Improvements that are part of the kitchen (walls, flooring, etc) are part of your "further investment in this building" and not Expense to be deducted. Think of it like this: the money is right there, every time you use that new sink or walk on that flooring. That is Expenditure (you paid for something) and not Expense (it isn't all gone already or used up already). Expense is Printer paper and Electricity. What you have is invested into the Basis, or total cost for the building you improved. If you only stated you repaired, for instance, that can sometimes qualify as expense. I teach this as the difference between replacing a broken window (repair expense), and putting in all new and better windows (improvement to the building).
Unless you have a kosher or otherwise split kitchen (multiple sinks, etc), it's pretty hard to state 1/3 is only for business and nothing personal happens there.
I recommend you google and read the results:
home office deduction 2019
