Nato said that its new headquarters will cost a billion euros (£850 million), a sum critics are calling unnecessary in a time of international belt-tightening.
Officials defended the project, saying maintenance costs for the "temporary" complex into which the military alliance moved in 1966 were excessive.
"We're not spending more, we're actually spending less," spokeswoman Oana Lungescu said. "This is a project that's been decided on years ago, way before the financial crisis.
"We even have concrete rot, some parts don't even comply with Belgian safety regulations," Ms Lungescu said.
Construction, scheduled to be finished in 2015, will cost about 460 million euros (£390 million), nearly 30% less than the original price, Ms Lungescu said.
The rest of the money will be spent to demolish an old Belgian air force base on the site, clear the terrain of bombs left over from the Second World War, and to pay for new security systems, equipment and furniture as well as the architectural costs.
Nato was formed in 1949 at the height of the Cold War. Its political headquarters was initially in London, but moved to Paris in 1951.
In 1966, French president Charles De Gaulle kicked the alliance out, complaining it was dominated by the United States, and Nato relocated to Brussels, the Belgian capital. It was offered the site of a former airfield just east of the city, from which Nazi bombers had struck at London during the war.
In more than 60 years of existence, Nato has never had a headquarters built specifically for its use, although it has expanded from 15 members, when it relocated to Brussels, to 28 today.
But the move comes as member governments are cutting defence spending and as the alliance is struggling in the Afghan war, its most ambitious mission yet. Over the past two years, defence spending by Nato's European members has shrunk by about 45 billion US dollars (£28 billion).
It also comes as questions are being raised about the continuing relevance of the Cold War alliance, and when Nato's ties with Russia are growing increasingly close.
"The new headquarters is not going to be vacant, but it certainly won't be the nerve centre of European defence and security policy that the old building used to be," said Marko Papic, senior analyst at Stratfor, a global intelligence analysis firm from Austin, Texas.