The cost of providing almost 900,000 new school places over the next 10 years will be more than £18bn, EC Harris has estimated.
The £18bn figure is 50 per cent greater than the £12bn estimated by the Local Government Association.
EC Harris head of education Marcus Fagent said the cost would be greater because around 40 per cent of new places would be achieved by expanding existing schools, rather than building new ones.
He said school expansion costs are increasing because easier sites have already been expanded, and more difficult and expensive projects now have to be delivered.
Mr Fagent added that high urban land costs, increasing construction costs and the need for more expensive special educational needs schools would also push up the cost of delivering the places needed.
The LGA called on the government to fully fund the cost of all new pupil places, with the scale of the problem too great to be solved at a local level.
Last year it warned of a £1bn school places “black hole”.
Mr Fagent said standardised designs and “bulk procurement of schools” in batches, flexible procurement models and building mixed-use school sites with housing and commercial uses could help to provide savings.