You will either be able to expense or capitalize (write off through depreciation deductions) the sofa.
However, you will have to use the lesser of your adjusted basis in the sofa (generally what you paid for it) and its fair market value on the date of conversion to business use.
For example, if you paid $800 for the sofa 5 years ago, it might only be worth $100 today and, thus, you would use $100 as the value when you convert it to business use.
