Any nonconforming residential building or structure lawfully under construction on the effective date of this chapter, pursuant to plans filed with and approved by the Construction Official and approved by all other municipal boards and agencies as required under law, may be completed and may be used for the nonconforming use of which it was designed, to the same extent as if such building had been completed and has been in use on the effective date of this chapter, provided that such building or structure shall be completed within two years after the effective date of this chapter.