In this contribution we present a low cost, extremely simple, and highly stable scheme to update a standard microscope into a holographic microscope. The proposed architecture is named as SMIM (incoming from the initials of spatially-multiplexed interferometric microscopy) and it is based on a common-path interferometric configuration which is adapted into a conventional microscope with some specific constraints to allow holographic recording. The main layout modifications are three: i) the use of a coherent light source instead of the broadband one included in the microscope, ii) the insertion of a properly placed one-dimensional diffraction grating needed for the holographic recording, and iii) the use of spatial multiplexing at the input plane to allow reference beam transmission in a common light-path with the imaging branch. As consequence of the input plane spatial multiplexing, the field of view provided by the used microscope objective is reduced in comparison with the field of view reported by the same objective lens in conventional white light illumination mode. However, complex amplitude distribution of the inspected sample is retrieved after off-axis holographic recording and conventional digital image processing of the recorded hologram. The proposed update is experimentally validated in a standard upright microscope by using coherent illumination incoming from a commercial grade laser diode, by inserting a one-dimensional precision Ronchi ruling grating in the microscope embodiment, and by dividing the input plane into the spatially multiplexed areas. Experimental results are provided for the different microscope objectives included in the Olympus microscope showing calibration (USAF resolution test) as well as biological (red blood cells and sperm cells) images containing complex (amplitude and phase) information.