"Can you tell me situations when it would not be subject to self-employment taxes?"

"When it's a one off."

I agree. That's why I listed various options in that specific way.

They are not "in the business" of buying a home on a farm, then breaking up the farm land. They are selling part of their personal property not in use as a farm, to be built on by others.

Here's what would make it fall under SE: They decide to develop and build spec homes ("spec" = "speculative" homes to be listed for sale). Each of these is part of their intent to generate a profit from a finished product, and they are doing it 6 times.

Or, they intend to build and then Rent everything out, to be the landlord, and they want to sell the properties later. That is another example of, Hey, I'm running a Business here.

Your client doesn't seem to have a business intent. They simply hope to make a profit from the property they invested in years ago.