Which is cheaper: Buying another home or adding on to what we have?

We own an older home, approximately 1920s, concrete block, a bit over 1K square feet. We're running out of space as our family has expanded, and are trying to weigh our options. We have almost no equity in this house, though we've made about $20K worth of improvements (new windows, roof, etc). Our credit is okay, but not perfect, and we're pretty sure if we sell our home we'll sell for a loss. That being said, there is the space factor. What should we do? Are there construction loans out there for folks like us? We have plenty of land and are in an unzoned township, so adding on isn't a factor, but we're worried about loan approval. What do you know about construction loans for home expansions? We just need an extra couple of rooms and then we're set. Any advice? :-)