Solar systems can supply space heating and domestic hot water to houses covering a large part of the demand by using solar energy. The use of long-term (seasonal) storage systems is fundamental to reach high levels of solar fraction for the application of these systems in cold climates. Building-scale systems are an attractive solution despite a higher specific installation cost of the tank, because they require less space and have smaller distribution networks which helps reduce thermal losses and costs. However, small-scale systems have restraints on reaching high levels of solar fraction. The integration of a heat pump unit in the system can be strategic to increase the performance level of the system and to enhance the use of solar energy. This study examines the integration of a modulating water-to water heat pump in a solar system equipped with a seasonal storage. The heat pump uses the water of the seasonal tank as the heat source, exploiting the residual heat stored in the tank at the end of the heating season. The system performance is assessed through a number of energy simulations using TRNSYS. A similar solar system without the heat pump is used as a reference system to compare the results and assess the impact of the heat pump integration. The results show that the solar fraction can be increased by approximately five percentage points by including a heat pump.