This study presents the spatial analysis of walking accessibility to a public facility in a hilly, multi-level urban environment. Our spatial database includes street topography, physical impedances, formal crossings, and designated access points in assessing the network distance of all private residential buildings to public open space in Hong Kong. Pedestrian movement of uneven speed on walkways of varying gradients is assessed. The study concludes that, compared with our method, conventional buffer analysis and network distance analysis over-estimate the walking accessibility of private housing to neighbourhood open space in Hong Kong by about 2–8%. Despite Hong Kong’s compact built environment, about 15% of the total number of residential blocks cannot reach a neighbourhood open space within 5 minutes of walking (equivalent to the local planning standard of 400 metres of walking on a flat terrain). These open space-deficient neighbourhoods comprise mostly the affluent or middle-class households in gated housing estates and in low-rise housing, but also some neighbourhoods of underprivileged families in old urban tenement buildings. Our assessment reveals the spatial bias of land use planning policy leading to these blackspots of open space shortfall in the territory. It requires urban planners to pay attention to the geographical barriers of a pedestrian network in redressing the inequitable distribution and achieving a pedestrian-friendly 3D city.