Thanks for your question, I am Sam and I am one of the UK tax experts here on Just Answer.
Why do you need to be onsite 24 hours a day to caretake the building - does it open 24 hours a day - or have amenties that are available 24 hours a day -
Explain why you could not perform your duties if you lived off site
Then I can advsie further
I do not need to be on site 24hours, the duties I perform are as a key holder when the building is in use until late at night, some cleaning duties when required and opening the building when required.
To perform the duties if not living on site would not be practical
Thanks for your response
Then as there is not anecessity - then HMRC will treat this as a taxable benefit -
Necessary examples are lighthouse keeper - or staff in a boarding school, care wadren in a care home etc
Your employers should be alerting HMRC of this benefit on form P11d, and the small reduction of rent you recieve is then liable to your usual rate of tax (so you still end up with a benefit - just suffer the tax - 20% if a basic rate taxpayer)
Your employers then should also provide you with a copy of the P11d so you can ensure your tax code reflects this taxabale benefit