The firm will continue to provide design and site supervision services as Balfour Beatty constructs a 28km dual carriageway on the A46.
This section will include 8 grade separated junctions and 22 bridges. It would complete the link between the M1 at Leicester and the Newark Relief Road.
Secretary of state for transport Geoff Hoon gave the scheme the green light last December following acceptance of the inspector's recommendations from the public inquiry.
The contract extension is worth £9 million over the next three years.
Scott Wilson chief executive Hugh Blackwood said: “We are very happy to have moved to the next stage of this contract which, once again, reflects our reputation for delivering high profile road projects.
“The A46 highlights the government's recently announced commitment to major expenditure in infrastructure projects. We look forward to continuing to work closely with Balfour Beatty and the Highways Agency.”
Scott Wilson has been working with Balfour Beatty and the Highways Agency on the initial phase of the Scheme since 2004.