You never know what your house or apartment or condo is worth, even to you. Maybe you can afford $400k barely, but you'll be hugely squeezed. Or maybe you'd get a bigger place if prices had dropped for your family size.

You hope conditions won't change. You can look at the current market trends to try to value it. But things will change in the next few years probably. You can guess, but you'll never "know with pretty good accuracy". The economy can go down, interest rates can change, major employers can come and go, there can be an earthquake or cancerous ground discovered there.