An apparatus and method for marking availability of a candidate coding block for merge estimation of a current coding block within a coding tree unit, CTU, which includes multiple coding blocks. Initially, the candidate coding block is marked as available. The candidate coding block is marked as unavailable when a predefined location of the candidate coding block is included within an extended merge estimation region, MER. The extended MER includes a current MER in which the current coding block is located and at least a portion of another MER, adjacent to the current MER.