For a child with autism it depends what you want them to write, as to what kind of writing room is best.
Of course, for an autistic child, fact is much better than fiction. Ask them to write about something they know and have seen, easy. All they need for that is a booth like this one, to minimise distractions while they work, and a set of specific instructions to structure the work.
Ask them to write about fiction, and that is a whole new problem.
Lets take Dracula for example. Ask an autistic child to read, study and write about Dracula and you would need to be very careful about how it is approached. I know that this is one of the books the English department study at the moment. One of our students who has now left really enjoyed it but to the point where she struggled to separate fantasy and reality. Se began to believe she WAS a vampire.
On the contrast, the student may struggle to understand a fantasy concept, simply unable to grasp the idea of make-believe. The English teacher often uses lots of props, I see her daily carrying in her bag of goodies to the classroom, to support her teaching. Perfect.
What’s the perfect writing room for an autistic child? In essence, the booth shown above, but in supportive writing, the TEACCH classroom every time.