It’s a two step process.
The appraiser first look at the physical layout, and compare it to the certificate of occupancy. If needed, then they can check further and compare the layout to the floor plans on file.
I was told the key is not “square footage”, but “room count”, so any construction that changes the room count needs a permit.
I got caught when they were checking a complaint on an illegal unit down the street, and knowing the area was zoned for two family, we had three mailboxes. The first mistake was my wife opened the door, and the code inspector barged in. Most folks here tell me the best way is not to let them in at all.
Anyway, I hired an expeditor (someone who works to correct violations) and was told the correct way to handle the citation was to make the basement look like the floor plan, basically empty, except for a washer dryer hookup, which was converted to a kitchen.
I told the guy to be more creative, and file plans for a home office, with a wet bar,and bathroom. Since this is not allowed living space, I had to take out the bathtub, and convert the area to a closet, and convert the kitchen sink to a bar sink. The bedroon is now designated the office. My theory was, since the citation was for “construction not in accordance with plans”, I said why not “file plans in accordance with what’s already constructed”.
The filing was originally turned down, but the expeditor said folks who get turned down usually gets a local politician to make a lot of noise, which we threatened to do in the appeals procees, and it was approved after appeal.
It cost me a few thousand dollars, mostly BS fees, less than it would’ve cost me to rip everything out (several thousand), and surreptitiously rebuilding it (another 15K), and I almost have a completed legal unit. I since put the bathtub back in, and all it takes to convert the wetbar to the kitchen is enlarge the hole for the sink, slip in a kitchen sink, put in a stove, and I’m set.