Dear Mary: We would like to enlarge our small two-bedroom bungalow. Our kids are reaching the teen years, and we need more space. We have always budgeted our money. We have $6,000 in our emergency fund and drive two paid-for cars, and our only debt is our mortgage. We have college savings accounts for our kids and tithe to our church. Our annual income is about $140,000. We have five different bids, and they all came in about the same at $160,000 to add three rooms upstairs, plus a stairway. Even though I know adding 900 square feet will increase the value of our home, I feel queasy about increasing our mortgage debt. What is your advice for us? —Sam and Edie Dear Sam and Edie: I can understand that you are nervous about taking on a big new monthly payment, one that could well double what you are paying …