The present invention encompasses a method, on a building site, of forming a stable and, in terms of weight, heavy modular unit under the utilization of prefabricated, in terms of weight considerably lighter, modular unit (12) transported to the building site, where said heavy modular unit (12a) can be adapted to serve as a complete modular-built cellar unit. Said modular unit displays a base slab (14) and double-walled wall sections (10, 11) anchored to the base slab. Inner and outer wall portions (10, 11) are mutually coordinated and secured to one another via supports (18) oriented within a free space formed and structured between the wall portions. The free space is completely filled with a concrete mass, which is caused to cure or set, in order in such instance to form the in terms of weight heavy modular unit. The present invention also discloses a heavy modular unit thus produced.