The anomalous Hall effect (AHE) in magnetic systems is typically governed by symmetry constraints that require the Hall response to be proportional to the out-of-plane magnetization component. Here we demonstrate the emergence of an unconventional in-plane AHE in a low-dimensional heterostructure. By interfacing a low-symmetry topological semimetal with a ferromagnetic insulator, we realize a system with reduced symmetry in which only a single mirror plane is preserved. When the magnetization acquires a finite component within this mirror plane, the remaining symmetry is broken, enabling a Hall response that depends on both in-plane and out-of-plane magnetization components. Measurements across multiple devices reveal a gate-tunable AHE, indicating electrostatic control of the underlying mechanisms. A minimal symmetry-constrained microscopic model shows that interfacial spin–orbit coupling and exchange interaction are responsible for the observed multidirectional AHE response. Our work establishes a pathway for engineering tunable, symmetry-driven Hall effects in low-dimensional quantum materials.