The GeN-Foam multi-physics code enables coupled thermal-hydraulic, neutronic, and thermal-mechanical analysis of fast reactor cores with 3-D unstructured deformable meshes. Advances within the Horizon2020 ESFR-SMART project, as well as trends towards higher-fidelity core-wise simulations motivated further developments of the code thermal-hydraulics, based on a coarse-mesh porous medium approach. These developments enable the single-phase characterization of the thermal-hydraulic impact of the inter-assembly gap and assembly wrapper windows in steady state and transient scenarios. The inter-assembly gap constitutes a non-negligible heat sink in accident scenarios in most SFR designs. Conversely, wrapper windows are an ESFR innovative feature meant to alleviate the adverse effects of sodium boiling. However, these will alter the single-phase thermal-hydraulics as well. This work presents development and verification efforts aimed to enable the coarse-mesh thermal-hydraulic analysis of these features. An analysis of the impact of these features is performed on a representative bundle of 7 ESFR assemblies.