To add on or buy is question

If Arlington has been living with a $240,000 mortgage at 5.25% for several years without being able to stash a bunch of money in savings accounts, they should not be thinking of increasing the mortgage payments significantly. If they have been saving money each month, they should figure out how much and subtract from that the extra it will cost to care for the new baby. What is left is the additional mortgage payment they could reasonably afford. Arlington should stick to that in deciding what is best: an addition, a bigger house, or making do with what they have. Also, a 2-story addition in Arlington doesn't have to cost $225,000. We are doing one in Bethesda for $100,000, but it took us several years to find a contractor for a reasonable price.