The group - Amec Foster Wheeler, Arup, Atkins, Grimshaw, Mott MacDonald, Jacobs and Quod - were awarded four-year contracts which commenced last month.
When the government confirmed that a third runway at the west London hub was its preferred airport expansion scheme it said a range of policy measures will enable it to comply with air quality requirements and mitigate noise impacts.
Heathrow's head of design, Barry Weekes, said the companies selected for their sustainability expertise have a "proven record in building complex infrastructure projects" which will allow the airport to "hit the ground running" on expansion.
He went on: "Their appointment is a significant milestone in what will be a fast-paced design and engineering schedule.
"We are now well on our way to delivering Britain's new runway, providing the additional capacity our country needs."
The new runway could be opened by 2025 and is estimated to cost £17.6bn.