Plots of land worth many millions of dollars that have been sitting idle for years will soon have embassies and high commissions on them.

The National Capital Authority's hardened stance against "fence sitters" on diplomatic land appears to be yielding results, according to evidence given to a national capital and external territories committee hearing in parliament on Thursday.

Iran, Russia and Bangladesh were moving to make use of land after lengthy inaction.

Iran's prime Yarralumla block had been sitting empty for about 20 years but now a deadline of March 31 loomed for construction to start.

The NCA had seen initial sketches for the Iranian mission.

The authority granted Iran an extension with the deadline for construction documentation and tender process to be finalised by Monday.

The NCA has signed a lease with Bangladesh and, according to NCA staff, the mission had been reminded it must build in the three-year timeframe.

Bangladesh was planning and designing its new mission after it signed a crown lease in January and put up the official sign for the high commission in April.

Russia was starting work on its block at Yarralumla and was confirming details about the fences it would use.

NCA chief executive Malcolm Snow said the new tough position had prompted positive movement from "missions that had sat on the fence too long".

Spots for foreign missions in elite suburbs near Parliament House were sought after and some had sat empty for decades.

Other nations wanting to build new embassies had since been sent six kilometres to the south to O'Malley.

Meanwhile, interest in foreign missions sub-dividing land to reduce their expensive running costs appear to have fallen flat.

Spain and Belgium declined to sub-divide after showing some interest, according to an answer to a question on notice to Canberra MP Gai Brodtmann, who had long been lobbying for the efficient use of prime land.