Dear Sir,
Since the LTCG has to be invested in a residential property or in specified bonds, if you propose to reinvest the LTCG in a commercial property (which has always meant to be a commercial premise and not a residential property used for commercial purposes), you cannot claim the capital gain tax exemption. Accordingly, the entire LTCG shall be taxed at 20% plus applicable surcharge & cess.
You should invest for purchase of a new residential house for exemption.