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Will having a secondary signee with bad credit on a mortgage raise or lower interest?
Generally speaking the lower credit score trumps.  In the case you cite, the lower credit score will prevail.  However, you may need to do exactly that in order to qualify for the loan income wise.   There are two factors when obtaining a mortgage, really all loans, but more so with a mortgage:  the likeliness to repay (credit score),  and your ability to service the debt.  This last one is a combination of income and debt-to-income ratio. If you don't have enough income to qualify for the loan or fail to meet the debt to income ration, you may have to use your GF's income to qualify despite her poor credit. You might want to see past posts about buying property with non-spouses.  It could work, but generally it requires a lot of legal work before closing on the deal.  Avoiding this will lead to tales of woe.