Land will be requisitioned to extend the line south from Tao Poon station to Rat Burana district.
The 23.6-kilometre-long extension has a budget limit of Bt124.95 billion. The route will run underground from Tao Poon station for 12.6km before emerging on Suksawat Road and running on elevated tracks for the remaining 11km. The last station will be at Khru Nai in Samut Prakan.
Land surveys are currently being conducted before plots are requisitioned for construction, the MRTA said.
The tender process will start around the beginning of 2021, it added.
The Purple line extension is due to open in 2027.