How do I allocate improvement expenses for a separate structure?

We purchased the house beside ours (4 feet away) in October 2021 to be used for business. It needs a lot of improvements (electric, plumbing, cleaning, painting) before we can move the business out of our home. So I still need to claim home office as 100% for 2021.

How do I set up the "separate structure" to expense the improvements made in 2021? 100% of these expenses are for the business, not part of our home. So are the utilities for that building.