Depreciation basis is the lower of (1) the cost of aquisition; (2) fair market value at the time of the conversion. So unless your house value decreased significantly, it is usually the orignal purchase price + any improvement.
Depreciation is only taken when the property is producing economic benefit. So if the house is only producing economic beneift 10 years after being bought, that's the only time you can take depreciation.
https://www.thetaxadviser.com/issues/2008/jul/convertingaresidencetorentalproperty.html
