Turtle's shell serves the purposes mainly of protection and maintenance of body temperature. For the sake of making this problem argument able, I will ignore that the shell is attached to the turtle by (among other things) its spine.
For protection, the turtle retreats its exposed body parts into the shell, escaping predators either when in danger or when sleeping -- in preparation for danger. The shell is also hard, making it unpleasant to bite. And like the Stegosaurus, I can imagine the plates maintain the reptile's body temperature by soaking up the sun.
All of these attributes -- providing safety from others, providing safety when sleeping, and providing a casing that heats and maintains heat -- cause me to call the turtle's shell its home/house.