We report on the quantitative investigation of lateral domain wall motion in BaTiO₃ single crystals subjected to a compressive unidirectional mechanical stress. Simultaneous to the mechanical testing, the single crystals prepared by the modified exaggerated growth method were characterized by scanning force microscopy and piezoresponse force microscopy (PFM) which allow both topographical details and the true three-dimensional ferroelectric domain configuration to be reproduced simultaneously. Stress induced domain formation is initiated at the sample surface followed by the forward- and lateral-domain growth both perpendicular and parallel to the direction of induced stress. Knowing the crystallographic orientation of the BaTiO₃ single crystal (from Kikuchi patterns) clearly associates our experimental observations with a 90° domain switching process, in accordance with a theoretical model. Additionally, 180° ferroelectric domain boundaries (a-domains) were detected with PFM which are not visible from the sample topography. The formation of these newly formed domains is driven by the compensation of the positive surface charge arising from the ferroelastic growth of C⁺ domains.