Ministers have been urged to put the brakes on new free schools and instead focus on upgrading current failing buildings.
A National Audit Office (NAO) report reveals that billions of pounds are being spent on the creation of new free schools, while the current schools estate – 60% of which as built before 1976 – fails to meet standards.
The report says that while the DfE is "making progress in improving schools in the worst condition", some £6.7bn is needed to bring existing school buildings in England and Wales up to a satisfactory level.
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Meanwhile, the cost of the DfE's free schools programme – which sees schools set up by individuals or communities using public funding – has sharply risen since the coalition government came to power in 2010.
That year ministers vowed to open 315 free schools at a cost of £900m by 2015, but the NAO found that, so far, 305 free schools have opened at a cost of £1.8bn.
The government says by 2021, £9.7bn will have been spent on 833 free schools.
Sir Amyas Morse, head of the National Audit Office, said having sufficient school places "in safe, high-quality buildings" was "a crucial part of the education system".
"The department has responded positively to start to meet the challenges it faces in relation to the quality and capacity of the school estate," the NAO boss said.
But he added: "Significant challenges remain, however, as the population continues to grow and the condition of the ageing estate deteriorates.
"To deliver value for money, the department must make the best use of the capital funding it has available – by continuing to increase the use of data to inform its funding decisions and by creating places where it can demonstrate that they will have the greatest impact.”
Meg Hillier, chair of the public accounts committee said: “This is taxpayers’ money that could be used to fund much-needed improvements in thousands of existing school buildings."
Opposition parties have already seized on the NAO's report, with Labour's Angela Rayner accusing the government of continuing "to waste millions opening free schools in areas that do not need them", while John Pugh, Liberal Democrat education spokesperson, said the watchdog's findings had shown the DfE's costs were "out of control”.
“Ministers are spending over £30m simply for plots of land, and their spending on free schools is pushing the department’s capital spending above all but four of the UK’s major housebuilders," he added.
“While many existing schools are struggling day-to-day in the face of harsh budget cuts, and many are unable to afford basic repairs and maintenance, it is disgraceful that money is being used so carelessly elsewhere."
But a spokesperson for the DfE pushed back against the criticism, saying: "As the NAO acknowledges, we have made more school places available, and in the best schools.
“The free school programme is a vital part of this – more than three quarters of free schools have been approved in areas where there is already demand for new places and the vast majority of those inspected are rated good or outstanding by Ofsted.
“The government is making a huge investment in the school estate of £23 billion up to 2021, to create a further 600,000 new school places, deliver 500 new free schools, and rebuild and refurbish buildings at over 500 schools.
“But we want to go further. That’s why we have set out plans to create more good school places, in more parts of the country, by scrapping the ban on new grammar schools, as well as harnessing the expertise and resources of our universities, and our independent and faith schools.”