Abstract Buildings encompass a significant share of overall energy consumption and new houses can promote a shift towards more sustainable societies. Current building regulations only focus on reducing operational energy; however, a life-cycle perspective is important to assess new houses and existing buildings under current (heating and cooling) operational habits. This article assesses the life-cycle non-renewable primary energy improvement potential of a new house compared to an equivalent existing (25-year old) house in the Portuguese context, analyzing alternative operational assumptions: four operation patterns, four heating systems, and two electricity generation mix scenarios. Results show that new houses can effectively reduce the primary energy of residential buildings, but attention should be paid to operational conditions. The new house embodied energy offset period of time is highly dependent on operational patterns. To reduce primary energy associated with new houses, attention should be paid to building material and components, in particular to heavy-weight construction elements, since embodied energy held the majority of the life-cycle impacts. Regarding operation, wood pellets boilers or heat pump systems can significantly reduce primary energy. We also recommend including future electricity generation mix trends in LC studies of houses, which is not common practice, but can influence life-cycle results significantly.