Fixtures vs. improvements
An improvement is defined as a change to an existing property that has made it better and cannot be removed. For example, built-in shelves constructed specifically for the space are likely to be considered an improvement. However, freestanding shelves, which can be taken down and brought to the next location, are trade fixtures.
What happens to trade fixtures not removed at the end of a lease?
A commercial tenant must remove the fixture either before or soon after the lease ends. If it's not feasible to remove it before the final day of occupancy, the tenant must make other arrangements with the landlord. The rules regarding a reasonable timeline for fixture removal vary by state, so it's in both parties' best interest to be on the same page for removal.
If a trade fixture is not removed, the items become the property of the landlord; this is known as accession. This might occur if a business owner goes bankrupt and must terminate a lease early. The business owner/tenant forfeits the right to remove the fixtures, which then become the landlord's real property. In this case, the trade fixtures then become regular fixtures.
This may seem like a windfall for the landlord, but that's not usually the case. The landlord might have been anticipating an empty commercial space to be taken over by someone in a completely different line of business than the previous tenant and must remove the fixtures at their own cost.