American officials will waste billions of dollars on extra facilities being built with U.S. tax money for Afghanistan's military unless they scale the size and number of the facilities to match actual needs, the top watchdog for Afghan reconstruction said Friday.

As part of the 2014 transition, the U.S. is building facilities worth $4.7 billion and transferring existing U.S. bases to hold Afghan National Security Forces troops. But officials didn't take existing U.S. buildings and future troop drawdowns into consideration, and may be left with expensive empty buildings, according to the Special Inspector General for Afghan Reconstruction.

"(The Department of Defense) is missing additional opportunities to reduce waste in the ANSF facilities construction program before the drawdown of U.S. and coalition forces is completed in December 2014," wrote IG John F. Sopko. "Given the size and scope of the remaining $4.7 billion ANSF basing construction projects, the U.S. government has the opportunity to prevent the construction of excess facilities and reduce wasted funds."

Officials also planned construction of permanent buildings with a 25-year lifespan for a surge lasting four to five years, according to the SIGAR report, meaning even buildings used for the Afghan surge will likely be standing empty in less than a decade.

The Afghan military currently has 352,000 troops, but will be reduced to 228,500 after a planned troop drawdown. But officials told SIGAR they can't change their construction plans until the International Security Assistance Force formally approves the reduction.

Even necessary buildings may cost taxpayers more if they don't meet a December 2014 construction deadline, SIGAR said. At least 36 buildings, worth almost $1 billion, won't be done before U.S. troops leave the country, and will become even more expensive because of the security and oversight costs of extra building time.

SIGAR recommended U.S. officials base building plans on potential ANSF reductions and identify underused facilities that the Afghan military could use instead of building new ones.

Underused and empty buildings have been a recurring problem in Afghanistan. Three Afghan Border Police bases in Nangarhar province were unoccupied or weren't being used for their intended purpose, according to a July 2012 report. SIGAR investigators in January found only 12 people occupying a $17.3 million border police headquarters facility built to hold 175, and said in July a $34 million unfinished command center in Helmand province will never be occupied.