Hello, I am Keith, one of the experts on Just Answer, and pleased to be able to help you with your question.
You can certainly write down the unit in your books. In any event such an asset is only worth as much as a buyer is prepared to pay for it which may vary widely from the book value. Such write down is a capital item and cannot be offset against profits for taxation purposes although the book keeping transaction will impair on the Profit and Loss Account as an exceptional item.
I do hope that you have found my reply of assistance.