Diamond semiconductor has attracted attention for use in next-generation high-power, low-loss, high-radiation proof power devices. Heavily B-doping of diamond produces low-resistivity contacts that enhance the maximum output current, however, unintended lattice strain cause defect formations, degrading the device performance. In this study, we investigated the doping-induced lattice strain of heavily B-doped (100) films fabricated by hot-filament chemical vapor deposition. Xray reciprocal space mapping indicated coherent, pseudomorphic growth between film and substrate. The perpendicular lattice strain as a function of B concentration followed Vegard's law, which accounts for substitutional incorporation. Synchrotron X-ray absorption fine structure showed no signature of lower-coordinated B-clusters. We believe that maintaining pseudomorphic growth is a key factor in suppressing electrically non-active B impurities.