I wasn't sure what the smart financial thing to do is, so I'm turning to the wise sages at /r/personalfinance!
My fiancée already owns her own home. She bought it for about $180k and has something like $45k in equity in it. She's five years into a 30-year mortgage at ~4.6% (she did a 10% down option. No PMI, but the interest rate was high-ish at the time). She would like to refinance to a 15 year.
We're getting married next year and need more space. We're looking at building a new house for around $350k to $400k with 20% down on a 30-year mortgage.
We both make about $100k per year with no debt other than about a $500/month car payment each (and she has her mortgage, of course). We both have excellent credit scores.
Ideally, what we'd like to do is refinance her house together now (to keep and rent out after we move) and also buy the new house together when it's done being built (say 7-9 months from now), but I'm not sure what the credit implications of this might be.
I think that the two of us buying her house now and then another house later should be equivalent to her refinancing her house now and then me buying the new house later, credit-wise, but I'm not sure and I was curious–from an interest rate standpoint–what's the best financial decision, or should it not matter?
Edit: For clarity. And to summarize: the goal is to keep her current house indefinitely and rent it out. I would like to have both our names on both the mortgages (so we both own half of each house). From a debt-to-income ratio hit that my credit will take by suddenly having half-a-house of debt, will that recover in 7-9 months when the new house is done and it's time to close so that we don't have to pay higher interest rates than whatever the best rate is at the time? Or is it better for her to be on that mortgage and me to have the mortgage on the new house?