If you owe more than your townhome is worth, you can bring money to close or do a short sale if you have a legit hardship. If your plan is to buy a house while interest rates are low, you will face obstacles in getting a mortgage on the new property. Underwriting requirements are strict and you would need to qualify for both homes and have a lease in force, and cash reserves of 6 months to one year. Banks are wise to strategic default scenarios, owners claiming intent to rent and once they get into the house they want, not worry so much about keeping the first place current. I'm not suggesting you are thinking of doing that, just that a lender with the new purchase will scrutinize the scenario. What i have suggested to my clients is to rent out your home for a year and rent a home for your family and in one year you will have a track record that could satisfy underwriting and look at buying with a history of renting your current home. Credit score, stable income and job history and debt to income ratio and cash reserves matter in this current mortgage environment.
best of Luck!