Last time one was in my house, was when we had an addition put onto he house. Its right there on the permit application, that the tax assessor gets to do a walkthrough at completeion, same as the building, electrical, and plumbing inspectors.
The funny thing is, the following spring (5 minths later) we got a similar letter, followed a week letter by the knock at the door. The assor handed me his ID to verify, then while standing on the porch had this breif conversation:
A: Wasn't I just here last Fall?
Me: Well, we had an addition put on the back of the house, so, someone was.
A: Right, your the one with the Room.... in the basement.
Me: Well, there are several rooms in the basement, buy one is rather unique.
A; [Smiling] Yeah, just to let you know I wrote that up as a closet o the records, because anyone can request a copy of the assessment records.
Me: Really? I didn't know that, Thanks.
A: Have you made any changes since the addtion?
Me: The deck on the back of the house was just finished a few weeks ago.
A: Mind of I go take some measurments?
Me: No Problem.
That 'Room' just happens to be a walk in vault 5 foot, by 12 foot, with concrete roof at 7 foot, and poured in place vault door. He marked it as a closet, so I think at least mine would be fine with some RC planes, and storage boxes.