What’s the best way to build yourself a nest egg? You might stick with your current home, pay down that mortgage over the next 30 years and stash your spare cash in stock and bond mutual funds. Call this the “small-house strategy” (though, in many parts of the country, a $400,000 home wouldn’t be exactly small). Alternatively, you could opt for the “big-house strategy” — trading up to a $1 million home and aiming to pay down the resulting $900,000 mortgage between now and retirement. At age 65, you would then cash in a big chunk of your home equity by swapping back to the equivalent of a $400,000 home.