In the past, mortgage lenders were relatively relaxed about additional borrowing on buy-to-let mortgages, as they were on residential mortgages. As a rule, as long as the additional borrowing did not take you above the maximum original loan to value percentage for your deal, not many questions were asked.
These days it is not quite as straightforward. Some lenders might lend you extra funds, as long as you are an existing customer who completed on your original mortgage at least six months ago. But they may reserve the right to offer you a different interest rate to your original loan.
No lender will encourage you to take on more borrowing than you need, or that you cannot comfortably afford. But in the right circumstances if further borrowing makes sense, some may be able to oblige.
Bradford & Bingley/Mortgage Express and NRAM
Bradford & Bingley/Mortgage Express and NRAM are closed to further business and unable to offer you additional borrowing. But they may be able to help you to remortgage to another lender that will advance you extra funds now, and might also provide additional funds further down the line. A mortgage broker such as MortgageForce can point you in the right direction.
Other lenders may make additional borrowing available for some or all of their mortgage loan types, although there may be restrictions on the amount or purpose depending on the type of loan you have. You may be looking to undertake home improvements on your rental property, or add to your existing portfolio, for example.