A first-principles-derived scheme is developed and used to investigate the diffuse character of the paraelectric-to-ferroelectric transition in ultrathin films made of BaTiO3, and to reveal its dependency on mechanical and electrical boundary conditions, as well as on thickness. It is found that such diffuse character does not require the presence of defects to occur, unlike commonly believed, but rather can originate from surface-induced dipolar inhomogeneities in defect-free films.