According to the Local Democracy Reporting Service, the cost of residential placements for looked-after children added to the council’s costs, with 10 unexpected placements added over the summer.
The number of children in care remained steady at around 457, but the complexity of cases and higher placement fees were making it difficult to keep costs down, the report said.
The council’s total 2025-26 budget, external is just under £359m.
Rotherham Council’s Strategic Director Children and Young People Services Nicola Curley said: “Increasing costs of children’s residential provision isn’t just being felt in Rotherham, it is a national issue.
“We have seen a rise in the number of children and young people with complex needs, meaning that some placements require additional staffing, which in turn increases the cost of placements.”
The council said it was also facing challenges in recruiting foster carers, meaning more children had to be placed in external, and more costly, settings.
Ms Curley said that the council was investing in authority-owned residential homes to increase capacity, according to the Local Democracy Reporting Service.
A growing demand for residential and nursing care for older people and those with mental health needs had also led to increased costs for the council in adult social care.
In addition, a nationally agreed 3.2% local government pay rise was set to cost the council £2.3m more than it expected when this year’s budget was set.
Despite these pressures, finance bosses said the council was still in a strong position overall.