The main structure, with pillars, beams, decks, and bracing walls, is site-cast concrete. The roof structure is laminated wooden trusses covered with metal sheets. All exterior walls are faced with red brick. This gives the facility a unified character. The windows appear as bands of varying heights in the brick wall.

Mellomåsen was the first facility in Nordland where a percentage of the building's cost was allocated to artistic embellishment. In the inner garden stands the work "Cormorants" by sculptor Ellen Jacobsen (b. 1945).