The £16.8m revamp of Scotland's national gallery will be delayed for months as it cuts back parts of the project so it remains on budget.
Sir John Leighton, director general of the National Galleries of Scotland, said work, which was due to start this month, was being put on hold because of higher than expected tenders for the cost of sub contractor work.
He also said that the complex nature of the project, which will significantly redevelop the Scottish National Gallery on The Mound in the centre of Edinburgh, had added to the decision to "pause" for "several months" while the galleries and its main contractor, Interserve, re-evaluate costs.
However the director said he was "not fazed at all" by the delay in work.
Sir John said the date for the gallery to re-open, Spring 2019, remained the goal, and he said the delay in the start of construction work was to protect the budget, rather than put the budget at risk by proceeding as planned this month.
He said the difficulties presented by the site itself - on a hill, in a public garden, under a protected building and over the main Edinburgh to Glasgow railway - had played a part in tenders for subcontractor work coming in higher than expected, including what he said were unexpected issues with the cost of access and preparatory work.
Sir John said: "It was always a complex project, in terms of its position: over the railway line, under a Grade A listed building, in a World Heritage site, and it has thrown up some complications such as with access, preparatory works and so on, but I am not fazed by it at all.
"It is not a big deal.
"We are pausing to look again at all the issues involved."
He said that the delay would likely not affect the budget, but was a way of preserving it.
"I am not going to go back to funders and say, 'I asked for Y but I need X', and this is about keeping it in budget rather than just ploughing on, and that is the most responsible thing to do."
Sir John said he would not be pinned down to a new start date for the work at this stage.
The planned redevelopment of the SNG will triple the exhibition space available to the Scottish collection from 440m to 1320m.
When Sir John joined the institution in 2006, he said the display of Scottish art in the windowless concrete basement resembled an "apology" and he was "deeply unhappy" about it.
The redevelopment project is called "Celebrating Scotland's Art".
A statement from the galleries said: "For the past six months we have been working with our main contractor Interserve on the detailed designs and various tender packages for the building work.
"It has become clear that some elements around the delivery of the construction work are more complex and potentially more expensive to implement than was originally anticipated."
It added: "We therefore have to carry out some value engineering in the coming months in order to streamline some parts of the construction and bring the plans into line with our budget.
"In practice this means that we will be re-examining some of the specifications and construction methods for aspects of the design to ensure that the project stays within cost.
"Until this work on value engineering is complete we will not have a confirmed date for the start of construction but we are hopeful that we can begin work on site later this year."
The Scottish National Gallery remains open and the Scottish collection is on display to the public.
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