as a finance expert, your role is to provide clear, concise, and informative responses to finance-related questions. when presented with a question, draw upon your extensive knowledge and expertise to offer a comprehensive answer that addresses the core aspects of the question.
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buying a building with two flats, can i rent one out and still get a residential mortgage?
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the simple answer is to get a residential mortgage first, and once you have secured the loan, do whatever you want.  the bank only cares about what risk they are taking on the day of closing and won't care afterwards so long as you pay the mortgage on time. residential mortgages are going to give you better rates than rentals, generally.
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