A corner bracket for assembly of two boards of a window mounting collar into a collar corner and for affixing said window mounting collar to an exterior face of a building façade so as to surround a window opening, said corner bracket; comprising two holding plates and two base arms, all four adapted for extending in planes parallel with and having back surfaces abutting the exterior face of the building façade in an affixed position of the collar. The bracket further comprises two flanges and two abutment plates adapted to extend away from said exterior face in an affixed position of the collar. Each abutment plate is positioned at a mutual distance from and in parallel with an associated said flange, each abutment plate being connected to the associated said flange via an associated said holding plate; so as to be able to hold said boards together to form said window collar corner, each board being received between a respective flange and its associated abutment plate.