You can only re mortgage your first property if you can show that you can afford the repayments.

If the lender knows you are buying to let, you pay a slightly higher interest rate.

The advantage of re mortgaging is you could buy the second one for cash at auction and get a good deal, or buy one in need of repair, where a mortgage wouldn't normally be given.

The lender will take account of the future rental income when deciding how much to lend.

There's probably some good info on here:

http://www.moneysavingexpert.com/mortgages

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