A buy to let mortgage is a mortgage on a property which is to be let out or rented, rather than occupied by the owner. A buy to let mortgage is exactly as it sounds - a mortgage that allows you to buy a property in order to let if out to a tenant.This type of mortgage is similar to most others however the buy to let mortgage is designed for people who buy a property with the intention of letting it out.Buy to let is purchasing a property, letting it to tenants and using the income from their ren.