The application of phase change materials (PCMs) in the building envelope can improve the thermal performance and the indoor thermal comfort conditions because they allow the proper storage or release of energy. In this work, three different solar assisted heat pump underfloor heating systems with and without the use of phase change material are designed, simulated and evaluated energetically for a building of 100 m(2) floor area. Different kinds of solar collectors (flat plate, photovoltaic, thermal photovoltaic) are coupled to a storage tank which feeds a heat pump for space heating purposes. The PCM is used on the underfloor heating system in order to increase the storage capacity and to avoid the operation of the heat pump during the peak electricity demand period. Furthermore, this work presents and compares the coefficient of performance, the electricity consumption, the solar coverage and the indoor temperature distribution of a building in Athens (Greece), with and without a PCM layer on the floor. More specifically, the PCM is placed on the underfloor heating system and different cases are examined by changing the collecting area, the collector type and the thickness of the insulation layer in the floor. The results prove that the use of the PCM layer on the underfloor heating system reduces the heating load by about 40%. The electricity consumption can be reduced between 42% and 67% using the solar-driven systems. It is also found that the system with the thermal PV consumes the lowest grid electricity. Moreover, the lowest simple payback period is found to be 9.61 years for the system with flat plate collectors without PCM. The simulation is conducted with TRNSYS for all the winter period.