Per IRS Publication 527 Residential Rental Property (Including Rental of Vacation Homes), starting on page 15:
Basis of Property Changed to Rental Use
When you change property you held for personal use to rental use (for example, you rent your former home), the basis for depreciation will be the lesser of fair market value or adjusted basis on the date of conversion.
Fair market value. This is the price at which the property would change hands between a willing buyer and a willing seller, neither having to buy or sell, and both having reasonable knowledge of all the relevant facts. Sales of similar property, on or about the same date, may be helpful in figuring the fair market value of the property.
Figuring the basis. The basis for depreciation is the lesser of: