Tridimensional prefabrication of housing has been the object of much literature and few realizations. Finally, enough buildings have been constructed that they can be analysed on site in order to confirm or deny the expectations drawn from integral or nearly integral prefabrication. As we have observed their foundations, it was surprising to find that many or most of their foundations were constructed in si tu, even though prefabricated solutions exist in the industry. This article presents available solutions for prefabricated foundations, it critically describes the foundations of 30 modular housing recently constructed in Catalonia, and it analyses the advisability (or non-advisability) of industrializing the foundations of this type of building.