Standard Cost Method
The standard-cost method uses an average price for basics such as the cost of food, heating, power, and includes an amount for outgoings such as rates, insurance, mortgage interest cost, repairs and maintenance. It is an average across the country and is inflation-adjusted annually.
If your income from boarders is less than the standard cost allowed, you will not have to file an income tax return, keep records of related expenditure, or pay tax on the income from boarders.