GST on Sale of Developed Land

Department may dispute that tax is payable. Post development there are certain registration/completion certificate which needs to be obtain from local authority. That can be proof that service portion is completed. If sold beyond that date, then developed land can be said to be immovable property entirely not coming under the ambit of GST. (Note: ITC reversal may be required as per Section 17 r/w Rule 42). 

Ideally, amounts collected towards land must not be taxed as GST law does not provide for taxing immovable property. Where values are significant, and considering ambiguity in dept eyes and possible issues in the future, suggest you obtain a written legal opinion.