The Backyard Beach is one of Phineas and Ferb's Big Ideas. It was, as the name states, a beach the boys built in the backyard. Many of the neighborhood kids came to it before it was destroyed by Dr. Doofenshmirtz's stolen lawn gnomes as they burst from the "ocean". ("Lawn Gnome Beach Party of Terror"). The Beach includes various activities, such as a Surfing Contest, a Limbo Contest, and an Arcade Machine within.
The beach was built actually behind the fence, temporarily replacing the houses normally located there.
This was the first time Candace didn't try to bust Phineas and Ferb.