Around here it's all gains.  But hypothetically you might have a gain on the sale of the land and a loss on the sale of the building.  Around me the land values jump significantly each year.  Lots of tear-down and rebuild a McMansion going on.

I ask the client, they might have an appraisal that breaks down the values.  If not, I'll pull up the county assessment and ask if they believe it fairly represents the relative values of land and building and just use that percentage.

Rick