This is from
Pay Related Social
Insurance (PRSI)
Contribution Rates
and User Guide. 2023
"Class K contributions.
Modified rate contributors, for example civil and public servants recruited prior to 6 April 1995, with self-employed earned income from a profession or a trade and any other unearned income are liable to PRSI at Class K on that income.
Employed contributors and occupational pensioners, under pensionable age, currently 66 years, whose only additional income is unearned income may be liable to PRSI on
such income at Class K."
I got caught out by this a number of years ago. I had 2 class A contributions in the year and 52 class K from my investment income.
The conversation to Class K only happens when a class A employee has unearned income as their only additional source of income.
I know for a fact that if this person has other income from an ARF no matter how small their unearned income will remain at class S.
If you had a small amount of other income e.g. earned self employment income you possibly might avoid your rental income converting to class K.
Say for instance you subitted 100 euro as earned self employment income in your tax return, you would then be in the situation where the rental income is not your only additional source of income. This should by the rules, prevent your rental income being converted to class K.