THE devastation of the Mackintosh Building in Glasgow has been apparent since flames ravaged the world-famous site and its unique interiors.

Now the director of Glasgow School Of Art has spelled out just how much money will be needed to restore it and how long that painstaking job will take - up to £35 million over four years.

Tom Inns was addressing MSPs at a special session of evidence at the Scottish Parliament less than a month after the fire.

The school has "a good sense" of the cause but has to wait until the joint police and fire service investigation has concluded, Professor Inns told MSPs.

Appearing before the Education and Culture Committee, he initially described the fire as an "accident" before correcting himself to call it an "incident".

About a tenth of the historic Mackintosh Building and a third of work by students' - which was hours from completion - was destroyed in the blaze.

The fire broke out at about 12.30pm on May 23 and the deadline for the final year students' degree show work was 5pm that day, said Professor Inns said.

The school had initially set a fundraising target of £1m to restore the building but is now looking to raise about £20m, he added.

Glasgow School Of Art is involved in a "complex negotiation" with insurers, but they will not fund the whole restoration, he said.

"We have been in discussion with the insurance company and a team of people appointed to deal with the immediate securing of the building, and the estimates are probably somewhere between £20-£35 million," he said. "In terms of the actual accident - or the incident - itself, that is still under investigation by the police and the fire service so they still have not reported back on that.

"We have a good sense of what happened but there are certain details still unknown, and we will have to reflect on what happened with that particular incident."

He added: "The assessment process I think was going to get frozen at 5pm that Friday afternoon and that was when all the work was going to be inside the degree show spaces and then the assessment teams would be going in on the Monday."

All 102 students affected have since been assessed under "mitigating circumstances" and awarded their degrees. They will also be able to bid for a share of the Scottish Government's £750,000 Phoenix Fund to recreate their work and recover from the setback of missing their final degree show, which would have been a chance to showcase their work to galleries and collectors.

Professor Inns said it would take 12 to 18 months to reopen the east wing of the building and 36 to 48 months to reopen the west wing.

He said he was "reasonably confident" they would be able to fully fund the restoration.

"I say reasonably confident because there are unknowns in that because we have an insurance policy on the building - the building is well insured," he said.

"The insurance will cover certain things, it won't cover other things. Obviously, it's a very complex insurance case.

"We will have to raise funds, so we will be setting up a fundraising campaign, so if all of these things come together we will be able to cover the cost of that restoration.

"I am an optimistic person."

The fund will be formally launched today and Professor Inns added: "We're looking to raise probably between £10-£20m.

"We have had some very generous contributions from individuals, one or two pledges from trusts and organisations, but it is too early to say exactly what that is, but I think we can be reasonably confident that if we worked that well and effectively we would be able to raise the funding.

"The Scottish Government said it would match-fund up to £5m to support the restoration. There is no more detail from the UK Government."

Conservative MSP Mary Scanlon said: "The UK Government has said: 'We don't know the precise extent of the damage or what the costs will be so we can't put a figure on it, but we will make contributions in the millions if necessary'. So they're waiting for detail."