The lender said it had entered discussions with the regulators to take on the next stage of its growth.
It was originally authorised by the former Financial Services Authority (FSA) in 2012 and offers investment products alongside its mortgage range.
In 2017, Castle Trust expanded its business with the acquisition of Omni Capital Retail Finance.
It now has approximately £750m of assets under management and £660m of bonds outstanding.
The lender said: “As a result of this growth, the company has applied for the licence to support the continued evolution of the business and enable it to further expand the product offering across both the mortgage, savings and consumer finance arms.
Group chief executive Sean Oldfield (pictured) added: “We are delighted with the progress of the business to date and today marks a significant moment in the evolution of the company.
“Making the application for a banking licence will allow us to continue our innovative approach to both investments and mortgages as the business continues to grow.”