West Ham will pay almost £450,000 extra per season if they decide to increase the capacity of the London Stadium to 66,000.
The figure was revealed in a joint letter from West Ham and the stadium's operators E20 to the London Assembly, outlining details of the 'peace' deal agreed by the two parties in November.
A bitter dispute about West Ham's use of the stadium was on the brink of going to a lengthy court case before the two parties came to an out of court agreement allowing the Hammers to increase the stadium's capacity.
BBC Sport reports that the capacity will initially rise from 57,000 to 60,000, with West Ham's Premier League match against Brighton on 2 January being the first where more seats will be available.
West Ham vice-chairman Karren Brady and E20 chief executive Lyn Garner have helped the two parties to establish a more productive working relationship which has allowed them to reach this new agreement.
Under the terms of the agreement, West Ham will not pay any additional rent on top of their current £3m annual fee if the stadium capacity does not exceed 57,000.
If the capacity is increased to 60,000, this fee will increase by £250,000 annually. A further £83,000 will be required to take the capacity to 62,500, and then an additional £33 for every seat after that, taking the final fee to £448,500 if capacity is maximised.
West Ham will probably keep the capacity at 60,000 for the rest of the season, but they must inform E20 by 1 June each year if they want to increase the capacity for the following season.
Despite their indifferent form at the start of the season, West Ham have been filling their stadium on a weekly basis, with an average home attendance of 56,903 - the third highest in the Premier League after Manchester United and Arsenal.