Russell Estate v Larson 2017 BCSC 113 contains a good summary of the law relating to amending court pleadings.
Pleadings are very important in litigation so as to give a concise definition of the issues to be tried and to allow the opposing party to have fair notice of the case against them to be met.
29 The applicants referred to the decision of British Columbia (Director of Civil Forfeiture) v. Violette, 2015 BCSC 1372in support of the application. In that case, the court considered the principles to be considered on an application to amend pleadings and noted:
 In Mayer v. Mayer, 2012 BCCA 77 at para. 215, the Court of Appeal affirmed that the fundamental purpose of pleadings is to define the issues to be tried with clarity and precision, to give the opposing parties fair notice of the case to be met, and to enable all parties to take effective steps for pre-trial preparation.
 Applications for leave to amend pleadings are considered on the same basis as applications to strike pleadings with the question being whether it is plain and obvious that the proposed amendments are bound to fail. In assessing that question, it is not determinative that the law has not yet recognized a particular claim. In its analysis, the court must be generous and err on the side of permitting an arguable claim to proceed to trial. See: McMillan v. McMillan, 2014 BCSC 546 at paras. 13-14, and cases cited therein.
[41 In Peterson v. 446690 B.C. Ltd., 2014 BCSC 1531 at para.37, this Court summarized the general principles arising on an application to amend pleadings as follows:
 Finally, the general principles arising on an application to amend pleading can be summarized as follows:
(a) Amendment to pleadings ought to be allowed unless pleadings fail to disclose a cause of action or defence: McNaughton v. Baker,  24 B.C.L.R. (2nd) 17 [(C.A.)].
(b) Amendments are usually permitted to determine the issues between the parties and ought to be allowed unless it would cause prejudice to party’s ability to defend an action: Levi v. Petaquilla Minerals Ltd., 2012 BCSC 776).
(c) The party resisting an amendment must prove prejudice to preclude an amendment, and mere, potential prejudice is insufficient to preclude an amendment: Jones v. Lululemon Athletica Inc., 2008 BCSC 719.
(d) Costs are the general means of protecting against prejudice unless it would be a wholly inadequate remedy.
(e) Courts should only disallow an amendment as a last resort: Jones, McNaughton, Innoventure S& K Holdings Ltd. et al. v. Innoventure (Tri-Cities) Holdings Ltd. et al., 2006 BCSC 1567.
30 Here, in my view, the question turns on prejudice. The position of the plaintiff that the new pleadings will raise matters going back 30 years raises no new prejudice. The entire action stems from matters going back 30 years.
31 The other prejudice argued is the potential expiry of the limitation period, the proximity of the trial date and the potential that the trial date will be lost and the fact that document discovery and examinations for discovery have been conducted on the basis of the amended counterclaim.