The design of low-energy office buildings requires specific attention to an energy efficient concept for providing good indoor air quality conditions. With this respect, mechanical ventilation shows undeniable advantages: it can be optimally controlled (infrared detection, CO2 control,...), heat recovery is applicable, outdoor noise and pollution penetration can be minimised. Another crucial challenge in low-energy office buildings is avoidance or, if possible, the minimisation of active cooling needs. Nighttime natural ventilation can play an essential part in an overheating prevention strategy. This paper illustrates these concepts through several consulting projects: * new office building to be occupied by the owner; * new office building by promoter to be rented by several companies; * advanced double facade building in polluted area downtown.