If you have a new income to declare, this should normally be done by 5th October following the tax year in which it started. The tax year runs from 6th April to the following 5th April.
You start renting out a property from 6th June 2017. This falls into the tax year ended 5th April 2018. Consequently, you should register by 5th October 2018.
Based on what you’ve said, it’s likely you should have registered by 5th October 2018. HM Revenue & Customs can impose late notification penalties and the scale of them will depend on your “behaviour”. They will look at how long your delay was, your eventual tax liability, whether you came forward without prompting etc.
The best thing you can do now, is register as swiftly as possible. To show HMRC that you are back on top of things, it’s probably a good idea to get your tax return completed well ahead of the deadline (31st October for paper returns; 31st January for tax returns filed online). This will also allow them to finalise any penalty they wish to impose for your late registration and hopefully HMRC will look favourably on you for completing it early.
If you would like an accountant to take care of your tax affairs, please contact your local TaxAssist Accountant who would be happy to help.