Large-scale seasonal thermal energy storage (sTES) systems play a crucial role in the transformation of district heating systems towards sustainable and renewable-powered systems. However, planning and optimization of sTES remains challenging due to complex interactions between design, hydrogeological conditions, and economic feasibility. Thus, this study investigates buried tank thermal energy storage (TTES) using a three-dimensional multiphysics model in COMSOL Multiphysics® focusing on insulation configuration, groundwater interactions, and storage volume. Results show that upscaling reduces geotechnical costs by 40 % and increases energy efficiency by more than 15 % for volumes between 100,000 m3 and 2,000,000 m3. Insulation distribution strongly influences performance: inhomogeneous layouts improve energy capacity efficiency by 2–3 % and reduce the levelized cost of stored heat by up to 4 €/MWh compared to homogeneous insulation. In contrast, when groundwater is absent, omitting insulation can be more cost-effective for medium-sized tanks (