Unless you plan on selling your residence and moving to a cheaper area after retirement hits, all you need to do is figure this as a different "expense" by your standard mortgage calculator, and calculate your time to retirement how you do now.
I'm actually doing the same thing, as we're looking to move from a townhouse to a single-family house next year. It basically means saving $700 less per month, BUT we plan on moving to a cheaper area at some point after that. I conservatively only consider the sale of that future home to be a net of the difference between the purchase prices ($100k) and add that to my portfolio at retirement. In reality, the value should increase over time.