If you have a plan for extension / or a new unit at the newly purchased plot on a later date, then do not declare now. Put a fence and let the construction be over. Till such time the site will be a construction area, and no Factories Act applicable!
The suggestion of Mr. Babu Alexander to keep in mind, if your factory goes for an extession.
Further, if you keep the area out of boundary out of existing facotory, for any construction inview of extension would come under B&OCW and cess @1% to be paid.