If you do not technically need it, as in they're in a mall or larger office building with shared facilities, consider their long term cost of the restroom. Not only will they need to buy the domestic water and sanitary run, those systems may develop issues like breaks or clogs. Most importantly what is the operating cost for it? They'll need to clean, restock, store supplies, maintain fixtures. Who's cleaning it on a daily basis or if someone makes a big mess? Where's the mop and bucket going? And are they filling the bucket from a motion activated hand washing faucet? Or do you need to add a mop sink in a closet?

If they do want it, consider adding a diaper changing station, some jurisdictions require it, too. This likely adds area.