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Work to make Britain’s high-rise residential buildings safe from hazardous cladding could cost up to £22.4bn, Britain’s spending watchdog has revealed.

The high-end estimate figure highlights the huge potential cost behind the UK’s cladding problem in 2017. Grenfell Tower Fire that killed 72 people.

At the low end, the estimated cost of remediating hazardous coatings was £12.6 billion, the National Audit Office said in a report on Monday.

The report also warned that work to fix problems in buildings taller than 11 meters was unlikely to be completed until 2035.

The NAO said 60 percent of buildings requiring remediation work had not yet been identified.

“There remains significant uncertainty around the number of buildings requiring remediation, costs, timelines and recovery of public expenditure,” NAO chief executive Gareth Davies said.

The majority of the cost of repairing thousands of buildings across the UK with cladding that could allow fire to spread is intended to be borne by building owners and developers.

But the UK government is preparing to fund up-front costs of £9.1bn and plans to reduce this to £5.1bn through a levy on the property sector.

there was violent arguments on apportioning the cost of the work between ministers, developers and rental property owners in the buildings.

Davies said putting responsibility for paying developers would help save taxpayers’ money. But he added: “This has also led to disputes, causing delays.”

ministers I’m calling right now To ensure the removal of hazardous coatings on all buildings higher than 11 meters that are thought to pose the greatest risk by 2035.

The NAO said 98 per cent of “tall” buildings over 18 meters had been identified and “progress” had been made in tackling dangerous cladding in the seven years since the Grenfell fire.

But according to the watchdog’s report, there was no mandatory registration for “mid-rise” buildings between 11 and 18 metres, meaning many of them went undetected.

As a result, the report stated that as of August this year, only 4,771 buildings higher than 11 meters that required improvement were identified.

That figure could be as low as 40 percent of the total of 9,000 to 12,000 mid- and high-rise buildings that the government estimates will need some remediation work.

Responding to the report, Building Safety Minister Alex Norris said the pace of improvement was “unacceptably slow” and promised to publish a “building improvement acceleration plan” soon.

“This government is taking action, delivering on our commitment to invest £5.1bn to remove dangerous cladding and ensuring those responsible pay the rest,” Norris said.

The NAO has warned that the Department of Housing, Communities and Local Government must “manage risks” to keep improvement spending below £5.1bn.

The NAO said it was unclear how much the building safety levy, which is meant to raise most of the funds, would generate. Builders won’t start paying the tax until the autumn of next year at the earliest.