We present a systematic study of the hydrogen induced anomalous expansion of the top layer spacing, d₁₂, using low-energy electron diffraction and surface x-ray scattering. After exposure of 6 L of hydrogen at 150 K sample temperature, the hydrogen atoms fully occupy the surface hollow sites and a lattice expansion of ∆d₁₂=+4.7% is determined in agreement with theoretical predictions (5.2%). Heating the sample above the hydrogen desorption temperature (T_D ≈ 340 K), leads to an almost complete relaxation of d₁₂ to the bulk value of 1.945 Å. Similarly, no expansion is observed for clean Pd(001) prepared by rapid cooling after quick thermal treatment to remove hydrogen. Hydrogen re-adsorption from the residual gas atmosphere and possibly hydrogen agglomeration in the near surface region leads to an expansion of d₁₂ in the 2%-3% range as observed in previous experiments. On the basis of our results, surface magnetism as a mechanism to trigger lattice expansion in Pd(001) does not need to be invoked.