There is no hard and fast rule. If he is in doubt then he probably needs professional advice/clarification. If he is wrong and it does not meet Revenue's criteria for being classed as his PPR then he could have some significant tax liabilities (e.g. liability for stamp duty clawback/payment on the original purchase since the property is rented out, inability to claim owner occupier mortgage interest relief on the mortgage, liability for CGT on any eventual resale gain, responsibility to register with PRTB, liability for income tax on rental income etc. etc.)