I converted some personal items to business use in my home office. What is the basis of these items for depreciation?

The basis for depreciation is the lower of the adjusted basis of the property (usually cost) or its fair market value on the date it starts to be used for business. For equipment (e.g., a computer, furniture), typically this means fair market value; usually the value of these items decline over time below what they cost. For the portion of the home used for an office, basis is usually the allocable part of the adjusted basis of the home (cost plus capital improvements), assuming the home’s value is more as is typically the case.