Hi.You should have registered for self-assessment when you started to let the property. That is, unless the net of expenses income was less than £2,500 per annum in which case any tax due could have been collected through your tax code if you had a salary or a pension and you were not already in the self-assessment system.It will be better for you if you some clean with the tax office before they find you which they undoubtedly will sooner or later as this will show a willingness to get your tax affairs in order. You need to draw up an income and expenditure account for each tax year that the property has been let and send them to the tax office and make an apology for not having dealt with the matter before. The tax office will assess any tax that is due, interest will be added and they may charge a penalty. There is information on the old and new penalty regimes here and here.You may be required to register for self-assessment but I would wait until HMRC tell you to do so. You might also consider having an accountant or tax adviser to handle the calculations and delaing with the tax office for you.Take a look here for information on letting income and tax. There is also a section on expenses. I hope this helps but let me know if you have any further questions.