The first TARDIS!
That makes alot of sense actually…
They mention how big those rooms are a lot. I remember there’s a Vangogh, or at least there was, a library, etc.
Duke, the dog from Bush’s Beans, said he has a printing press in his basement.
A selling point is its spacious interior.
I do not recall another strip where all or any of the kids went into Snoopy’s house.
Perhaps it’s like the Tardus, where it’s bigger in the inside than the outside. Or perhaps magic causes the doghouse to be linked to a much larger space (instant transportation). Or maybe there’s a downstairs area, in which the huge rooms are located underground.