MWAA has posted documents for their upcoming board meeting; it includes an agreement with UA to build a 20,000 sf addition to Concourse C at IAD for the new Polaris Lounge.

http://www.mwaa.com/sites/default/fi...al_airport.pdf

The location would be right near the tunnel to the AeroTrain; it would require some modifications to gate C18. There is a diagram in the presentation linked above.