In New York, a party's request for a court order modifying a marital separation agreement is governed by the same contract law principles that apply to other types of contracts. Claims to set aside or modify provisions in matrimonial settlement or separation agreements must be asserted through separate, plenary actions, and may not be pursued by way of motions in a divorce cases.

In the case Frieland v. Frieland, 606 N.Y.S.2d, 200 A.D.2d 484 (1st Dept. 1994), New York's Appellate Division for the First Department summarized the procedure that a party who wishes to set aside a marital separation agreement must follow:

The stipulation of settlement was a separation agreement that was incorporated but not merged into the judgment of divorce. As such, it survives as a separately enforceable contract that cannot be set aside by motion but only by a plenary action.

The impropriety of attacking the terms of a separation agreement as unfair through a motion in a matrimonial matter - rather than through a separate plenary action - is notably one of the principals of matrimonial law upon which all four of New York's judicial departments are in full agreement. E.g. Kellman v. Kellman, 559 N.Y.S.2d 549, 162 A.D.2d 958 (4th Dept. 1990)(refusing to consider defendant's claim that the parties' separation agreement was "unfair" and holding that "Defendant is relegated to a plenary action to rescind or enforce the agreement under principles of equity"); Granato v. Granato, 859 N.Y.S.2d 132, 51 A.D.3rd 589 (1st Dept. 2008)(separation agreement may not be set aside by motion); Dombrowski v. Dombrowski, 657 N.Y.S.2d 208, 239 A.D.2d 460 (2nd Dept. 1997)(rejecting challenge to terms of separation agreement because "the parties may challenge the terms of such an 'independent contract' only by way of a plenary action").

A party seeking to set aside a marital agreement must establish the existence of compelling circumstances which would permit the Court to set it aside. Such circumstances may include fraud, collusion, mistake, or duress, or that the agreement is unconscionable or contrary to public policy. The mere fact that a party to the agreement is unhappy with its terms and provisions is not sufficient.

Fraud, in particular, involves a detailed analysis of the particular circumstances which led to the parties' execution of the Agreement. Because the pleading requirements for fraud cases in New York are more stringent than for other types of cases, a party seeking to set aside a marital agreement should set forth in his or her complaint specific details which give rise to the conclusion that the Agreement was entered into in circumstances that suggest fraud.

A review of case law suggests that courts are more likely to set aside marital separation agreements when one of the parties establishes that he or she was induced to enter the agreement by the other party's misrepresentation of one or more material facts. For example, in a recent decision by the Second Department, the court held that the Defendant had presented a triable issue of fact as to whether the parties' marital agreement should have been set aside where the circumstances presented to the trial court below suggested that the Plaintiff had submitted appraisals that materially undervalued certain marital property. Reiner v. Reiner, 874 N.Y.S.2d 143 (2nd Dept. 2009).

Another factor that courts consider is the length of time between the execution of the agreement in question and the assertion of a claim to set aside the subject agreement. In Ricca v. Ricca, 870 N.Y.S.2d 419 (2nd Dept. 2008), the Appellate Division for the Second Department ruled that by waiting until two years after the agreement was executed, plaintiff was estopped from attacking the agreement as unfair:

In any event, since the plaintiff accepted the benefits of the stipulation of settlement, and substantially complied with its terms for almost two years, he ratified the stipulation by his conduct (see Korngold v Korngold, 26 AD3d at 359; Brennan v Brennan, 305 AD2d at 525).

Id.

In summary, recent case law suggests that parties interested in obtaining judgments setting aside marital settlements should: (a) proceed as quickly as possible after a separation agreement has been signed; and (b) present detailed facts setting for the manifest unfairness, or other fatal deficiency, mandating such relief.