Existing park homes longer than zoning allows

Hi all,

We own a park in Iowa. The park was built in the early 70’s. Lot’s are 40 wide by 80 long and were plotted out prior to the park being built.The zoning requires there be 10 feet of space on the light to the front and rear of the home. Meaning the home can be no longer than 60 feet, but most of the homes that are in the park prior to purchase, majority of them are home from the 70’s are longer than 60 feet, sum up to 76 feet.The city is not approving any homes longer than 60 feet according the the code that the park was under when built. Current code won’t allow it either.Do I have any case against the city for not allowing longer homes since they have basically allowed it in the past?

Please help.

I did not find the right solution from the internet.

References:-

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Thanks