In the absence of any other information, I’d think that @Tropical_Willie‘s advice is sound. But that might be moderated depending upon:
– the specific product/s that you stock and ship from the warehouse;
– the number of individual shipments in and out of the warehouse per day;
– how long you’ll be able to have (or required to have) both facilities open;
– staffing, machinery and automation considerations;
– security, traffic, supervisory and other considerations;
– the location of the facility and weather conditions prevailing (i.e., heating or cooling issues);
– the budget that you have to work with.
It could be that “the most efficient way” (depending on the various considerations above, and others that I haven’t even thought of yet), might be to simply close the old facility except for the movers preparing and loading the goods for storage in the new facility, and do all shipping and receiving at the new facility, or doing the move overnight or over a weekend, even if it requires hiring a temporary crew – and paying overtime to the existing staff – to work through a weekend and some evenings to do the move all at once. (You didn’t ask for “the cheapest way”, after all.)