Yes. The cost index is per sqm.
I received a number of quotes from different companies ranging from Baht4.2m, 3.6m, 2.8m and 2.25m for a 182sqm one storey house. The costs came out to slightly under the Medium costs for Line 5. As you will note, the costs vary greatly and the company that quoted 4.2m would not build with AAC blocks, only red brick, so it was the first to be knocked off the list.