A memorandum on the extension was signed at Parliament by Saithip Chaowalittawil, acting secretary-general of the secretariat, and Sino Thai president Pakpoom Strichamni. The document does not mention financial issues.
Project adviser Chotjutha Ajsorn said after the signing the extension was required because state property where building will take place was occupied by a school, offices and communities.
Due to bureaucratic complications, original buildings on the land could not be removed in time, he said.
Yothinburana School, a notable building in the area, would have its land expropriated as early as the beginning of next year. But it would take until the middle of next year at least to demolish the old building, he said.
He said Sino Thai had struggled to find sufficient space to dump soil dug up during the construction process within a 10 kilometre radius of the construction site, resulting in delays.
The soil, however, has now been sold and was expected to be removed from the site within 20 days, he added.
With about 15 per cent of the job completed, the construction is expected to be completed in early 2018.
While Sino Thai is allowed to extend the construction period two more times, it has not been decided how long those periods would be.
The Bt12-billion project began in 2013 and was originally expected to be completed this year.
After Sino Thai raised concerns about the delays, National Legislative Assembly president Pornpetch Wichitcholchai set up an investigative committee.
The long delay also concerned Prime Minister General Prayut Chan-o-cha, who used his power under Article 44 of the interim charter to remove Charae Panpruang as secretary-general of the secretariat. It was speculated that Charae was removed because of delays in clearing up land hand-over issues.