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Unfortunately, they can try to sue you for this and there are some attorneys who do this to try to just extort money from people just enough to make it not worth hiring a lawyer, which you will have to hire to defend the suit. The first thing to do is get the local building inspector out to examine the ramp based on your building age and see whether or not it is within the guidelines for ADA based on the code back at the time when your building was built. If you are found within code you can choose to either engage a lawyer which is required to represent you as you are a business
or you can choose to offer them a nuisance settlement
of maybe $1000 or $1500 to go away. You can go to court and fight them but general an attorney will cost you at least that amount. Once you settle you can bring the ramp into code which should be minimal to avoid another extortionist coming after you.
These are really your legal choices but if you are being sued you have to decide quick because you have to get an attorney to answer the suit if you are not going to settle to get them to withdraw the suit.
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