It may depend on how your business is incorporated, or not. This year's business improvements apparently can't be written off as one time but will need to be depreciated.
But it depends on whether you, personally, own the building or a corporation owns it. And if you use this space for yourself. You can still divide it up into 'ours' and 'business' but you usually can't do that after the fact.
We, personally, don't own the inn. We own the corporation that owns the inn. We rent our space from the corporation so improvements to the building can be written off. Including the new entry we just had done..