What catches landlords out?
Qualifying income is gross, not net. A landlord looking at profit after mortgage interest and agent fees can be well under the threshold in their own mind and well over it in HMRC's.
It also combines. Property income and self-employment income are added together for the test, so a modest portfolio alongside modest freelancing can cross a line neither would cross alone.
And it is per person, not per property. Jointly owned property is apportioned to each owner's share.