If the garage is on a separate title and there is no mortgage on the garage, then there is no mortgage on the garage. In which case the situation is that whilst the house may be sold by the mortgagee in possession, the garage is still owned by the borrower.
It may be an error of the solicitor when the borrower brought the property if the solicitor failed to register the lenders charge at some stage when the property was bought.
The way of dealing this is to ask the lender to confirm they have no interest in the garage. Then, you buy the garage from the repossessed borrower and you buy the house from the lender but, and this is the important part, both of the contracts are conditional on each other. Hence, you don’t buy the garage unless the borrower sells the house and you don’t buy the house unless the borrower sells the garage.
I would get your solicitor to write to the mortgagee in possession (details will be on the title deed of the house) and also send a copy of the garage plan title and asked the lender to confirm that they have no interest in the garage. It’s as simple as that. Then you proceed as I mentioned earlier. It’s messy, but not insurmountable. It won’t take a lot of effort to sort this.
Can I clarify anything else for you?
Please rate the service positive. It is an important part of the process by which experts get paid. It does not cost you anything but helps us greatly.