Redevelopment powers under this chapter may not be exercised by any political subdivision unless so authorized by a local law relating thereto, which local law may limit but may not expand those redevelopment powers established by this chapter as to the local political subdivision to which the local law is applicable. Such local law, and all amendments thereto, shall become effective only if approved in a special election by a majority of the qualified voters voting of each political subdivision directly affected, which special election shall be held as provided in that local law, but in conformity with the requirements for special elections pursuant to Title 21.