K.S.A. 60-215.Amended and supplemental pleadings

Subsection (a) gives every party one free amendment. A pleading not requiring a response can be amended as a matter of course within 21 days of serving it. A pleading that does require a response can be amended as a matter of course within 21 days after a responsive pleading is served, or within 21 days after a motion under 60-212(b), (e), or (f) is served, whichever comes first. Beyond that one free shot, subsection (a)(2) requires either the opposing party's written consent or the court's leave -- and the court should give that leave freely when justice requires it. When a party does amend, subsection (a)(3) gives the opposing side the later of the time remaining to respond to the original pleading or 21 days after the amended pleading is served.

Subsections (b)(1) and (b)(2) handle amendment once trial has started. If a party objects that evidence offered at trial falls outside the pleadings, the court may allow the pleadings to be amended, and should do so freely when amendment would help present the merits and the objecting party can't show real prejudice; the court can grant a continuance to let that party respond to the new evidence. When an issue outside the pleadings is tried by the parties' express or implied consent, subsection (b)(2) treats it as if it had been pleaded from the start -- a party can move to amend to match the evidence at any time, even after judgment, though failing to make that motion doesn't change the outcome of the issue tried.

Subsection (c) is where the stakes of relation back show up -- an amendment that would otherwise miss the deadline gets treated as filed on the date of the original pleading. That happens when the law governing the statute of limitations itself allows relation back, when the amendment merely adds a claim or defense arising from the same conduct, transaction, or occurrence already set out (or attempted to be set out) in the original pleading, or when the amendment changes the party being sued and, within the time allowed for commencing the action against that party (including the service period under K.S.A. 60-203), the new party received enough notice of the action to avoid prejudice in defending on the merits and knew or should have known the suit would have named it but for a mistake about the proper party's identity. Subsection (d) lets a party, on motion and reasonable notice, serve a supplemental pleading describing something that happened after the original pleading was filed, even if that original pleading was itself defective in stating a claim or defense, and the court can set a deadline for the opposing party to respond.