Typically your tenants pay a percentage based upon the amount of space they are leasing, and your lease should reflect this. If a tenant occupies 1 of 4 equal units in the bulding and it is 10,000 square feet, then they would pay 25% of the total NNN charges for the building 2500/10000 = 25%.
Whether or not these charges should be built into a base rental number or separated out is a matter of marketing and preference. if they are separately charged, you can typically adjust them more frequently to respond to unusual circumstances, i.e. you don't have snow for 5 years, then have a heavy snow one year and have to de-ice, sweep your parking areas, etc If your maintenance cost goes up that year, and the charges are not separated, you have no way to collect the added expense from your tenants.
With the internet, and the ability to price shop in a second, if your asking rents are not comparable to your competitors, you may not get a second chance at a good tenant. If all of the competition is marketing the space at $10 per square foot but the operating expense charges are extra, and you are marketing at $14 per square foot, with the charges included in your base rent, you may never see some of those leads.
Hope this helps.
Sep 17, 2013