1) Why not ask the question you asked here? "How much of this role will be new feature development?"
2) Don't assume that the /manager/ really has the low-level information that will dictate how your time will be spent. Ask to meet the coders you'll be working with (not a bad idea in any interview process!) and pop the questions:
"What state do you feel the codebase is in? How does new feature development tend to get divvied out across the team? Which part of the product are you most proud of? ... and least proud of?"
HTH