Multiferroic BiFeO₃ exhibits a morphotropic phase boundary at large compressive strain that merges metastable phases of tetragonal (T) and rhombohedral (R) character resulting in giant ferroelectric and electromechanical responses. To utilize this functionality in devices, it is essential to understand the response of these ferroelectric phases to the environment of a nanoscale heterostructure. Here, the emergence of ferroelectricity near the morphotropic phase boundary in BiFeO₃ is explored directly during thin-film growth, using optical second harmonic generation. It is found that the epitaxial films form at the growth temperature purely in the T phase with zero critical thickness for the spontaneous polarization. Signatures of monoclinic T-like and R-like phases only appear upon sample cooling. The robustness of a single-domain configuration in the high-temperature T phase is furthermore demonstrated during growth of capacitor-like metal | ferroelectric | metal heterostructures. Here, a reduction in tetragonality, rather than multidomain formation, lowers the electrostatic energy in the few-unit-cell thickness regime. For this lower tetragonality, density-functional calculations and scanning transmission electron microscopy point to the stabilization of a novel metastable monoclinic structure upon cooling toward room temperature. The synergistic combination of strain and electrostatic phase stabilization in BiFeO₃ heterostructures hence provides a basis for designing new ferroelectric phases and ultrathin ferroelectric devices.