What would cause a decrease in accumulated depreciation?
A decrease in accumulated depreciation will occur when an asset is sold, scrapped, or retired. At that point, the asset's accumulated depreciation and its cost are removed from the accounts. (The net of these two amounts—known as the book value or carrying value—is then compared to the proceeds to determine if there is a gain or loss on the disposal.)
Some accounting textbooks state that the cost of an expenditure that extends the useful life of an asset should be debited to the accumulated depreciation account instead of the asset account. Such an entry will also reduce the credit balance in the accumulated depreciation account.