defining a main axis and comprises a cartridge comprising a mixing chamber traversed by the main axis, wherein the mixing chamber comprises intakes of the incoming stream and an outlet for an outgoing stream. The mixing unit comprises means for mixing the incoming steams to form the outgoing stream contained in the mixing chamber, and an additional casing extending out of the mixing chamber. According to the invention, the mixing unit comprises a single-piece common part which delimits the mixing chamber, while forming at least a portion of the additional casing. The common part forms a separation wall between the cartridge and the additional casing, extending in a plane that is orthogonal to the main axis, wherein the intakes and the outlet are formed through the separation wall.