The county council is set to face a shortfall of £750,000 due to the much lower than expected selling price of the former Bennerley School site in Cotmanhay.
Derbyshire County Council had expected to make £1.05 million off the sale of the site - but it has been forced to revise this down to less than a third of the original estimate at £300,000.
The authority had budgeted for the £1.05 million windfall in February 2017, but now faces a hefty shortfall in funding which it had not planned for.
The site, off Cotmanhay Road, returned to the county council in 2014 after the school merged with the former Ilkeston School – now Ormiston Ilkeston Enterprise Academy.
Students and staff are now based at the King George Avenue buildings in Ilkeston.
Demolition of many of the former school buildings took place in 2016.
Some of the buildings will remain and are the base for groups and companies such as radio station Erewash Sound, which is in the former media block.
The main block is now occupied by the Breadsall Pupil Referral Unit.
The former school site is the preferred location, as of April this year, for the county council’s new £15 million 40-bedroom care home to replace the current Hazelwood Home for Older People in Skeavingtons Lane, half a mile away.
A new location is being sought due to the cost of essential repairs at the current site – in the region of £3 million, with the total council budget for maintenance at all its 22 homes standing at only £4 million.
Council officers say that the former Bennerley school site would also be able to cater for any need for further care home bedrooms in the future.
The authority aims to start construction in December 2019 and finish by spring in 2021.