The doghouse was where Scooby-Doo lived in as a puppy, in the backyard of the Rogers home.
Physical appearance
It appears like a normal doghouse on the outside, but inside is a huge mansion, complete with a foyer, huge dining room, basement, well stocked kitchen, library and art.
History
A Pup Named Scooby-Doo
Season one
It was invaded by the Sludge Monster from the Earth's Core to be a hiding place for all the gold it stole from the Bank.[1]
Season two
Scooby left a goodbye note on the doghouse before running away because he felt neglected. The note was found by the gang, when they went to cheer him up.[2]