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system
is it commonly possible to buy an option for a mortgage at a specific interest rate?
user
i think the answer to this is just "no."  it's not commonly available to have the option to obtain a mortgage at a fixed amount and fixed rate, especially over a timeframe like the 5 yrs you mentioned in your question.  there would be several practical problems with such a thing, including but not limited to: as was noted in a comment to your question, it is common to be able to "lock" a rate over a period of days to weeks.  this isn't the same as what you asked though, because it's much shorter term and it's typically tied to having an offer accepted on a specific house.
assistant