I apologize for any miscommunication. Your original comments didn't mention that this is a dispute between you and the city. Regardless, if the city is requesting that you install handicapped parking, then upon your refusal, unless you can show a compelling legal rationale for your position, the city will file a lawsuit to force your compliance -- which is essentially the same as what I described with a private plaintiff.
Anyway, you ask for a precedent in Florida case law for this issue, which I will be happy to provide. The analytical framework set forth in Johnson v. Gambrinus Co./Spoetzl Brewery, 116 F.3d 1052 (5th Cir.1997), has been adopted by the U.S. District Court for the Southern District of Florida (which encompasses Broward County) provides as follows:
- " The plaintiff has the burden of proving that a modification was requested and that the requested modification is reasonable.  The plaintiff meets this burden by introducing evidence that the requested modification is reasonable in the general sense, that is, reasonable in the run of cases.  While the defendant may introduce evidence indicating that the plaintiffs requested modification is not reasonable in the run of cases, the plaintiff bears the ultimate burden of proof on the issue.  If plaintiff meets this burden, the defendant must make the requested modification unless the defendant pleads and meets its burden of proving that the requested modification would fundamentally alter the nature of the public accommodation." [numbering added] Id. at 1059.
No. 4, above is the exception that you must prove to the city, because this exception is what the city would need to defend itself, if a future private person were to sue both the city and you for allegedly maintaining a public accommodation in violation of the ADA: that the accommodation required would fundamentally alter the nature of the public accommodation.
This seems to me to be a relatively easy proof for you, given your comment that you would have to demolish a portion of the existing structure. However, you will almost certainly have to hire an expert in disability access to provide a report of the modifications required, and the cost, so that the city will have something that it can rely on to grant you a waiver of the requirements local ordinance, state and federal law.
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