OPINION
¶ 1 Appellani/cross-appellee City of Tucson challenges those portions of the trial court’s judgment determining that the City’s “vacant lot” ordinance, Tucson Code § 3-59(a)(6)(b), was unenforceable against appellee/cross-appellant Whiteco Metrocom, and that four of Whiteco’s billboards were not subject to removal. Whiteco cross-appeals the portion of the trial court’s judgment ordering it to remove eleven of its billboards. We affirm in part, and vacate and remand in part.
¶ 2 In this action for equitable relief, the City sued Whiteco, seeking the removal of several billboards, claiming the billboards violated the City’s vacant lot ordinance. The City also claimed some of the billboards were impermissibly expanded, relocated, or changed, and thereby lost the protection of the nonconforming use statute, A.R.S. § 9-462.02. The trial court granted Whiteco’s motion for partial summary judgment, declaring that § 9-462.02(B) prohibited the City from enforcing the vacant lot ordinance against Whiteco. After a bench trial on the remaining claims, the court entered detailed findings of facts and conclusions of law, permitted Whiteco to retain three billboards, ordered Whiteco to remove the second advertising face from another billboard, and ordered Whiteco to remove eleven of the contested billboards because they had lost their nonconforming use status.
APPEAL
A. Vacant Lot Ordinance
¶3 The City first argues that the trial court erred in refusing to enforce the vacant lot ordinance. The relevant facts are undisputed. The billboards that are the subject of this action were erected on undeveloped land before the City enacted its vacant lot ordi *393 nance. In 1985, the City enacted § 3-59(a)(6)(b), 1 which provides:
Billboards are prohibited on any developed property. Any existing billboard must be removed before a certificate of occupancy will be issued for a development on any undeveloped parcel which is subject to the requirements of section 23-409, sections 23-534 through 539 or the Uniform Administrative Code, Section 302.
¶ 4 After the ordinance was enacted, Whiteco sued the City in federal court, claiming that the ordinance was unconstitutional and exceeded the authority granted the City by § 9-462.02(A). The City stipulated that it would not enforce the ordinance against Whiteco while the lawsuit was pending.
¶5 In 1993, the Ninth Circuit Court of Appeals declared the ordinance constitutional and enforceable.
Outdoor Systems v. City of Mesa,
A municipality shall not require as a condition for a permit or for any approval, or otherwise cause, an owner or possessor of property to waive the right to continue an existing nonconforming outdoor advertising use or structure without [paying for or relocating the use or structure].
¶ 6 After subsection (B) was enacted, but just before it became effective, the City sued Whiteco seeking removal of the billboards. The City moved for partial summary judgment, claiming that subsection (B) could not be applied retroactively to bar enforcement of § 3-59(a)(6)(b) as to parcels developed after the ordinance was enacted but before the effective date of subsection (B). Whiteco also moved for partial summary judgment, claiming that subsection (B) prohibited enforcement of § 3-59(a)(6)(b) against the billboards in issue because the City’s cause of action had not been litigated to judgment prior to the effective date of subsection (B). The trial court granted Whiteco’s motion and denied the City’s.
¶ 7 At the outset, we note that the City has attempted to raise new arguments on appeal. Below, the City made a threadbare argument that subsection (B) should not apply retroactively because building permits for each contested property at issue were issued prior to the effective date of subsection (B). This argument, at least implicitly, is reflected in the City’s argument on appeal that subsection (B) impairs the City’s vested rights. On appeal, however, the City argues for the first time that subsection (B) affects substantive rights; that application of subsection (B) violates Arizona judicial policy and the separation of powers doctrine; and that statutory amendments imposing monetary obligations on the government may not be applied retroactively. Several of these arguments were first raised in the City’s reply brief. Except for the City’s vested rights argument, which is the only argument implicitly raised below, we will not address the City’s other arguments regarding subsection (B) which were raised for the first time on appeal.
ABC Supply, Inc. v. Edwards,
¶8 We review the application of subsection (B) to these facts de novo.
Schwarz v. City of Glendale,
*394
¶ 9 Municipalities do not have vested rights in their municipal powers because the legislature has virtually unlimited authority over them; “[i]t may incorporate or disincorporate them; it may add to or take away from their territorial area; it may grant to or ' take away from them such powers as it may see fit.”
Udall v. Severn,
¶ 10 The City’s power to enact zoning ordinances derives exclusively from the state.
Levitz v. State,
¶ 11 The City cites
In re Dos Cabezas Power District,
¶ 12 Finally, “[a] cause of action depending solely on statute is not a vested right protected by the Constitution.”
Brown Wholesale,
¶ 13 The City’s action is wholly dependent on § 9-462.01 which was modified by § 9-462:02(B). Subsection (B) prohibits the City from requiring the removal of billboards as a condition of issuing a permit or “otherwise caus[ing]” their removal other than by purchase, condemnation, or relocation. This broad language withdraws the City’s power to remove billboards under its vacant lot ordinance as presently written. Because the City had not prosecuted its lawsuit to a final judgment before the effective date of subsection (B), it was without power to continue the action seeking to require the removal of the billboards under Tucson Code § 3-59(a)(6)(b).
*395 ¶ 14 We conclude that the City did not have vested rights in the powers granted by the state under §§ 9-462.01 or 9-462.02, that the City’s cause of action was not a vested right because it was entirely dependent on the grant of authority in § 9-462.01, and that the filing of the lawsuit did not convert the cause of action into a vested right. Partial summary judgment in favor of Whiteco on this issue, therefore, was proper.
B. Second Advertising Face
¶ 15 The City contends that the trial court erred in ordering Whiteco to remove the second advertising face of a billboard located at 2844 North First Avenue, rather than ordering Whiteco to remove the billboard entirely after the trial court concluded that the addition of the second advertising face “was improper and not permitted.”
¶ 16 The billboard was constructed as a single-faced billboard pursuant to a permit issued by the City. It became a nonconforming use after the City amended its sign code to prohibit billboards within a certain distance of one another. Whiteco later added a second advertising face to the billboard without obtaining a permit.
¶ 17 The City contends the billboard lost its nonconforming use status because “[t]he addition of a second advertising face to the billboard was a substantial change and resulted in virtually the creation of a new sign,” and the addition, therefore, “effectively created an illegal sign subject to removal.” Whiteco argues, however, that the Tucson Code allows billboards to have two advertising faces.
3
This argument is irrelevant as to whether the billboard lost its nonconforming use status; the relevant inquiry is whether the property remains unchanged or whether a change constitutes a reasonable repair or alteration.
See Arizona Foundation for Neurology & Psychiatry v. Sienerth,
¶ 18 Although adding the second face to the billboard was an impermissible expansion of the nonconforming use, the Tucson Code does not require that a billboard be removed when it has been expanded.
See
Tucson Code § 3-91(“No nonconforming sign shall be
moved,
altered,
4
removed and reinstalled, or replaced,
unless it is brought into compliance with the requirements of [the sign code].”) (emphasis added). This ordinance is vastly different from the ordinance in
State ex rel. Western Outdoor Advertising Co. v. Missouri Highway and Transportation Commission,
¶ 19 In actions for equitable relief, we defer to the trial court’s discretion in fashioning the remedy.
Scholten v. Blackhawk Partners,
C. Illegal locations
¶20 The City next challenges the trial court’s ruling that three billboards, located at 3400 North Fairview Avenue, 401 South Park Avenue, and 5513 East Golf Links Road, could not be removed because the billboards were constructed “in conformity with permit specifications approved by the City” and, therefore, “the City is estopped from asserting any violation of [a] zoning or building code in existence at the time of the issuance of the permit[s].”
¶ 21 The City first contends it cannot be estopped from removing the billboards because “Millboards constructed at illegal locations obtain no vested right[s] and are therefore subject to removal,” citing
Outdoor Systems v. Arizona Department of
Transportation,
¶ 22 The elements of equitable estoppel are: “(1) the party to be estopped commits acts inconsistent with a position it later adopts; (2) reliance by the other party; and (3) injury to the latter resulting from the former’s repudiation of its prior conduct.”
Valencia Energy Co. v. Arizona Dep’t of Revenue,
¶23 To estop the City, therefore, the trial court would be required to find that the three general elements of estoppel applied, that a “serious injustice” would result from the City’s conduct, and that the public interest would not be “unduly damaged” by the application of principles of estoppel. Valencia Energy; Freightways. This balancing test requires the court to analyze, for each billboard, a variety of factors, including the presence of long-term leases, recent investments or expenditures on the billboard, any benefits derived from the billboard owner’s lengthy use of the billboard, the presence of other billboards in the area or within the city, the character of the neighborhood, and the alleged violation which created the nonconforming use. Even if estoppel would have been appropriate at the time the permit was originally issued, the trial court should address whether estoppel is still appropriate given the current circumstances.
¶ 24 The trial court’s findings of fact and conclusions of law do not address these issues, and therefore, we are unable to uphold the trial court’s judgment that the City should be estopped from asserting that the permits for the three billboards were issued in violation of then-existing zoning or building code regulations. “Where possible, when a trial court in a non-jury case fails to make or makes insufficient findings of fact and
*397
conclusions of law, a reviewing court should remand the case to the trial court for further findings.”
Miller v. Bd. of Supervisors,
CROSS-APPEAL 5
¶ 25 Whiteco contends the trial court erred in requiring WTiiteco to remove eleven of its billboards, claiming that the City should be estopped from seeking removal of any of the billboards, or alternatively, that replacement of the structural supports of two of the billboards and relocation of four of the billboards did not justify their removal.
6
We are bound by the trial court’s findings of fact unless they are clearly erroneous, but review questions of law de novo.
Scottsdale Unified Sch. Dist. No. 48 of Maricopa County v. KPNX Broadcasting Co.,
A. Estoppel
¶26 Whiteco first asserts that the City should be estopped from seeking the removal of any of the challenged billboards, because they “have been in place for the last 20 years or more ... [and n]ot once during this extensive period of time did the City ever indicate that there was any problem with the billboards or the sign permits the City’s own Sign Code staff had issued.” As to each of these billboards, however, the trial court found that WTiiteco either constructed the billboard in violation of a permit, or relocated or removed and replaced the billboard without a permit. It was Whiteco’s conduct, therefore, that triggered the termination of the nonconforming use status; the City did nothing upon which Whiteco could rely. Under these circumstances, estoppel does not apply.
See United Bank v. Mesa N.O. Nelson Co.,
B. Structural Changes
¶ 27 WTiiteco next argues that the trial court erred in ordering it to remove two billboards, one located at 1090 North Swan Road and the other at 2699 East Valencia Road. The two billboards had been constructed with a “twin I-beam support structure,” which Whiteco had replaced with a “uni-pole structure” without first obtaining a permit. The City’s sign code provides that “[n]o nonconforming sign shall be moved, altered, removed and reinstalled, or replaced, unless it is brought into compliance with the requirements of [the sign code].” Tucson Code § 3-91. WTiiteco contends, however, that changing the support structure is “merely a ‘reasonable repair or alteration’ protected by A.R.S. § 9-62.02.”
¶ 28 Under similar facts, Division One of this court held that “such a change in the structural support system of the billboard does not constitute a ‘reasonable alteration’ within the meaning of [§ 9-462.02].”
Gannett Outdoor Co. v. City of Mesa,
¶ 29 Whiteeo next contends the court in
Gannett
never directly decided whether “replacement of a billboard’s supporting structure was a permissible repair or alteration.” As to the issue of alterations, this is clearly incorrect.
See
Gannett,
8
Although the court in
Gannett
never considered whether replacement of the support structure was a “repair” under § 9-462.02 because
Gannett
did not assert that its activities were repairs, we have considered the issue. We conclude that the complete replacement of the support structure is not a “reasonable repair” under § 9-462.02.
See Park Outdoor Advertising Co. v. Commonwealth,
¶ 30 Under the Tucson Code, Gannett, Park Outdoor Advertising, and the public policy favoring removal of nonconforming uses, these billboards lost their nonconforming status when Outdoor replaced their twin I-beam support structure with a uni-pole structure. The trial court properly ordered Whiteeo to remove them.
C. Relocated Billboards
¶ 31 Whiteeo next argues that billboards located at 1090 North Swan Road, 10 North Kolb Road, 7111 East Broadway Boulevard, and 3656 East Grant Road did not lose their nonconforming use status when they were moved from one location on a parcel to another location on the same parcel. The Tucson Code prohibits moving a nonconforming billboard unless the billboard is brought into compliance with the sign code, see § 3-91, and it is not alleged that any of the billboards now comply with the sign code. Whiteco’s actions, therefore, caused the billboards to lose their nonconforming status unless Whiteco’s actions were protected under § 9-462.02.
¶ 32 Relying on
Outdoor Systems v. City of Mesa,
¶33 Whiteeo attempts to distinguish a case upon which the City relies,
People ex rel. Department of Public Works v. Ryan Outdoor Advertising,
¶ 84 Although § 9-462.02 does not specifically prohibit new “placings” of nonconforming billboards, it only protects Whiteco’s right to make reasonable repairs and alterations. Ryan Outdoor supports the City’s argument that relocating a billboard is not a routine repair or alteration and goes beyond the protections of § 9-462.02. Furthermore, the holding in Gannett suggests that the removal and replacement of a sign for any reason, even if re-erected in precisely the same location, is a violation of the sign code, and not protected by § 9-462.02. Whiteco’s relocation of the signs, therefore, was not a reasonable repair or alteration.
¶35 Further, because § 9-462.02 does not restrict the City’s power to prohibit Whiteco from moving or removing and replacing the billboards, the prohibition in the ordinance is valid and enforceable.
Cf. In re Estate of Tovrea, 173
Ariz. 568, 573,
¶ 36 The trial court did not err in finding that moving the signs was not “reasonable repair or alteration” of the nonconforming use and in ordering the removal of these billboards.
CONCLUSION
¶ 37 The trial court’s judgment is vacated and remanded as to the billboards located at 3400 North Fairview Avenue, 401 South Park Avenue, and 5513 East Golf Links Road. In all other respects, the judgment is affirmed.
Notes
. Originally, the ordinance was numbered as Tucson Code § 3-39(3)(6)(b).
. The issue of retroactivity is complex and problematic. See generally Jill E. Fisch, Retroactivity and Legal Change: An Equilibrium Approach, 110 Harv. L.Rev. 1055 (1997), and Jan G. Laitos, *394 Legislative Retroactivity, Wash. U.J. Urb. & Contemp. L. 81 (1997). Whiteco’s argument that subsection (B) pertains to remedies, and may therefore apply retroactively, has merit, but we do not decide the case on that basis.
. Whiteco did not cite to any authority or refer to specific portions of the record supporting its argument, and therefore failed to comply with Rule 13, Ariz. R. Civ.App. P., 17B A.R.S. Whiteco's alternate argument, that the City should be estopped from removing the billboard, similarly fails to comply with Rule 13. We reject this estoppel argument on the same grounds as those discussed in Whiteco’s cross-appeal.
. We note that the Tucson Code provision prohibiting alterations directly conflicts with § 9-462.02. The City does not, however, argue that the addition of the second billboard face was an "alteration”; it argues that the addition was an expansion or addition, citing Sienerth.
. Whiteco failed to designate in its Combined Answering Brief and Opening Brief which arguments related to the cross-appeal. We address those arguments challenging the trial court’s grant of partial judgment for the City in this section.
. Other than its estoppel argument, Whiteco has not raised on appeal any other challenge to the removal of the six billboards located at 3655 East Golf Links Road, 6300 East Golf Links Road, 6427 East Golf Links Road, 6450 East Grant Road, 630 West Speedway Boulevard, and 1302 South Park Avenue.
. Whiteco's claim that Gannett is distinguishable based on its procedural history may stem from its quotation of an unrelated portion of the decision in Gannett concerning whether the superior court exceeded its jurisdiction in issuing its order. Division One’s conclusion that the superior court exceeded its jurisdiction, however, did not impact its conclusion that a change in the structural support system was not a reasonable alteration.
. The first paragraph of Gannett states:
In this appeal, we consider the issue of whether the replacement of an existing multipole billboard structure with a new mono-pole structure of the same dimensions constitutes a "reasonable alteration” of non-conforming property within the meaning of A.R.S. § 9-462.02. We conclude that such a change in the structural support system of the billboard does not constitute a "reasonable alteration” within the meaning of the statute.
