The spatial development strategy of Slovenia requires the urban densification of town centres, which opens the way to various practices, including the utilization of garret spaces and roof terraces. This approach also offers the possibility to improve their energy and construction, and make technical improvements. “The urban parasite” we used to rehabilitate a run-down and disused terrace of an old apartment block took advantage of the neighbouring construction site. Making use of three cranes, the collaborative effort of their jibs enabled us to lift the huge metallic parts of the “parasite” from the bank of the Ljubljanica River over the high surrounding roofs and inner courtyards on to the terrace of the extension of “Rdeča hiša”. The result is a studio flat of 70 square metres, with a kitchenette and bathroom, hanging like a weightless pavilion over Ljubljana, flirting with Ljubljana Castle. A lightweight facade membrane was put over an innovative metallic structure standing on just two columns, enveloping the vacant inner space. To the general satisfaction of the owner, the total building costs amounted to just about € 35,000. “Urban parasite” is evidence of the versatility and innovative utility in apparently useless urban areas.