I'm going to refuse to stay at grandma's house for dinner
What is your reason for not wanting to eat at your grandma's?
Grandma simply can't cook.
That definitely sounds nasty, I can see why you're refusing. If possible, turn down the dinner request politely.
Nah. I'm over 18 and should be able to to make my own decisions.
It is fine to not want to go over your grandmothers to eat if that is how she cooks. Just try your best to not be rude about it when you are in the action of your refusal.