We investigate the structural and superconducting properties of single-phase YBa2Cu3O7+y powder samples oxygenated under high-pressures of 4–6 GPa using KClO3 or AgO as oxydizing agent. A structural refinement of neutron and synchrotron x-ray diffraction data, supported by thermogravimetric analysis and Raman spectroscopy, indicates a large increase of oxygen concentration up to y=0.4 with respect to optimal doping, y=0. The corresponding structural changes smoothly follow the trend observed in the underdoped region, y < 0. Notably, we observe a progressive occupation of the initially empty oxygen sites between adjacent CuO chains, while the CuO2 planes are unaffected. According to a bond valence sum analysis of the copper and oxygen sites, 3/4 of the extra holes increase the valence of Cu in the chain site up to 3+, while the remaining 1/4 are transferred to the CuO2 planes. As a result, we estimate an increase of hole density, p, in the planes up to 0.27 hole/Cu, corresponding to the heavily overdoped region of the superconducting dome. A vanishing electronic specific heat at low temperatures rules out the possibility of unpaired holes, so the extra holes are expected to contribute to the superfluid density. In spite of this, neither the onset of the superconducting transition, Tc, nor the superconducting fraction change as compared to optimally doped samples. Instead, the transition is broadened, which suggests a nonuniform distribution of superconducting holes in the planes and thus the formation of overdoped regions with reduced Tc.