Uracil and its derivatives strongly absorb UV light but photodamage is hampered through effective non-adiabatic decay channels. As a first step towards a quantum dynamical (QD) study of the decay route of the photoexcited ππ* state to the underlying nπ* state, here we present our procedure to build a reliable reduced-dimensionality model of the decay process, and we discuss its theoretical foundation. We established the three most important nuclear coordinates for the decay process and we computed the S1 and S2 excited-state potential energy surfaces of Uracil and 5-fluoro-Uracil in acetonitrile and in water at TD-DFT level, describing the solvent in the frame of polarizable continuum model. Through a property-based diabatization we obtained the diabatic ππ* and nπ* states’ energies and coupling and we fitted them to analytical functions of the nuclear coordinates. We show how these diabatic models can be utilized for QD simulations of the ππ* → nπ* decay.