Abstract: Construction activities adjacent to or beneath existing buildings during urban underground development require reliable information on pile foundations to ensure construction safety and control settlement. However, investigations of existing pile foundations in complex urban environments are frequently constrained by incomplete foundation records, restricted site conditions for surface geophysical surveys, and the inherent ambiguity of interpreting a single geophysical method. To overcome these limitations, this study proposes an integrated borehole geophysical investigation approach combining borehole magnetic surveying, borehole ground-penetrating radar, and parallel seismic testing under drilling constraints. These methods provide complementary constraints on the bottom of the reinforcement cage, pile-shaft interface, and pile tip by exploiting magnetic anomalies, electromagnetic reflection and diffraction responses, and elastic-wave velocity variations, respectively. To validate the proposed approach, it was applied to the Xiangjisi Road West Extension Project in Hangzhou, where a shield tunnel passes beneath existing buildings. Borehole magnetic surveying identified the bottom of the reinforcement cage at a depth of 9.50 m. Borehole GPR delineated the pile-shaft boundary and pile-tip diffraction, indicating a pile-tip depth of 9.75 m, whereas PST identified the pile tip at a depth of 9.25 m based on first-arrival travel times and velocity variations. Integration of the three datasets, together with drilling verification, identified a pile length of 9.50 m, consistent with the drilling result. The results demonstrate that the proposed integrated borehole geophysical approach effectively reduces the interpretational uncertainty of individual methods and improves the reliability of existing pile foundation investigations in complex urban environments.