Enlargement of existing structure devoted to nonconforming use. Except as set forth in this provision, an existing structure devoted to a use not permitted by this chapter shall not be enlarged, extended, constructed, reconstructed, or structurally altered in any way unless same may be required for normal maintenance or to prevent damage or injury. In a single instance after the passage of this chapter, a single-family, detached residential dwelling may be enlarged through construction or reconstruction to an extent not to exceed 50% of its preexisting size, provided that said enlargement is limited to residential use. Accessory structures shall not be considered when applying this provision.