Remortgage / Bank Loan

One of our clients rents out 2 properties and wants to renovate a third.

They approached the bank who have given them a remortgage on their own home as an interest only mortgage to the value of X

They had outstanding loans with the same bank so the bank deducted the outstanding and deposited a balance of Y in their account.

They will be using Y to renovate the third property.

Am I right in thinking that because the remortgage is on their own home that I cannot claim any of the interest back as an allowable expense on the self assessment?

Any help much appreciated.