The growing demand for environmentally sustainable energy alternatives promotes the incorporation of different renewable energy technologies. District heating systems, implemented extensively in central and northern European nations, are essential for the conversion and usage of renewable sources. The increasing hydrogen production has created important possibilities to recover the waste heat. However, the determining the cost-optimal dimensioning of energy systems in buildings, considering the utilization of waste heat from hydrogen generation, is still in its early stages. The present work investigated the cost-optimal dimensioning of energy systems which supply energy for a kindergarten and an apartment using the 25-year life cycle cost. A ground source heat pump is base heating and there is district heating backup. In addition, photovoltaics was installed for electricity generation, and some was used for the heat pump. The analysis had two district heat tariffs: the monthly district heat price provided by a company and the renewable district heat price obtained from hydrogen production waste heat. The results indicate that, for the ground source heat pump, the optimal dimensioning covers 20–30% of the total dimensioning heating power. The kindergarten's higher peak heating power results in the power fee imposed by the peak power dominating the overall cost of district heating. Thus, there is a negligible influence on dimensioning when adjusting the district heat energy tariff. However, for the apartment, it has a significant impact of 10% due to a reduced peak power.