If you choose to rent a room in your property under the Rent a Room Scheme, you can earn up to £7,500 per tax year and not have to declare this on a Self Assessment tax return. If you earn over the £7,500 allowance, you will have to pay tax on the income, but only on the amount over £7,500. Under the Rent a Room Scheme, you cannot claim any expenses.
You can choose to use income and expenditure and claim for the income and expenditure incurred in renting out your room but it is rarely more tax-efficient. Speak to your accountant if you need help deciding which is the better option.