Sometimes an existing air conditioning unit is installed in the location where you want to build a new deck.  Your best option is to move the unit to another location out of the way.  This might cost you a few hundred dollars but will save you a lot of headaches in the long run.  Most AC manufacturers recommend maintaining a minimum 60" uninterrupted clearance above the compressor to allow for hot air to exhaust from the top of the unit.   Without proper air flow your AC unit can overheat and break down.  You must also consider access for repairs of the unit.  If your deck is very high you may be able to leave it in place but keep in mind that AC units can be very noisy and hot.