Will property value improve with attached or detached ADU?

[Ethan Fisher]

I am planning to add a ADU to my existing home. Though ADU will generate additional revenue, I would like to make a right decision between attached vs detached ADU as which option will help improve my overall property value if I ever to sell my property in the future? Some of my friends have made ADU attached with a thick (double) wall between main house and ADU. Idea is when they are ready to sell the house, they can put a door in the separation wall and make ADU as part of main house (additional rooms) making it as bigger square footage home.