Unique properties of domain walls in ferroelectrics have been recently discovered but methods of their controlled engineering remain underdeveloped. Here we investigate the domain structure of Bi0.9La0.1FeO3 epitaxial thin films with piezoelectric force microscopy and transmission electron microscopy. We show that with increasing Bi0.9La0.1FeO3 thickness, a transition from randomly-oriented 71 degrees domain walls to 109 degrees domain walls stripe occurs. At intermediate thicknesses, 71 degrees and 109 degrees domain walls coexist together with non-ferroelastic 180 degrees domain walls. TEM cross-section images show complex domain structure due to the interplay between electrostatic constrains imposed by the SrRuO3 electrode and mechanical constrains imposed by the DyScO3 substrate.