An amendment is to be granted unless it would have been struck under R 21 intially

Spar Roofing & Metal Supplies Limited v. Glynn, 2016 ONCA 296:

 

[43]        The respondent's submission is in effect a submission that the court should consider the merits of the factual and legal basis for the proposed amendment at the pleading stage and not at a later stage of the proceedings. That is not the law under r. 26.01. As stated in Todd Archibald, Gordon Killeen & James C. Morton, Ontario Superior Court Practice, 2016 Edition (Markham, ON: LexisNexis Canada, 2015), at p. 1151:

The amended pleading must be legally tenable. It is not necessary to tender evidence to support the claims nor is it necessary for the court to consider whether the amending party is able to prove its amended claim. The court must assume that the facts pleaded in the proposed amendment (unless patently ridiculous or incapable of proof) are true, and the only question is whether they disclose a cause of action. Amendments are to be granted unless the claim is clearly impossible of success.

Put another way, an amendment is to be granted unless it would have been struck out under r. 21.01(1)(b) if it had been pleaded originally: 1317424 Ontario Inc., at para. 7. A motion to strike out a pleading on the ground it discloses no reasonable cause of action or defence must not, however, be conflated with a motion for summary judgment under r. 20.04: see Andersen Consulting v. Canada (Attorney General) (2001), 150 O.A.C. 177 (C.A.), at paras. 34-37; Griffiths v. Canaccord Capital Corp. (2005), 204 O.A.C. 224 (Div. Ct.), at para. 10.