Can you attach a cycle shelter to an existing fence without additional support?

In most cases, not structurally. A boundary fence is built to hold back the wind and keep people out, but it wasn’t meant to carry a roof that lifts hard in a gale. So we design the shelter with its own posts and foundations down the open side, and we only fix back to a wall where it’s solid enough to take the load. The fence sets the line the shelter follows. It doesn’t hold the shelter up.