delivered the Opinion of. the Court.
Petitioner the Town of Carbondale ("the Town") filed suit against Respondent GSS Properties, LLC ("GSS") concerning GSS's construction activities and use of agricultural chemicals on a tract of land located above the, Town's water supply. The Town asserted, inter alia, that its local watershed ordinance prohibited the use of such chemicals. Shortly before trial, GSS moved to amend its answer to state a preemption defense. In particular, GSS asserted an operational conflict between the Town's ordinance and various state statutes. Before the court ruled on GSS's motion to amend, GSS reasserted its operational preemption defense in a motion for partial summary judgment.
The trial court denied GSS's motion to amend, subsequently denied GSS's motion for partial summary judgment, and did not permit consideration of the preemption defense at trial. On appeal, the court of appeals reversed and remanded. See Town of Carbondale v. GSS Props., LLC,
We now reverse the court of appeals. We hold that the fact that the operational preemption defense was raised in a summary judgment motion did not, in and of itself, constructively amend GSS's answer. We also hold that, because the Town would have been prejudiced by the untimely defense, the trial court did not abuse its discretion in not allowing GSS to proceed with the defense at trial. Finally, we hold that GSS's operational preemption defense is waivable because it does not challenge the subject matter jurisdiction of the trial court. In other words, the trial court would have retained jurisdiction over the lawsuit regardless of whether state law or the Town's ordinance was applied. Because we conclude that GSS's operational preemption defense was not successfully raised, we do not consider the merits of that defense.
I.
In 1999, GSS purchased a fifty-five acre tract of land in Pitkin County, Colorado for the purpose of creating a ranching operation and mountain resort. Shortly thereafter, GSS began construction on the land and instituted a weed management program intended to eradicate noxious weeds and dandelions from the property.
On June 15, 2001, the Town filed suit in Pitkin County District Court, alleging that dirt from the construction site polluted Nettle Creek, the primary source of drinking water for the Town, and that GSS's use of herbicides, fertilizers, and pesticides threatened to cause additional contamination. The Town further alleged that GSS's activities constituted a public nuisance and sought to enjoin these activities under the Town's watershed protection ordinance until GSS implemented "the necessary application and storage controls to prevent contamination of the Town's public water supply."
On June 22, 2001, the Town and GSS entered into a stipulation to resolve the dispute over GSS's use of herbicides and other agricultural chemicals. However, the Town's newly elected board ultimately rejected this compromise in May 2002, stating that its goal was to preclude the use of all agricultural chemicals on the property. The trial court subsequently denied GSS's motion to enforce the stipulation. GSS had filed its answer on September 19, 2001, and the time for amending pleadings under the case management order had lapsed in January 2002. The court had set the case for trial in October 2008.
In June 2003, approximately four months prior to the commencement of trial, GSS filed a motion to amend its answer to state an affirmative defense that the Town's ordinance was preempted by state law. Specifically, GSS asserted that there was an operational conflict between the application of the Town's watershed ordinance and various state statutes. The Town opposed GSS's *678 motion to amend, arguing that the new defense would require significant additional discovery and the retention of additional experts. Before the trial court ruled on this motion and approximately three months before trial, GSS filed a motion for partial summary judgment, reasserting, inter alia, the preemption defense. 1
On August 4, 2003, the trial court denied GSS's motion to amend, reasoning that the amendment would "substantially change the nature of the trial and put the trial date in jeopardy." GSS then sought relief from this court pursuant to C.A.R. 21, which we denied. |
On August 15, 2003, the Town responded to GSS's summary judgment motion. In pertinent part, the Town argued,
The Court ruled in its Order of August 4, 2003 [denying GSS's motion to amend the answer], that these defenses may not be asserted by GSS in this case at this june-ture due to the close proximity of trial. ... As such, the Town does not presently intend to further respond to the preemption issue unless the Court determines that GSS' preemption defenses may go forward.
Two months later, on October 3, 2003, the trial court also denied GSS's motion for partial summary judgment. The order did not specifically address GSS's operational preemption defense, although it "recognize[d] that the issues raised in the [summary judgment] motion may be appropriate to address at mid-trial or other appropriate stages of the trial to the court." Because GSS had asserted a number of issues in its summary judgment motion, the court's order was ambiguous as to whether GSS could argue its operational preemption defense at trial. __
The ambiguity, however, was resolved at the outset of trial. When counsel for the Town attempted to discuss the preemption issue in his opening argument, the trial court immediately stopped him, saying,
Excuse me, my order having denied the amendment to the answer and the Supreme Court's refusal to grant an Order of Prohibition puts this issue to rest, does it not? The question is not one of whether the Town has authority.... That has been litigated. That is done. That is the law of the case.
Later, the trial court referred to GSS's operational preemption defense as an "interesting and complex question which was not successfully raised in the pleadings in this case and will not be reached by this court." Counsel for GSS did not argue preemption or attempt to introduce evidence on the issue at trial.
Following the bench trial, the court concluded that GSS violated the Town's watershed ordinance and ordered GSS "restrained from storing, mixing, applying or disposing of pesticides, herbicides, fertilizers or chemical compounds on the property in any manner that would pollute the Town's water supply as defined in the ordinance." 2
GSS appealed and the court of appeals reversed, ruling that GSS should have been permitted to present its operational preemption defense at trial. The court remanded the case to the trial court because it determined that additional fact-finding was necessary before ruling on the merits of GSS's preemption defense. We now reverse the court of appeals.
II.
The court of appeals gave two grounds for its decision. First, the court reasoned that although the trial court was within its discretion to deny GSS's motion to amend its answer to add the operational preemption defense, 3 GSS had constructively *679 amended its answer by raising the defense again in its motion for partial summary judgment. The court also found "no basis" in the record for concluding that such a construe tive amendment would prejudice the Town. Second, the court raised the question of whether the affirmative defense of operational preemption could actually be waived. We disagree with both grounds.
A.
The Town contests the court of appeals' holding that GSS's operational preemption defense was incorporated into the answer-despite the trial court's previous denial of GSS's motion to amend-because GSS raised the defense in its motion for partial summary judgment. The court cited two of our decisions to support this proposition: Bebo Construction Co. v. Mattox & O'Brien, P.C.,
Previous court of appeals' opinions have also suggested that an affirmative defense asserted for the first time in a summary judgment motion will be treated as incorporated into the pleadings. See, e.g., Trujillo v. Farmers Ins. Exch.,
In Bebo, we noted that "in some instances, an affirmative defense asserted for the first time in a motion for summary judgment will be deemed to be incorporated into the defendant's answer."
Additionally, it is significant that in both cases, the trial court-hearing no timeliness objection from the opposing party-found the affirmative defense meritorious and granted summary judgment. Bebo,
Bebo and Cox do not stand for the proposition adopted by the court of appeals in this case that asserting a defense in a summary judgment motion is, in and of itself, sufficient to amend the pleadings. Such a "rule," as the court of appeals put it, see Town of Carbondale,
Properly understood, Bebo and Cox do not apply to the circumstances of this case. First, unlike the situation presented by those cases, the opposing party-here, the Town-made a timeliness objection to the newly raised affirmative defense at every opportunity. The Town initially objected to GSS's motion to amend on the ground that it would require significant additional discovery and the retention of additional experts. The Town again raised its objection in its response to GSS's motion for partial summary judgment, arguing that GSS had waived its operational preemption defense and stating that it would not address the merits of the preemption defense because the trial court's denial of GSS's motion to amend barred GSS from raising it. Thus, in no way can it be said that the Town consented to the amendment of the pleadings.
Second, unlike the plaintiffs in Bebo and Cox, the Town would have been prejudiced if GSS had been allowed to proceed with its operational preemption defense. To determine whether an operational conflict exists, a trial court must determine whether the effect of the local ordinance materially impedes or destroys the state interest. Bd. of County Comm'rs v. Bowen/Edwards Assocs.,
Finally, we note that, unlike in Bebo and Cox, the trial court here did not reach the merits of GSS's preemption defense. While the trial court's summary judgment order was ambiguous as to whether preemption was one of the issues it considered appropriate to re-address at trial, the court cleared up this ambiguity when it referred to GSS's preemption defense as an "interesting and complex question which was not successfully raised in the pleadings in this case and will not be reached by this court." In short, none of the factors important to our decisions in *681 Bebo and Cox-i.e., no timeliness objection, no prejudice, and a ruling on the merits-is present here.
If the opposing party objects to an untimely defense, as occurred in this case, the trial court has discretion to disregard the objection and allow amendment of the pleadings. See C.R.C.P. 15(a). However, prejudice to the opposing party is a reason to deny amendment. Polk,
If a defense is not raised in the answer or through a successful amendment of the answer, it is waived. See Duke v. Pickett,
B.
In addition to its reliance on Bebo and Cox, the court of appeals stated that the "application of the rule allowing affirmative defenses to be raised for the first time in a summary judgment motion is particularly appropriate in cases where, as here, the defense is preemption." Town of Carbondale,
As noted above, affirmative defenses must be timely pleaded, and failure to do so results in waiver. See C.R.C.P. 8(c); Duke,
*682
We have never considered whether preemption is a jurisdictional-and therefore a non-waivable-defense. Courts considering the matter have held that the waivability of a preemption defense depends entirely on the nature of the alleged preemption. If, as in most cases, the: alleged preemption would simply alter the applicable substantive law governing the case, then preemption is waivable. See, e.g., Saks v. Franklin Covey Co.,
Conversely, if preemption "affects the choice of forum rather than the choice of law," then preemption is akin to a jurisdictional challenge and therefore is not waivable. Gorman,
This distinction between preemption based on choice of law and preemption based on choice of forum is consistent with the general distinction between waivable and non-waivable defenses recognized by Colorado law. Compare Triebelhorn,
Having adopted this distinction, we conclude that GSS's preemption defense is waivable because it concerns the choice of law to be applied by the trial court, not whether the trial court had jurisdiction to hear the parties' dispute. GSS does not argue that the trial court was an improper forum for its defense, only that the substance of the Town's ordinance was preempted by state statute. This is a preemption defense based on choice of law. Consequently, GSS's defense is waivable and, as noted above, was in fact waived in this case.
We recognize that preemption is a somewhat unique affirmative defense because it is oftentimes a private litigant like GSS, not the state itself, that seeks to enforce state law. Similarly, we note that in cases involving express or implied preemption, there will be little or no prejudice to the opposing party from an amendment to the answer to add an untimely preemption defense because, unlike operational preemption, those forms of preemption are primarily matters of statutory interpretation. See Bowen/Edwards Assocs.,
*683 Finally, we recognize that preemption involving federal law may raise a separate set of issues, including, for example, the application of the federal Supremacy Clause. These issues, however, are not present in the case at bar.
IIL.
We hold that the fact that GSS raised the operational preemption defense in a summary judgment motion did not, in and of itself, constructively amend GSS's answer. We also hold that, because the Town would have been prejudiced by the untimely defense, the trial court did not abuse its discretion in not allowing GSS to proceed with the defense at trial. Finally, we hold that GSS's operational preemption défense is waivable because it does not challenge the subject matter jurisdiction of the trial court. Accordingly, the judgment of the court of appeals is reversed, and the trial court's judgment and order are reinstated.
Notes
. GSS also argued that (1) the water court had exclusive jurisdiction over certain claims alleged by the Town, (2) the Town failed to state a claim on which relief could be granted because the Town did not allege contamination of the water source, negligent application of the chemicals, or other violation of the water protection ordinance, and (3) the Town's claims of erosion were moot.
. The trial court also found GSS negligent in causing erosion that contaminated the Town's water supply and awarded damages to the Town in the amount of $8,389. However, GSS did not appeal that portion of the trial court's judgment.
. Preemption of a local ordinance by state law can take one of three forms. The first two arise when the state legislature, either expressly or impliedly, demonstrates its intent to occupy a
*679
given field. As their names suggest, express preemption occurs where the statute's express language demonstrates this intent, and implied preemption occurs where the legislative intent is implied "by reason of a dominant state interest." Bd. of County Comm'rs v. Bowen/Edwards Assocs.,
. GSS offers a number of reasons for why it did not raise its operational preemption defense earlier in the proceedings, including (1) the complaint did not allege that GSS's use of chemicals on the property violated the local watershed ordinance, (2) the Town only later argued for a total prohibition of chemicals on the property, and (3) the Town repeatedly represented to the trial court that settlement remained a realistic possibility. GSS's arguments are unavailing. First, the complaint was clearly premised on the local watershed ordinance: It recited the language of the ordinance, particularly highlighting the sections that protected against the introduction of pollutants or contaminants into Nettle Creek. Second, the complaint pursued an injunction to "restrain[] the use ... of chemical herbicides or pesticides" unless GSS implemented the necessary safeguards to prevent contamination and the Town approved those safeguards. This conditional language served as fair warning to GSS that the Town might find no safeguard safe enough. Finally, the possibility of settlement does not excuse insufficient pleadings.
. A party's ability to raise the defense of lack of subject matter jurisdiction is not completely unbounded. In particular, the doctrine of issue preclusion prevents a party from raising the defense in a collateral attack against a final judgment. In re Water Rights of Elk Dance Colo., LLC,
