For the next 20 weeks it will discuss road and rail access to the terminal.
Opponents say the plans will make the M25 a death trap, but the British Airports Authority, Heathrow's owners, say that one of Europe's biggest construction projects would have little impact.
Thirty million people a year would use the proposed terminal and opponents argue the roads' network could not cope even when part of the M25 is widened to 12 lanes.
The local authorities which oppose the planned terminal say that is not true. Only two weeks ago Surrey CC committed another £500,000 to oppose BAA at the public inquiry.