Some situations require you to hang on to tax records for longer than the six-year retention period advised by CRA. Here are a few examples:
-If you receive property as a gift or inheritance, you are deemed to inherit it at the fair market value (FMV) at the time of the gift or death. In such events it is recommended you keep a copy of the schedule from the donor’s or deceased’s return indicating the reported proceeds for as long as you own the property.
-Purchase agreements, T3s, or other documents showing the adjusted cost base (ACB) of an investment. For example, if you bought a stock in 1990 and sell it in 2016, you will need to hang on to the ACB information until 2021.
-Various tax election forms have relevance for much longer than the 6-year CRA window. For example, in 1995 a one-time election (Form T664) could be made to to bump up the tax cost of your property by a maximum of $100,000. If Form T664 was previously filed, and you later want to claim that property under the principal residence exemption, CRA requires that she file Form T2091 (even if no tax would be payable)
For the full list of examples, please read the following Advisor.ca article.