The increasing importance of shallow geothermal resources in the decarbonization of heating and cooling systems requires the correct management of all the project stages. One of the fundamental steps in this process is determining the space energy demand, which plays a significant role in the subsequent geothermal design. In the context of Spain, different tools are available for the estimation of the mentioned parameter. For evaluating these procedures, this research applies the principal energy demand calculation tools and uses the outcomes for the later design of the shallow geothermal system. Results show how the Spanish official tools (HULC and CE3X) provide lower energy demand values adjusted to the construction conditions of the building that allow the optimization of the geothermal well field. On the contrary, simpler, and more intuitive applications (regular spreadsheets and GES-CAL) assume higher heating energy demands, which in turn implies an oversizing of the geothermal scheme. Even though all the procedures ensure to cover the energy requirements of the building, the most precise tools manage to reduce the initial investment of the system and its operating costs, in addition to reducing the global CO2 emissions because of the lower power of the associated geothermal heat pump.