I dissent for the reasons given below.
The majority opinion has three holdings: First, it concludes that under Government Code section 66413.51 the developer here was not entitled to approval of its final subdivision map because its application for a vesting *282tentative subdivision map was not made within the statutory time constraints. The developer, however, conceded this point in its opening brief to this court, as it did below. Accordingly, the majority’s holding pertaining to section 66413.5 is irrelevant to a resolution of this case.
Second, the majority holds that the city in question is not equitably estopped by its conduct—participation in and encouragement of the developer’s efforts to clear conditions imposed on the tentative map—from withholding final subdivision map approval. Because of my conclusion that the city was required under section 66474.1 to approve the developer’s final map, I need not reach the estoppel issue.
Third, the majority concludes that the city, after its incorporation and notwithstanding its adoption and readoption of the county’s land use regulations, retained discretion to withhold ministerial approval of the developer’s final subdivision map. I disagree. In my view, the city lacked such discretion.
I.
The property in dispute consists of 14.46 acres within the coastal zone of the County of Santa Barbara (County). In 1993, the County adopted the Goleta Community Plan, which designated the parcel as lying within an affordable housing zone. Two years later, the County approved development of the site with 105 residential units, half of them affordable housing. The County prepared and certified a final environmental impact report for the proposed development. The proposed development, however, was never built.
In June 1999, Oly Chadmar Sandpiper General Partnership (Sandpiper) bought the parcel. On July 4, 1999, a petition to incorporate Goleta as a city received its first signature.
On November 18, 1999, Sandpiper submitted to the County its vesting tentative map application calling for construction of 109 residential units, with 20 percent affordable housing. The County deemed the map application complete on January 1, 2000.
On November 6, 2001, voters residing within the boundaries of the proposed city passed a measure to incorporate Goleta.
On January 15, 2002, the County board of supervisors approved Sandpiper’s development plan and its vesting tentative map for the 109-unit project.
On February 1, 2002, Goleta’s incorporation as a city took effect. Under section 57376, the County’s subdivision ordinances automatically became the *283city’s subdivision ordinances for 120 days. The city council did not adopt superseding ordinances, and the only textual change it made to the county ordinances was to substitute “City of Goleta” for “County of Santa Barbara” and “City Council” for “Board of Supervisors.” Ten days later, the City of Goleta imposed a 45-day moratorium on approvals of all developments, including this one. Thereafter, the City of Goleta, on March 25, 2002, exempted Sandpiper from the moratorium because its project included affordable housing and based on a finding that the project posed no threat to public health, safety or welfare. (§ 65858, subd. (c)(1).)
In June 2002, after the statutory 120 days had elapsed and the City of Goleta had not enacted its own subdivision ordinances superseding those of the County, it readopted the County’s subdivision ordinances without change. (§ 57376.) In November 2002, a County employee acting as the city’s agent submitted Sandpiper’s final subdivision map to the Goleta City Council for its approval, having found the map to be technically correct and consistent with the city’s subdivision ordinance, which provided that the city “shall approve the map at its next regular meeting.” The city council held four regular meetings, and then denied final map approval on January 6, 2003. Its resolution denying final map approval stated, “There is not a reasonable probability that the project will be consistent with the [city’s] general plan,” then still in preparation.
II.
The issue presented is simple: Did the newly incorporated City of Goleta have discretion to deny approval of the developer’s final subdivision map that substantially complied with a tentative vesting map previously approved by the County? The answer is “No.”
In 1998, the Legislature enacted section 66413.5, as part of the Subdivision Map Act (§ 66410 et seq.; Map Act), in response to uncertainty as to what rights were retained by a developer who had obtained county approval of a tentative vesting subdivision map once the acreage of the proposed subdivision became part of a newly incorporated city. (Off. of Local Gov. Affairs, Enrolled Bill Rep. on Sen. Bill No. 186 (1987-1988 Reg. Sess.) Aug. 24, 1988, p. 3; Sen. Corns, on Local Gov. and Housing & Urban Affairs, Joint Hearing Rep. on New Cities and Land Use (Nov. 1987) Background, pp. 24-25.) In 1980, an Attorney General’s opinion had concluded that under general provisions of the Map Act a new city’s approval of a final conforming map was a purely ministerial duty. (63 Ops.Cal.Atty.Gen. 844, 848 (1980).) In 1985, the Legislative Analyst reached a similar conclusion (Legis. Analyst, Vesting Tentative Maps: Incorporation or Annexation, Opn. No. 15919 (July 15, 1985) p. 8), in an analysis prepared for Senator Joseph *284Montoya, who later authored the bill that became section 66413.5. (Sen. Bill No. 186 (1987-1988 Reg. Sess.) § 1.)
Section 66413.5, as signed into law, only partially addresses the approval duty a newly incorporated city has as to tentative vesting maps approved by the county before the city’s incorporation. Under that statute, when the county has approved a tentative vesting map both before a petition to incorporate a new city receives its first signature and before an incorporation election has been held, the newly incorporated city must approve a final conforming map. Section 66413.5 thus provides a so-called safe harbor for only those projects receiving county approval within a limited timeframe. Notably the Legislature did not expressly state what incorporation meant for projects falling outside the safe harbor of section 66413.5. (See § 66413 [on annexation to a city of area included in a tentative vesting map that has not yet received county’s final map approval, “all procedures and regulations . . . of the annexing city shall be deemed to commence as of the effective date of the annexation”].) Therefore, section 66413.5 is irrelevant here, as the developer has consistently maintained.
The majority reasons that because Sandpiper does not come within the safe harbor created by section 66413.5, Goleta, once its incorporation took effect, had absolute discretion to deny approval of Sandpiper’s final map even though the final map conformed to its county-approved tentative vesting map. The majority reaches that result by relying on the County’s subdivision ordinances as adopted by the City of Goleta. I read section 66474.1 to control the application of those ordinances.
In language that could not be any clearer, section 66474.1 states: “A legislative body shall not deny approval” of a final subdivision map “if it has previously approved a tentative map for the proposed subdivision and if it finds” the final map “is in substantial compliance with the previously approved tentative map.” When all the conditions pertaining to the tentative map have been met, approval of the final map is a purely ministerial duty. (Youngblood v. Board of Supervisors (1978) 22 Cal.3d 644, 648 [150 Cal.Rptr. 242, 586 P.2d 556] (Youngblood); Great Western Sav. & Loan Assn. v. City of Los Angeles (1973) 31 Cal.App.3d 403, 410 [107 Cal.Rptr. 359] [“governing body’s function is administrative, ministerial and mandatory where the final tract map” complies with local laws and conditions imposed on tentative map].) In Youngblood, “we held that a county lacked discretion under the Map Act to deny a final subdivision map if the application showed the development substantially conformed to the tentative map and its attendant conditions.” (City of West Hollywood v. Beverly Towers, Inc. (1991) 52 Cal.3d 1184, 1190-1191 [278 Cal.Rptr. 375, 805 P.2d 329].) The rationale underlying the statutory scheme at issue is that once a “tentative map is approved, *285the developer often must expend substantial sums to comply with the conditions attached to that approval,” which “will result in the construction of improvements consistent with the proposed subdivision, but often inconsistent with alternative uses of the land.” (Youngblood, supra, 22 Cal.3d at p. 655.)
The majority here concludes that because it was the County, acting through its board of supervisors, that approved Sandpiper’s tentative subdivision map, Goleta’s city council is not subject to section 66474.l’s mandate that “the legislative body” may not deny approval of a final map “if it has previously approved a tentative map for the proposed subdivision.” (See maj. opn., ante, at pp. 278-279.) The majority’s conclusion might have force had the City of Goleta not put itself in the shoes of the County by adopting and readopting the County’s subdivision regulations as its very own, a point I discuss in detail below.
A newly incorporated city that comprises formerly unincorporated land is required by statute, “immediately following its organization and prior to performing any other official act, [to] adopt an ordinance providing that all county ordinances previously applicable shall remain in full force and effect as city ordinances for . . . 120 days after incorporation, or until the city council has enacted ordinances superseding the county ordinances.” (§ 57376.) California’s Map Act permits local ordinances to supplement its statutory procedures. “Of course, if a local ordinance supplements procedures in the Map Act, and does not conflict with them, the local ordinance does not ‘modify’ the Map Act.” (Griffis v. County of Mono (1985) 163 Cal.App.3d 414, 425, fn. 14 [209 Cal.Rptr. 519], italics omitted.) Here Goleta, by operation of state law, adopted the County’s subdivision ordinances as its very own the day after its incorporation as a city took effect. Four months later, Goleta readopted the County’s subdivision ordinances. When Goleta denied approval of Sandpiper’s final, conforming subdivision map on January 6, 2003, some 11 months after Goleta’s incorporation took effect, it had not enacted subdivision ordinances of its own, as it could have (§ 57376), either to supersede those it took over from the County or to supplement the state law procedural requirements imposed by the Map Act.
Nor did the City of Goleta seek to avail itself of an urgency provision in the Map Act permitting a local public entity that has approved a vesting tentative map to “condition or deny” final approval on findings that “[a] failure to do so would place the residents of the subdivision or the immediate community, or both, in a condition dangerous to their health or safety.” (§ 66498.1, subd. (c)(1), italics added.)
Only on January 6, 2003, when the City of Goleta denied approval of Sandpiper’s final subdivision map, did the city council give a nod to this *286urgency provision, citing the project’s potential inconsistency with Goleta’s “future general plan.” Such potential inconsistency, the council concluded, “would be detrimental to the health, safety, and general welfare of the City of Goleta.” But a mere assertion of detriment does not comply with the urgency provision’s requirement of danger to the health or safety of the residents of the subdivision or the community. Moreover, Goleta’s concern about detriment to the health and safety of its residents was a complete “about-face.” Nine and a half months earlier, on March 25, 2002, when the City of Goleta exempted Sandpiper’s project from the moratorium on development approvals, the city had concluded that the project posed no threat to public health, safety or welfare.
For the reasons stated above, I would reverse the judgment of the Court of Appeal.
All further statutory references are to the Government Code.