How is Zakat to be paid on a house that has not yet been sold, which was invested for commercial purposes, given the difficulty of estimating its market value?

The zakat on trade goods follows the original asset with which they were purchased, if that asset was zakatable. The house must be appraised when the hawl (one year) passes on the money with which it was purchased, and a quarter of a tenth of its value must be given, regardless of the purchase price. Experts can be consulted to appraise the house at the time zakat becomes due.