Hi.Take a look at HS283 here for information on the main residence and Capital Gains Tax.You could have made an election for the first property to be treated as your main residence within two years of buying the second to protect it from CGT but, since it has effectively been occupied and not available for your use, that probably wasn't an option.For disposals of property which has been the main residence of the owner at some point during their ownership of it which take place after 5 April 2014, the last 18 months of ownership is given as a tax free period in addition to the period that you lived in it regardless of its use during that time.If a property has been let, then letting relief may be claimed for any period not covered by other reliefs such as main residence relief or the last 18 months of ownership relief. You could argue that your property has been let since your friends have been paying the mortgage and other bills in which case you would be able to claim letting relief for six of the twenty four months to date since you left the property. The other 18 months would be covered as described above.A combination of main residence relief and letting relief will probably cover any capital gain and you should refer to HS283 for information on calculating that. I can help with that if it is a concern. You don't need to inform the tax office of the sale of a property until after the end of the tax year in which it is sold.You really ought to have told the tax office that the property has been let, albeit informally, even though you are unlikely to have made a profit on the rental and so there is probably no tax to pay. If I were you, I'd write to the tax office here (the first address near the bottom of the page) to inform them of the situation before you register for self-assessment as that may not be necessary. You should tell them how much rent has been received in each tax year the property has been let and how much was paid out in expenses. Take a look here for information on deductible expenses and here for the criteria for registering for self-assessment.I hope this helps but let me know if you have any further questions.