What is the top-up amount being used for? It not to purchase/renovate your PPR then it does not qualify for mortgage interest relief. If it is being used for such purposes and your lender does not grant additional relief automatically then you should contact Revenue to apprise them of your change in circumstances. I don't think that you need to or should file a new claim for mortgage interest relief though as that could confuse the system. Just write to Revenue outlining the situation.