I lived in a row house that had had a fire about 10 years previously that spread to other houses in the row via the attic space. There were original brick firewalls between the units on the other floors, but at some point in the past drywall had been put up to separate the common attic space (which used to be the norm), and no one realized until 3 kids nearly died in a different row house than where the fire started.
They can depending on the type and construction of the multihousing units. However separating the attic space is usually the cheapest way to meet code.
When I worked at DreamWorks Animation someone found a crawlspace in the building and decked out the secret room as a lounge. We figured when corporate found out they would rip it out and fire them. Instead, they had it fixed up to code–or at least they installed a sliding bookcase so we didn’t have to crawl through a duct.
I lived in a room like that in college. It was a fraternity house, and a few years before I got there, some guys were roughhousing, one got tossed in the closet, and his arm went through the back wall and found a space. They opened it up and found a roughly 8x10 space, low ceilings because it was the corner of the roof, but you could mostly stand up in the middle, and it was big enough for a couch, bookshelves, and stereo. We kept the desks in the front half of the room.