The maid’s quarters you have seen probably are separate residences designated for workers as long as they are employed. That space is her personal dwelling, even though the employer might own all the furniture. As such, her doorways are exempt from mezuzah.
However, a room inside a Jewish home that is informally designated for the use of a domestic helper could still be considered the dwelling of the Jewish owner or renter. Therefore, some authorities advise affixing a mezuzah on such a door without a berachah.
 Teshuvos V’Hanhagos 1:650; Unpublished ruling of R. Asher Weiss; Agur B’ohalecha 30:42; Cf. Aruch HaShulchan Y.D. 286:3.