Seasonal thermal energy storage systems are important assets to close the temporal mismatch between renewable solar energy generation and thermal energy demand of buildings. Thermochemical storage is an attractive solution for this purpose, as it has a high volumetric energy density and no heat loss over long-duration energy storage. However, thermochemical storage is not yet a mature technology, as its system-level operational potential and efficiency, for example in fulfilling buildings’ space heating demand, has not been investigated. To this end, we developed an optimization-oriented model of aqueous sodium lye (NaOH-H2O)-based closed sorption thermal storage system and integrated it into a techno-economic optimization framework, formulated as a mixed-integer linear programming problem, to determine its optimal size and operation. Its application was demonstrated on a single-family house case study, located in Switzerland, integrating a solar PV, a heat pump and the sorption storage device in its energy system. The optimization aims to minimize the annual costs and CO2 emissions of the energy system, by varying optimal components capacities and their energy dispatch plan. Results were evaluated and compared to a reference system integrating a water tank as a seasonal heat storage. Both the thermochemical and water storage display seasonal and shorter-term charging and discharging cycles, but the thermochemical storage achieves the same level of thermal self-sufficiency as the water one with 7.5 times smaller storage volume. Furthermore, the water storage efficiency tends to decrease with the storage size – and consequently energy storage length, while the thermochemical one remains constant at 73% regardless of its size, highlighting its potential as a seasonal storage solution.