If you are receiving income from the letting of furnished accommodation that forms part of your main residence, this is tax-free provided that, before deducting any expenses, it does not exceed £7,500 (for 2020/21) in any tax year.
The property must be used as your main residence at some point in the tax year. The relief is also available to those running bed-and-breakfasts or a guest house.
If the gross rent exceeds £7,500 per year you have a choice of either paying tax on the excess or claiming property expenses in the normal way.