In general, for recapture of depreciation for a rental, you have to pay tax on the amount of depreciation you have taken on the building.
A Section 179 recapture would mean that you took the entire amount of value in the very first year of an asset, instead of a portion over 5/7 years, according to the asset type. If you take the entire amount at once and do not hold on to it as a business asset over its entire useful life, you have to pay back the portion that you were not entitled to.
