Light pollution in recent years has drawn surging public concerns in Hong Kong. The improper management of the light sources causes pollution, particularly light trespass on the community level, which obstructs the residents. Yet more studies are expected to quantitatively and systematically assess the severity of the artificial light trespass problem, and to offer practical solutions for mitigation and urban management. This study was conducted on a typical high-density, mixed-use neighbourhood in Tsim Sha Tsui district. The study consists of on-site measurement and model simulation. Three illuminance measurements were conducted to evaluate the impact of light pollution on the streets and light trespass on the nearby buildings. It was found that the impacts on the buildings (mean illuminance of 3 lx) are less significant than on the streets (mean illuminance of 188 lx) for the target study area. Indoor measurement at various height levels showed a mean illuminance level of 0.6 lx, which is much lower than the international limit of 25 lx. Meanwhile, the modelling work, consisting of correlation, verification and regression analysis, was carried to bridge luminous flux from light sources and illuminance received at the target. A three-dimensional (3D) model was developed to mimic the actual situation and was used to identify the primary light sources causing impacts. While a growing number of research is found on measuring and modelling the intricate light pollution in such a micro scale that elevates explicitly the importance of extrapolating the light pollution effect vertically from street level to building level, direct simulation is still lacking. This study also refined and summarised the research methodology to serve as a feasible and organised protocol for setting up the measurement and modelling work. This can be adapted in future assessments on light trespass issues under similar circumstances elsewhere, like scientific research, urban planning, or property development projects.