In the current work a building integrated solar thermal system with seasonal storage is optimized with the use of TRNSYS modeling software in order to evaluate different integration options of the solar collector array. The model calculates the space heating needs during the heating period as well as the annual domestic hot-water needs of a typical single-family detached home in the city of Thessaloniki, Greece that has been built according to the latest building code. The contribution of the solar system and the thermal load covered by the auxiliary conventional system are determined, for both space heating and domestic hot water, considering the respective simulation periods. The relative solar fractions as well as the combined annual solar fraction are also calculated. A parametric analysis on the impact of various solar collector areas and types, building integration type, as well as the volume of the seasonal storage tank is also presented in order to optimize system design and obtain a seasonal combined solar fraction of at least 39%. (C) 2018 Elsevier Ltd. All rights reserved.