If you borrow an additional amount of money on your own mortgage to fund a purchase of a buy to let,can that portion of the interst be claimed as an expense?
Yes that's perfectly fine Georgie. It makes life easier if your client can get the re-mortgage part as a separate account, otherwise the easiest way of calculating it would be as a proportion of the mortgage, e.g. if your client had a £100k mortgage on their house and then re-mortgaged another £20k, look at the total interest and apportion as 20/120.