Can somebody explain how rental income is taxed when there is a mortgage on a property. The mortgage is for a rental property. I have been told that all of the rental income can be written off against the mortgage interest and that any refurbishment cost can also be written off. Is this true? If so do I have to complete tax forms. I am also PAYE worker so normally my tax returns are completed by my employer. If I have no tax liability is it necessary to complete tax forms?