We present some results concerning boundary optimal control problems and related initial-boundary value problems. We prove the existence and uniqueness of the solution of a parabolic saddle point problem, as well as the existence and uniqueness of a penalized and an iterated penalized saddle point problem. Moreover, we derive semidiscrete error estimates for the finite element approximation of the penalized saddle point problem, and semidiscrete error estimates for the penalized and unpenalized heat equation with nonhomogeneous boundary data under minimal regularity assumptions. Finally, we use the above results for the analysis and finite element analysis of boundary optimal control problems having states constrained to parabolic partial differential equations.