We present a new block budgeting algorithm which speeds up timing closure in timing driven hierarchical flows. After a brief description of the addressed flow, block budgeting challenges are detailed. Then, we explain why existing budgeting approaches are not adapted to fulfil these challenges. A new block budgeting algorithm is proposed. In order to derive relevant block constraints, this algorithm analyzes the design flexibility. This flexibility aware budgeting (FAB) approach is then compared to some previous ones. Experiments based on commercial EDA tools and real designs show up to 55 % reduction in hierarchical flow run time and lead to a good flow timing closure.