CRA states that it will accommodate failures to make a GRE designation in the first estate return

To be a graduated rate estate, the estate, amongst more substantive requirements, must designate itself as a GRE in its first T3 return. CRA stated:

If the designation for the trust to be a GRE is omitted in error, an adjustment request containing the elements required for the designation and the rationale for the omission from the original filing may be submitted to the CRA.

No time limitation was stated.

Neal Armstrong. Summary of May 2016 Alberta CPA Roundtable, Q.18 under s. 248(1) - graduated rate estate – para. (d).