Single-crystal metal-halide perovskites hold significant promise for optoelectronic applications due to their tunable physical properties and the possibility of low-cost, low-temperature synthesis. Compared to their polycrystalline counterparts, they exhibit reduced defect densities and enhanced stability. Their intrinsically soft lattice facilitates integration with conventional semiconductors via heterostructures. However, their high ionic mobility can lead to interdiffusion processes that compromise the integrity of adjacent layers, making the formation of well-defined interfaces a critical challenge for device optimization. Here, we exploit the temperature dependence of the perovskite growth kinetics to demonstrate a one-pot, space-confined growth method for synthesizing vertical 2D/3D lead-halide perovskite heterostructures in single-crystal form. The process leverages differences in precursor solubility to drive sequential crystallization and create well-defined interfaces. Structural and optical analyses confirm the formation of stable, phase-separated heterostructures, which are promising for optoelectronic applications.