In a bedroom community with smaller properties, an addition can increase the value of your house, says Gerald Barrett, an agent with Christopher J. Barrett Realtors in Wakefield. “I don’t think you’d get 100 percent of your money back,” he says, “but maybe 75 percent, and your house won’t sit on the market for long.” Building an addition can seem like a daunting prospect--especially if you plan to do some of the work yourself or even act as your own general contractor. Here, what you need to know to get started.