Along with costs, expansion work has been adjusted in line with inflationary pressures.
The proposed larger western extension is “not currently viable” but the Durham Miners’ Association (DMA) – which has its headquarters in the building – said it had not “reduced” its ambitions for the site.
Ross Forbes, program director of the DMA, said: “We have had to backtrack a bit on the full plan, but we are continuing to build the larger extension on the east side. With fair winds, we should reopen in the spring.
“It would have provided more space for teaching and community, but we will continue after that in a second phase.”
The changes were made in consultation with Durham County conservation officers and the smaller extension “has been designed with the same architectural quality, detail and features as the wider scheme”.
The National Lottery Legacy Fund has provided key funding for the rebuilding costs since 2020, according to the Local Democracy Accountability Office.