Hexagonal boron nitride (h-BN) thermal neutron detectors have demonstrated the highest detection efficiency among all solid-state detectors (at 58% for a detection area of 1 mm2 and 53% for a detection area of 9 mm2). However, scaling up the detector size of vertical h-BN detectors is challenging due to increased dark current, capacitance, and surface recombination with the increasing detection area. Here, we report the demonstration of a 29 mm2 thermal neutron detector fabricated from a freestanding 10B enriched h-BN epilayer of 90 μm in thickness with a detection efficiency of 50% by employing a lateral device geometry. The lateral detector geometry takes advantage of the unique layered structure of h-BN which naturally provides higher in-plane carrier mobilities than those in the vertical direction. Moreover, due to the reduced area of metals in contact with the h-BN material, the detrimental effects associated with the surface recombination at the metal contacts and device capacitance were reduced, which resulted in improved charge collection efficiency and signal to noise ratios. This work laid the ground work for scaling up to large size neutron detectors based on h-BN.