Grandmum's Cottage is in the Poconos region in Pennsylvania (The Doom Funnel Rescue reveals the zip code 725-5068.) The cottage seems to be located close by a lake in the backyard.
The house was built by GrandDad after he and Grandmum's honeymoon, and it still stands to this very day.
Jason and Michelle are spending there for the summer while their parents are away.
The cottage appeared in every episode of 3-2-1 Penguins!