When you let a property to a third party for a market rent it should be relatively straightforward to calculate your taxable rental income, or loss if applicable. A market rent is that which a landlord can expect to receive in accordance with rents charged for similar properties in the same area. If your rental income exceeds the allowable expenditure incurred the profit will form part of your income and if the allowable expenditure exceeds the income then, assuming you meet all criteria, you will have a loss which you can carry forward and offset against future rental income.
There are some circumstances where a market rent is not charged. For example, a parent may let a property to their child for a lower amount, as parental duty dictates. In this instance the expenses incurred, such as mortgage interest, landlord insurance, etc., may be greater than the rent received. If this is the case, then the expenses are not sustained wholly and exclusively for business purposes and in strict terms should not be claimed. However the good people at HMRC do allow expenditure to be claimed up to the value of the rent received, therefore resulting in no profit no loss. As a result, however, any actual ‘loss’ will be lost.
Please note: This article is a commentary on general principles and should not be interpreted as advice for your specific situation.