Foster's Home For Imaginary Friends is a mansion where many imaginary friends live in until they are adopted again by new children. As stated in the first episode, it was originally Madame Foster's own house, until she opened it up as a place for homeless imaginary friends to stay.
Probably the most unique building in the neighborhood, the mansion can easily be spotted, passing through the Gothic gate with "Foster" decorating the upper arc, with a massive willow tree bigger than the house itself in the garden, Foster's Home for Imaginary Friends is a very tall Victorian building painted in many shades of bright orange and red, with tall windows, balconies, three visible chimneys, two of them tied together. Decorating the very top is a long flag with the Foster's seal on it.
Inside, the mansion is as random and different as it is outside, being very colorful and almost childlike in design, yet at the same time quite regal and Victorian. With big chandeliers, tall staircases, pillars and expensive decoration (including a collection of Madame Foster busts to replace any broken bust in the house). The halls are lined with colorful doors, most of them leading to the rooms where the imaginary friends sleep. The doors don't all quite match, greatly varying in size and shape. Although the rooms themselves have relatively similar design and furniture, with a few variations. As stated by Madame Foster in "House of Bloo's" and as demonstrated in "Dinner is Swerved," it's fairly easy to get lost inside the confusing halls of Foster's. The mansion was built with several trapdoors and secret passages.
As seen in "Dinner is Swerved," the mansion seems to defy several laws of physics, with the halls and stairways leading to places physically impossible to reach, such as taking several staircases down and ending up in the highest floor, or falling through trapdoors and ending up on the rooftop.
There is also an old system of communication throughout the house, with several pipes leading to megaphones which can easily carry the voice of whoever is speaking into the microphone throughout the entire mansion, mostly used by Mr. Herriman to give orders to Frankie no matter where she is and make announcements to the house.
- Foyer - The entrance to Foster's Home for Imaginary Friends, the hall opens in a wide room with tall ceiling and a large staircase, turning to the halls and the bedrooms, it was revealed the main stairway is also a trapdoor.
- Bloo, Wilt, Eduardo, and Coco's bedroom - a plain, square bedroom with a two-story bunker bed where Bloo sleeps in the lower mattress while Eduardo sleeps in the upper. Wilt sleeps underneath the bunker (since he willingly lent his own bed to Bloo) and there is a straw nest under some kind of wooden construction that Coco sleeps in.
- Dining room - A very long room with tall ceiling and high chandeliers and a long table with a several chairs, where all imaginary friends have their meals, according to Mr. Herriman's rigid schedule. A pair of doors lead to the kitchen.
- Kitchen - Where all meals are prepared, the kitchen is proportionally big as the house itself, being pure white from floor to ceiling with several ovens and sinks.
- Mr. Herriman's Office - Where "no imaginary friends want to get sent" according to Wilt in "House of Bloo's". The room is decorated in Victorian motifs, with quills for writing and dial telephones. The room also has a terminal to the mansion's communication system. The walls are covered from floor to ceiling with file drawers. The vault keeping the Foster's fortune can also be found in this room. The room is directly connected to the waiting room.
- Stables - Some imaginary friends are equines, such as pegasi and unicorns, and thus, they live in common stable built in the mansion gardens. In an interesting coincidence, some of the equines resemble horses from another one of Lauren Faust's shows, My Little Pony: Friendship is Magic.
- Duchess's room - Due to its host's ego, this room was built solely to the use of Duchess, being one of the most luxurious rooms in the house, furnished with a writing desk and a wide bed with a tall canopy, it also possess a telephone of her own, possibly connected to the mansion's communication system. In "Dinner is Swerved," it was revealed that there is a trapdoor in the middle of the room, where Duchess probably disposes of any unwanted visitors (and oddly enough, it leads to the rooftop, even though it leads meters and meters downway).
- According to a tweet by Craig McCracken, the house's address is revealed to be at 1123 Wilson Way located somewhere in the Pacific Northwest. The address was also revealed in several other episodes.
- It is possible that the house is located in Eureka, California due to being nearly identical in appearance to the Carson Mansion, a Victoria mansion that is found in real life in Eureka, though this has not been confirmed.
- The foster home's hours of operation are from 7 a.m. to 7 p.m. as revealed in the episode, Bloo's Brothers.