However, any lot in the City of Taunton considered buildable under MGL c. 40A, § 6, as of July 1, 2005, shall be exempt from the shape of lot requirement in said lot's configuration and size as of July 1, 2005. In addition, an existing lot considered buildable under MGL c. 40A, § 6, as of July 1, 2005, that has not been altered as to shape or size since July 1, 2005, shall be exempt from the shape of lot requirement for the purpose of subdividing to create one additional buildable lot.