South Kelsey Street was located in Hill Valley, California. 1121 South Kelsey Street was where Dr. Emmett Brown's apartment was located, until the building burned down in 1945.
When the building burned down, the blueprints for Doc's inventions were destroyed in the fire. When Doc demonstrated his Fance-O-Dance mechanical automatic dancing shoes for his son, Verne, in 1992, they burned out, and Doc couldn't fix them without the blueprints. So, Marty and Verne traveled back to September 27, 1944 to retrieve the blueprints from Doc's apartment.