O’Rourke pipped Skanska to win the Leadenhall Building after earlier fighting-off Balfour Beatty, Lend Lease, Mace and Sir Robert McAlpine.
The property giant said the “main construction contract was agreed on favourable terms (vs budget and timetable).”
The development cost of the scheme was estimated at £340m when British Land launched its joint venture agreement with Oxford Properties in December.
But revised figures in today’s results put the cost at £318m.
Sources close to the contract race said O’Rourke was very keen to win the job and had put in a “highly competitive” price which saved money and cut down the build time.
Laing O’Rourke and British Land declined to comment on the final contract value.