Don't give up a 2.9% first mortgage on instinct; the blended-rate math makes this call. The answer turns on the ratio between what you still owe and how much you want to pull out. Take the favorable case: you owe $500,000 at 2.9% and want $100,000 out. Leave the first alone and take a second mortgage for the $100,000, and even at a higher second-mortgage rate your blended rate across both loans stays far below a full cash-out refinance, because the large 2.9% balance dominates the blend. Refinancing $600,000 at current rates just to access $100,000 is an expensive way to get it. Now flip it: you owe $100,000 at 2.9% and want $400,000 out. The small low-rate balance barely moves the blend, and the $400,000 gets borrowed at current rates either way. There, a full cash-out refinance of the whole thing can beat stacking a large second on a small first. For most people sitting on a large, low-rate first mortgage and needing a moderate amount of cash, the second mortgage wins even though second-mortgage rates run higher. Run both paths side by side on your actual balance and cash need before deciding, because the ratio determines the winner.