Neighborhoods of standardized concrete block buildings form a significant part of the post-socialist build heritage. It is very important to renovate the houses to suit the needs of the current user. The reconstructions typically affect the apartment layout (e. q. uniting the kitchen with the living room, bathroom renovation) and also the indoor environment (thermal insulation, window replacement, installing air conditioning units). One of the alterations that has a major impact on both the apartment layout and the indoor environment is extending the loggia (the standard width is 1,2 meters). The construction companies offer several designs, enabling expansion ranging from 0,2 to 2,5 meters. This article examines the possibilities of loggia extension on a specific example a mass housing neighborhood Velká Ohrada in Prague, built in the years 1988-1993 in the standard system VVU ETA (one of the most frequently used systems in the Czech Republic). The aim of this research is to determine the optimal width of the loggia. Several viewpoints are considered: the functionality of the layout and the indoor environment, particularly the interior lighting conditions and the overheating in summer. The research proves that it is possible to extend the loggia of precast panel building enough to majorly improve the comfort level of the apartments (from the functional and overheating point of view) while still meeting the lighting requirements.