Student Lets and Council Tax

Hi. I currently let a five-bedroomed flat to students and have done for the last eight years. The flat has always been fully occupied and although each student signs an individual contract, we have supplied the council with those students' details on a single sheet which included the total rent.

This academic year has been problematic and the mix of students turned out to be toxic with the result that all of the students notified to the council have now left. We have re-advertised and so far have two students in residence.

The council who have followed these proceedings have now sent me a bill for council tax saying that I am renting individually and that the property is an HMO. (The property is registered as an HMO with the licensing dept. of the council)

If the property is for the sole use of students why do I now appear to be liable for council tax for the period subsequent to the original group leaving in December?