Complex oxide thin films exhibit intriguing phenomena due to the coupling between multiple degrees of freedom through interfacial structural engineering. Atomic tailoring of structural parameters determines unique band structure and phonon modes, regulating emergent magnetic and electrical properties of oxide films. However, the construction of different strained and oriented domains in one intact oxide thin film is impossible using conventional means. Here we report the fabrication and quantitative structural analysis of La_0.7Sr_0.3MnO₃ (LSMO) homostructures assisted by atomic-flat freestanding membranes. Pristine substrates and suspended membranes regulate the epitaxial strain and orientation of subsequently grown films. Our results demonstrate an ultrathin transition layer (~ 4 atomic layers) between freestanding membranes and LSMO films is formed due to the strain relaxation. This work offers a simple and scalable methodology for fabricating unprecedented innovative functional oxide homostructures through artificially controlled synthesis routes.