Hydrogen blending in residential gas networks requires assessment of gas transport across connected rooms and of explosion consequences if a flammable cloud forms. This study used FLACS-CFD to compare hydrogen blending ratios (HBRs) of 0, 5, 10, 15, and 20 vol%, the upper bound corresponding to the blend level demonstrated in distribution networks, in an accident-informed multi-compartment apartment. Two independent scenarios were evaluated: a 30 s release through a 6 mm opening at 0.025 barg and an explosion of a prescribed uniform cloud at an equivalence ratio of 1.05. Across 16 monitoring points, the maximum predicted fuel concentrations were 0.06575–0.07111 vol%, corresponding to 1.31–1.49% of the HBR-specific lower flammability limits. From HBR 0% to 20%, the maximum monitored concentration increased by 8.2%; the local maximum increased by 71.6% above the source but decreased by 16.3% outside the living room window. Under the independent explosion condition, maximum overpressure increased from 0.872 to 1.246 barg, maximum monitored pressure impulse increased by 21.3%, and the ignition-to-peak-pressure interval decreased by 12.7%. Analytical inventory estimates and comparisons with published data provided physical context for the calculated HBR-dependent trends, whereas the absolute values remain scenario-specific. By extending the comparison to connected apartment compartments, this study provides a numerical basis for selecting measurement locations and response metrics in future residential hydrogen-blending safety studies.