Test for depreciating as part of structure vs. sta

[sqjehd]

For residential rental real estate, what's the test for determining
whether an asset is depreciated over 27.5yrs as part of the structure
vs. depreciated as a "standalone" asset?

For example, I'd expect a refrigerator to be depreciable as a
standalone asset. But what about a built-in bookcase? New kitchen
cabinets? A door? A window? A toilet? A furnace? A new wood
floor? Etc.