"Ho un piatto."
Isn't it also correct to say "There is a plate."? In the last lesson duolingo suggested "There is water in the sugar." for "Ho acqua nello zucchero."
I would rather stick to the most literal translation "I have".
"There is" literally means "C'è".
While "There is water in the sugar" can be understood, in context, as the water is in your sugar, "there is a plate" would be difficult to be understood as the plate is yours.
I hope that helped.