The API magazine have done a couple of articles on car parks. The latest was in their November 2009 issue.
They had a case study from someone who purchased one – he did it because he didn't have a car space for his job and so he bought one for use at that time. He purchased it with cash, was able to sub-lease it if he wanted to, had to pay body corporate rates on it, it came with its own title.
I looked into it (was going to do it from my superannuation fund at the time), couldn't see that the numbers worked and wasn't sure whether or not it could be insured. So decided to stay clear of them.
