@Scot ******
I took a grab since it was slightly cheaper however the guy had no idea where he was going and refused to listen to the gps a 35 min journey took more than an hour and a half. With grab it’s hit or miss but I’d take a taxi personally. I’ve attached a photo that I hope helps you find it. If you have the google maps app and search muang thong Thani immigration it will show you the correct place that you can then show to the taxi. When you get there it’s almost like a big compound with several buildings. Follow the signs that say 90 day extension and you’ll find the right building.
*****
day extensions are on the 3rd floor. Hope this helps pal