A let to buy mortgage lets you keep your current home and rent it out while buying a new property to live in. It involves two mortgages at once: a buy to let mortgage on the home you are moving out of, and a residential mortgage on the new home you are moving into.
People often use let to buy when they want to move but cannot sell, or want to retain a property as a long-term investment. Lenders assess the expected rental income on the existing home and your income and outgoings for the new residential mortgage. Because two mortgages are involved, affordability, deposit requirements and tax treatment can be more complex, so it is worth taking advice before proceeding.