A crucial way to reach future sustainable society concern the path towards nearly-zero energy buildings because of large amount of energy at stake. The present work proposes an approach for the optimal integration of small-scale technologies (renewable and traditional) to enhance the pathway of existing and inefficient buildings towards low-carbon system in a cost-benefit effective manner. Operation optimization as well as an innovative combined design and operation optimization are investigated with the goal of selecting the capacity of the technologies to be installed depending on the expected operations. The renewable technologies are integrated with proper storage units, such as batteries and latent thermal storage, which allows reducing the space required for the installation. Two different non-linear programming approaches are used with the aim of finding an optimal solution. The optimization allows reducing operation costs of 22% for RES fed dwellings. The combined operation and design optimization leads to a reduction of installation and operating costs of 7%. In the analyzed case, the adoption of the advanced optimization approach shows that latent heat storage is more suitable to be installed than electric storage (−2.5% cost).