Timmy's Treehouse is a clubhouse that is built in a tree in the Turner family's backyard.
This wooden structure is built on top of a tree in Timmy's backyard. Here is also where he, Chester, A.J., and, sometimes, Sanjay and Elmer hang out. Tootie sometimes comes around, wanting to play with them, but they will not let her up because she is a girl. Timmy's fairies sometimes use their magic to change the place around and make it more interesting.
In the episode, "Boy Toy", Timmy, Chester, and A.J. fled inside the treehouse, which they called their "Anti-Girl Fortress", when Tootie wanted to play with them. Cosmo and Wanda changed it into a metallic room with a chair and screens like the bridge of a spaceship, and Timmy and his friends launched magic water balloons at Tootie. She was eventually able to get inside anyway, and took doll-form Cosmo and Wanda to her house to lure Timmy there. Timmy later returned to the treehouse after retrieving the dolls, where Tootie was outside waiting to throw a water balloon at him.
Timmy also used his his tree house as a radio broadcasting station in "MicroPhony", encouraging adults to take their children to the beach with them instead of hiring Vicky to babysit. Eventually, Vicky caught onto Timmy by following a helicopter-form Cosmo all the way back to the treehouse, where she barged in and tied Timmy up with wires. She then used a Magic Microphone Timmy had wished up to disguise her voice as Double T (Timmy's radio persona), and promptly insulted all the adults in Dimmsdale while also saying all parents are morons (the word 'moron' cannot be said on the radio. It can only be said on television). An angry mob was formed outside the treehouse, but Timmy's friends and the other neighborhood kids rushed to his defense. When the kids all claim they are Double T, the parents find themselves unable to ground all the kids. However Vicky claims they can thanks to her babysitting service. Before they can pay her, Timmy comes out and admits he was Double T, but only so he could get everyone's family together for family fun. When Mr. Turner claims Double T said that parents were morons, Timmy states that Vicky said that to get him in trouble. Vicky demands proof, but the FCD appear and demand Double T. The kids claim Vicky is Double T, so they arrest her. Vicky tries to tell them Timmy is Double T, but Timmy points the microphone at Vicky, causing her to say it in Double T's voice. The FCD tell Vicky she can't say 'moron' on the radio before taking her away to jail.
On the premiere night of "Crash Nebula vs Crimson Chin" in the episode "Sleep Over and Over", Timmy hosts a sleepover at his treehouse with Chester and A.J., but a disagreement about which is the better superhero results in a fight between Timmy's two best friends, who each decide to have sleepovers at their own places instead, while simultaneously inviting Timmy to their places. After bringing his friends back together by having them rescue him from junkyard dogs, Timmy successfully hosts the sleepover at his treehouse even though they missed the premiere of the special. Timmy decides to find out what his cat costume does, but upon activation, it attracts the junkyard dogs, causing them to find Timmy and his friends and attack them.
In the episode "Timmy's 2-D House of Horror", the treehouse was the location from where Timmy wished a movie called The Haunted Volcano to be more realistic, causing a lava flow to burst from the treehouse and land on Vicky's House. Timmy didn't mind this at first until he found out that Vicky and her family were moving into his house.
Timmy tried to hide in the treehouse in "Dread 'N' Breakfast" after his home had been turned into a bed and breakfast, and several of Timmy's adversaries showed up. Timmy planned to hide in the treehouse until they went away, but Tootie found him and ripped through the roof to get him. Timmy manages to escape her using a zip-line from the treehouse.
Timmy goes to his treehouse in Whittle Me This to get away from Catman and his group, as he did not want to get involved in what was going on. They manage to track him there, with Crocker, who almost manages to steal Cosmo until Chloe rescues him by pushing Crocker into his own machine.
According to a gag in the episode "Boy Toy"'s script, on top of not allowing girls, they also apparently do not allow kids whose names sound like bodily noises)
The treehouse's interior changing into the "Anti-Girl Fortress" resembles the Starship Enterprise from Star Trek. Timmy also calls the water balloons they shoot at Tootie "phasors", a term for the laser guns seen in Star Trek.