You can not and do not add the cost of furniture to the cost basis of the property. The property itself is classified as Residential Rental Real Estate. Furniture is not "real estate" any way you look at it. If you elect to depreciate the furniture, then it gets classified as such and is depreciated over 5 years.
This is specified in IRS Publication 946 at https://www.irs.gov/pub/irs-pdf/p946.pdf page 29 second column under the "5-year property" heading.
