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Can future rental income be applied to present debt-to-income ratio when applying for second mortgage?
Generally speaking, no they won't. In this case, though I haven't done it myself, I was recommended to put the mortgage on the real estate after it's been leased out and has a contract on it.  Then, yes, they will use it for that.  But, ex-ante don't expect any bank to count on income from it because, at that point, there's zero guarantee you'll get it leased, and even if you do, at what rate.