Laser-metal additive manufacturing (AM) inevitably yields residual stresses due to the coexistence of both high thermal gradients and stiff mechanical constraints and thus, favoring part warpages as well as cracking potential [1]. Although some strategies have been developed to control the accretion of residual stresses by reducing thermal gradients, they fail to prevent the stress concentrations at the built basement and impact other physical fields involved in AM [2]. Thereby, this work focuses on the optimal design of the inner structure of the substrate to reduce its local mechanical stiffness and, consequently, the stress accumulation during AM. Both selective laser melting (SLM) and directed energy deposition (DED) processes are considered to examine the effectiveness of the substrate design. In detail, the SLM group consists of two T-shape parts while three structures with increasing geometrical complexity (single-wall, rectangular and block parts) are included in the DED set. All components are first printed on the standard plates and the modified substrates with grooves or movable tenons, respectively, accompanying in-situ measurements in order to validate the coupled thermo-mechanical model for AM used in this work. The comparison between the numerical predictions and the measured data presents a good agreement. Next, the validated finite element software is employed to analyze the complex interactions between the structure-dependent thermal and mechanical response of AM-parts made on substrates with different structural designs. The results show that the stress concentrations and related cracks appear at the interface between the AM-builds and the typical solid substrates. Contrariwise, the designed substrates successfully avert the generation of residual stresses and cracking, while not compromising the thermal and metallurgical evolution and resulting mechanical hardness. This provides a possibility to separately solve different physical problems, widening the AM process window.