The place was owned by Winnie. There's a sign written "Pups only", indicating that this is an exclusive place for the Werewolf kids; a rubber tire swing can also be seen outside the house. In Hotel Transylvania, Winnie's parents, Wayne and Wanda, were more likely simply frequent guests of Hotel Transylvania, so it was most likely built after 2012, when Wayne became a tennis coach as of Hotel Transylvania 2.
At Dennis' 5th birthday party, while Vlad argues with his relatives about the plan of changing Dennis into a vampire, Dennis leaves the hotel quietly and Winnie follows him; she leads him to her treehouse. Inside, Winnie prepares some tea and a dead pigeon for Dennis, which he doesn't eat, but Dennis after a heartfelt conversation, says he doesn't think he's a monster; Winnie puts him down immediately and says that he's the nicest boy she knows and that she has 300 brothers. They are interrupted by Bela, who destroys the treehouse and finds them.
Bela grabs Winnie and Dennis, planning to use them as hostages and then destroying Hotel Transylvania. But he throws Winnie to the ground after she bites his hand; seeing this, Dennis' fangs suddenly appear, and his roar throws Bela to a tree far away. Bela then roars to Bat Cronies, a battle thus starts.