The projected cost of creating specialist primary school spaces in Barnsley has more than doubled to almost £7.4m.
Space at The Mill at Worsbrough Bridge is set to be converted to provide 40 places for children with special educational needs and disabilities, helping more pupils learn closer to home.
Children are expected to start at the school in September 2028 but a council report said the refurbishment was more extensive than originally anticipated.
Barnsley Council plans to convert an empty building to provide specialist support for pupils who are on the autism spectrum and have communication needs.
A local authority report said surveys and design work revealed more extensive refurbishment, access works and specialist equipment requirements were required, pushing the total budget up from just under £3m.
Building works and specialist fit-out costs have risen from £1.8m to £3.6m, while the estimated cost of a new site entrance and vehicle access route has risen from £320,000 to £760,000.
Fees, overheads, risk and contingency allowances now total £2.34m, compared with £420,000 previously.
Officers describe the overall figure as a cautious estimate rather than a final construction price.
The project would be paid for entirely through a ring-fenced government grant, without additional council borrowing, according to the Local Democracy Reporting Service.
