Open and honest communication is the key here. You have to be honest and let them know that they hired you because you know what you are doing. While you are perfectly willing to build what they want, show them why it's not the option. Use facts and examples all that you can.

Sometimes you don't get to vote who you work with, and you simply have to suck it up and design what they want. In the end, it is their money and their desires and you really don't have much you can do about it.

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