HBG has lost a £47 million residential scheme in London to InteriorExterior six months after being lined up for the deal.
Sources at rival firms say the swap is due to a rethink by developer Berkeley Homes about the scope of the Acton project.
One insider said: 'The number of homes has varied a lot and the client has changed to a management contract.That's far less appealing to traditional builders.'
InteriorExterior will convert Bromyard House, a block formerly occupied by the Department for Work and Pensions, which has relocated nearby.
Plans approved by Ealing Borough Council in October 2001 covered 410 homes but the designs were reworked last November to include an additional 570 units.
Final changes agreed in April reduced this number to 526, although the proportion of affordable homes has increased from 30 to 35 per cent.A number of smaller, new-build blocks are also included in the deal.
The four-storey building will be expanded to include an additional level on the roof and a split-level underground car park.
Laing O'Rourke and Kier were HBG's main rivals when the job was first tendered but, following the change in contract form, Schal and InteriorExterior became frontrunners. Insiders say InteriorExterior just pipped Schal to the deal.
Construction is scheduled for completion in late 2007.