Final answer:
The basis of the new office building, if § 1033 is elected, is $420,000.
Step-by-step explanation:
Under § 1033 of the Internal Revenue Code, nonrecognition of gain from an involuntary conversion allows the owner to defer recognizing the gain if they reinvest the insurance proceeds in a replacement property. In this case, the new office building is considered a replacement property.
The adjusted basis of the destroyed office building is $420,000, which is the owner's basis in the replacement property. So, the basis of the new building is $420,000.