How to handle home office improvement deductions/depreciation when closing business

In 2020, I:

  • made $25K income in a sole proprietorship (software development)
  • spent $20K building an external structure to use as a home office
  • decided to close the business
  • qualified to take the "Safe Harbor Election for Small Taxpayers" (I think)

Can I deduct the $20K home office improvement costs? If not, what is the proper way to handle them?

 

If I depreciate over 39 years, but then close the business the same year, do I need to do anything about the unclaimed depreciation?