Second/Vacation Home Loan with Intent to Add Units

Hi BP, I'm looking to purchase an out-of-state property that is currently a single family home, and rent it out long term, so not an Air Bnb or anything short term. I'd like to use a Second/Vacation Home Loan to save on the downpayment (put down 10% instead of 20%). One of the requirements of the Second Home Loan is that it must be a 1-unit property (which it currently is), however I have plans to convert it into a duplex. Is this violating the loan agreement if I purchase it as a SF then convert it later? Thanks!