Speaking to Windpower Monthly, the head of offshore wind for the Crown Estate, Huub den Rooijen, said it expects 5-6GW of capacity, beyond the 5GW already financed, by 2020, giving a maximum installed capacity of 10.7GW.
He said that the lower figure is to be expected as greater clarity about the technical nature of projects and government policy emerges.
Den Rooijen's comments came as the Crown Estate announced plans to revise agreements with Round 3 concession holders. Rooijen declined to clarify exactly what was to be revised, saying the details "would not be shared with the public".
"This is the appropriate moment to step back and look again at the agreements and see if they are futureproof. We want to release some of the administrative burden," he added.
Over the past year a number of UK offshore projects have been curtailed, with the Crown Estate admitting more may fail to progress.
Last November energy secretary Ed Davey, speaking at the RenewableUK conference, said that the government was aiming to hit an installed capacity of 16GW by 2020. "I know that some have concerns that we are only committed to the low end of that range, so let me tell you clearly, that is not so," he added.
The Crown Estate made the claims as it announced that it is moving from the allocation of development zones and towards a model where developers select specific projects within them. Of the possible 33GW that was touted in the Round 3 leases of 2010, it said that around 18GW is within the long-term build out.
It said the halving of the capacity figure was to be expected "as government policy evolves" and more in-depth studies reveal technical difficulties with some projects.
The 33GW figure was based on "very preliminary" assessments, Rooijen said.
When asked whether the Crown Estate should have done more to assess the nature of the zones before auctioning them, Rooijen said: "We could have done it differently, but we wanted to leave it to the market to decide what is best. There's an easy pitfall of saying 'we know best'."
The Crown Estate said that the 18GW figure was open to revision and that government policy would dictate whether all of it would be built, or indeed if further capacity could be added.