Replacing metal spiral stairs from deck to garden

We have a typical deck off the parlor floor with metal spiral stairs leading to the garden. We are considering replacing it with stairs that either go straight down (but this would cut into the side of the garden a fair bit), or stairs that double back on themselves with a small landing to save space. Has anyone done this before and what’s a ballpark estimate? I’m assuming no DOB approval is needed since it’s an existing structure?