The term light well, better known in Croatian language by its German term “ lichthof” is defined as a space within a building serving to bring air and light to rooms surrounding it. In the Croatian residential architecture they first appeared at the turn of the 19/20th century due to the great building depths where entrance halls and some service rooms had to be orientated to light wells. The Zagreb Building Regulations stipulated the number, size and light well characteristics. The examples of this type of residential buildings dating from the beginning of 20th century in Zagreb (Croatia) show a whole array of plan types. The Italian examples from the same period show the whole richness of this typology used in very luxurious apartments. At the end of the 1950s this type at least in Croatia disappeared due to the small building depths as well as to the allowed use of artificial ventilation and illumination in some dwelling parts (corridors, bathrooms, toilets). But at the beginning of this century especially in the POS projects (State subsidized housing) this type reappeared as an answer to the great building depths but this time of inadequate dimensions and with too many rooms orientated to it. It appeared as a sign of the time and of its values. Today this negative space (not saleable) is reduced to the minimum and is degraded and as well degrading the spaces that caused its reappearance