A let to buy mortgage works by allowing you to borrow money to buy a new home to move into, while your existing residence is let out to tenants.

The new mortgage lender will calculate the maximum that they are prepared to lend you and not take your existing mortgage into consideration as a commitment as long as the rent covers the existing mortgage payment and/or fits the new lenders let to buy calculations.

A deposit maybe required for the new mortgage however this maybe released from the existing property by remortgaging or a secured loan.  * Be aware that you must ask your existing lender for permission.