If you open a Room Details pop up and there is nowhere to put an Untenantable Date, the Room Details will appear as follows:

This appears this way because the Room has no Type. A type must be entered for all Rooms, even if the Room Type is a house.

Select a Room Type from the drop down list:

Click Update and double click on the line of the Room to re-open the Room Details pop up. You will now be able to enter the 'Will Be Unavailable From' date: