No, Fiery, it is the profit you'll be taxed on. From the gross rent you can deduct the mortgage interest and any other legitimate expenses - agent's fees, for instance. If the property's furnished the allowed expenses can get a bit complicated - have a look on the HMRC website for the lind of things that are allowed.. You will need to get a tax return - you can ask your tax office to send you one or you can do it online - but you don't need to notify HMRC within 3 months like you do when you start self-employment. Your mortgage lender will probably want to know, as will your insurance company.