okay IF you entered the country on a visa exempt entry stamp <-(meaning you did NOT buy a visa from a thai consulate before you came here and were stamped in FREE for 45 days) you do NOT go to the main immigration office in Bangkok out at the Chaengwattana Government Center to get your extension :O
Instead you go to the alternate immigration office at Muang Thong Thani at this location;
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6,100.
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3&entry=gps&shorturl=1
You want Counter K once you get inside.
This is the building, and the arrow is where you enter
Keep in mind that ALL immigration offices in the country are closed Monday, Tuesday, Wednesday and Thursday this coming week because of the Songkran holiday. The only day they're open next week is Friday (and it will probably be a madhouse there).