Estimate the income tax on your rental profit after expenses, with the 20% tax credit for mortgage interest and the option to use the £1,000 property allowance.
Rental profit is added to your other income and taxed at 20%, 40% or 45%. Mortgage interest is not deducted. Instead you get a 20% tax credit on the interest.
Property allowance
If your rental income is small you can use the £1,000 property allowance instead of claiming expenses, but then you cannot claim finance cost relief.
Worked example
See it with real numbers
With £24,000 rent, £4,000 expenses, £6,000 interest and £40,000 of other income, the extra tax on the rental profit is about £4,746.