When it was built, Mellomåsen Hospital was an extension of the old Rønvik Psychiatric Hospital. The natural formation Mellomåsen serves as a visual divider between the two hospitals, with Mellomåsen Hospital located on the ridge east of Rønvik Hospital.

Mellomåsen consists architecturally of interconnected units that form a U-shape, with the main entrance in the middle and departments extending to both sides. Between the two rows of buildings lies a sheltered garden. The entire facility is situated along the top of the ridge, and the various brick-clad parts with sloping roof surfaces rise one and two stories above the ground. The facility is relatively low, which provides easy accessibility and good contact with the terrain.