Finite element approximations of Dirichlet boundary control problems governed by parabolic PDEs on convex polygonal domains are studied in this paper. The existence of a unique solution to optimal control problems is guaranteed based on very weak solution of the state equation and as control space. For the numerical discretization of the state equation we use standard piecewise linear and continuous finite elements for the space discretization of the state, while a dG(0) scheme is used for time discretization. The Dirichlet boundary control is realized through a space-time -projection. We consider both piecewise linear, continuous finite element approximation and variational discretization for the controls and derive a priori -error bounds for controls and states. We finally present numerical examples to support our theoretical findings.