I'm 45 and want to build my dream home. Should I be getting into a 30-year mortgage? —J.L., Location withheld.

With mortgage interest rates close to historic lows, a 30-year mortgage is tempting. But you'll be 75 before you pay it off. Will you be able to afford the payments 10 years into retirement, when your income may have fallen substantially? How much money do you have saved, or will you have saved by then? To answer those questions, you'll need to run the numbers (calculators at CNNMoney.com/retirement can help). "If you can comfortably handle the payments while employed and in retirement, then a 30-year mortgage is a good, low-cost way to afford your dream house and enjoy the quality of life it will bring you," says Andy Tilp, a certified financial planner and principal at Trillum Valley Financial Planning near Portland, Ore.

If you'd rather not worry about making payments during retirement, consider a 15- or 20-year mortgage. With the current low interest rates, these loans, too, are more affordable than usual. (In late July 2011, the average fixed rate for a 15-year was 3.99% vs. 4.78% for a 30-year.) Of course, the shorter term means your monthly payment will be higher than it would be with a 30-year mortgage. Let's say you get a $200,000 loan at current rates. With a 30-year your monthly payment would be $1,046, vs. $1,478 for a 15-year.

Bottom line: Choose the shorter loan if you are on track to save plenty for retirement. Otherwise, you may be better off with the lower payment on a 30-year mortgage, which would allow you to save more of your monthly income for retirement. But if you're not on track for retirement saving, and you can't afford the new house very easily, the prudent move is to ask yourself: Can I downsize my dream a bit?

-- Walecia Konrad

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