The Tories have called for an inquiry into the estimated £2.3bn cost of the Ministry of Defence's new Whitehall HQ.
The MoD says the project will save money
The party want the National Audit Office (NAO) to look into the private finance refurbishment project, the cost of which will be repaid over 30 years.
Leader David Cameron said British people were on tight budgets and did not want a "big-spending government".
The MoD said the NAO reviewed the on-budget project in 2002 and found there had been "effective procurement".
A spokesman said the Conservative's figure was an estimate and the deal would save £18m a year.
The NAO said the MoD had "achieved appropriate risk allocation" and the management of the project had been good, the spokesman noted.
He confirmed the private finance deal's cost today was £748m, but taking into account running costs and inflation over the 30-year repayment period, it could reach an estimated £2.3bn.
'No expense spared'
In an interview with GMTV, Mr Cameron described the "fantastic" new 10-floor MoD building in London as having "no expense spared".
The building features a restaurant, coffee bar, large plasma screens, a gym and "quiet rooms" where staff can relax.
"Most people are on tight budgets in this country," Mr Cameron said. "They want some help from this government, they don't just want big-spending government, big-taxing government wasting their money."
Shadow defence secretary Liam Fox told the Sunday Times the figures "stretched credibility".
Members of the armed forces have criticised military accommodation
News of the cost of the MoD building came as the government acknowledged more needed to be done to improve housing for the armed forces following criticism of conditions by a senior army officer.
However, the MoD defended the costs saying the building, which has been in use for two-and-a-half years, was more efficient because staff were now on one site rather than spread across 20 different ones as they were in the early 1990s.
The spokesman said the site was not just used by civil servants, but also by the serving members of the armed services of all ranks.
He added: "The 2004 modernisation of the UK's top level defence headquarters - home to 3,300 military and civilian staff - was completed early and on budget it allowed MoD to dispose of five other central London buildings that alone means £18m-a-year, or £540m over the life of the building, can be reinvested where we need it most."
'Cramped and decaying'
Last week adjutant-general Lt Gen Freddie Viggers condemned the armed forces cramped and decaying living quarters in barracks, saying soldiers and their families deserved better.
And last month, outgoing Army head Gen Sir Mike Jackson said some forces accommodation was "frankly shaming".
Defence Minister Derek Twigg said he accepted accommodation was "not perfect" and improvements were needed.