Tax implications of letting house I used to live in

[whackin]

My partner bought a home in 2006 and we have lived in it since. We are now moving to my home as my partner is now earning much less than he used to and we hope to let out his apartment. The apartment is about €100K in negative equity and the repayments on it will be more than he would be able to rent it for.
Is is likely that revenue could ask him to pay tax on the "income" from it even though it is significantly less than the cost of mortgaging it? Do they turn a blind eye to these things? Would there be much of a chance of him being refused interest relief on the mortgage in practice?

 

[mathepac]

... Is is likely that revenue could ask him to pay tax on the "income" from it even though it is significantly less than the cost of mortgaging it? ...

All net income (after allowable expenses) is taxable so the answer is "Yes".

... Do they turn a blind eye to these things? ...

Fortunately for other tax-payers they don't

... Would there be much of a chance of him being refused interest relief on the mortgage in practice?

In practice he is not allowed mortgage interest relif as the property is no longer his PPR; as is the case with other tax-payers who have investment properties, it will be disallowed.