Mortgage interest tax relief can only be claimed by owner occupiers on loans used to buy/renovate their Principal Private Residence. You cannot claim it on an investment property/loan. You can however offset interest on the loan used to purchase/renovate the investment property against rental income from that property. Note that you obviously can't claim owner occupier mortgage interest relief and offset the same interest against rental income (e.g. if the mortgage to purchase/renovate the investment property was secured on the PPR). The purpose for which the loan was used is used to determine what tax treatment applies, not the property on which the loan is secured.