A base extension assembly for measuring angles on a radiused corner includes a base that has a blade slot extending therein to insertably receive a blade of a combination square. The base has a channel is integrated therein to receive an anvil of the combination square. The base has a width that is greater than the width of the anvil such that the base extends beyond a radius of a corner is measured with the combination square thereby enhancing accuracy of the combination square. A pair of plates is each of the plates is positionable on the base to retain the anvil of the combination square in the channel.