The incident happened around 11 a.m. at a property at West 28th Street and Gale Avenue, according to the Long Beach Fire Department.
Officials say the explosion originated from the ADU, which is located on the property behind the main home.
Following the explosion, there were multiple small fires at the scene, but firefighters doused those quickly, said Jake Heflin, Long Beach Fire Department spokesperson.
Yulissa Gutierrez, who lives about four houses away from the scene, said the explosion felt like an earthquake.
"We just heard like a loud explosion, a very loud sound. Felt kind of like an earthquake, except we peeked outside the window and we saw a lot of things coming from the air onto the floor. It was like pieces of the house," Gutierrez described.
1 injured after explosion on home property in Long Beach, officials say
One man was taken to a local trauma center with severe burn injuries. Further details on that individual's condition were not available.
Family members who live in the main home on that property told ABC Los Angeles affiliate KABC that their uncle, who lives in the ADU, was inside the structure at the time of the explosion. His two children were not home.
It's unclear what caused the explosion.