Diffraction of X-rays in conditions of total external reflection (grazing-incidence diffraction–GID) is a powerful method for defects’ diagnostics in subsurface layers and thin films. In a given paper, the dynamical model of diffuse X-ray scattering in crystals, which contain subsurface dilatation-centre-type defects, is developed. Diffuse scattering near the specular reflected and specular diffracted coherent peaks provides the main information about subsurface lattice distortions. New approach developed for calculation of the diffuse scattering amplitude is based on the distorted wave Born approximation (DWBA). Fourier components of lattice distortions are expressed by exact formulas within the scope of the continuous approximation, which takes into account the image forces caused by surface boundary. The intensity maps calculated for diffuse grazing waves scattered by subsurface defects randomly distributed in a flat layer are presented. For different scanning schemes, the conditions of predominantly diffuse component registration in grazing diffraction are defined.