This invention is in the field of automated 3D printing of buildings or structures and method of its operation. A 3D printer having an extendable boom arm with an extruder for extruding a concrete-based chemical solution moves with translations! and rotational motion in an XOY plane. The extendable boom arm is mounted such that it is capable of height adjustment in a XOZ plane. The invention also regards a method for automated 3D printing of a building or structure using the 3D printer.