If you let property and your gross rental income, plus any self-employment income, is over the threshold for the year, HMRC wants your records kept digitally and a short update sent every quarter. This is the short version. The full answer, a checker that runs in your browser, and the dated sources are on MTD Facts.
Qualifying income is the gross figure, before expenses, with property and self-employment income added together.
You send a quarterly update of income and expenses through software by 7 August, 7 November, 7 February and 7 May. The tax return itself still follows at the end of the year.
Check your own position on MTD Facts