The Thai source for this is from a website on the BTS Skytrain extension into Samut Prakan, where the mass transit project is listed coupled with the move of the government offices. If I understand that short text correctly, the study will be finished within the next one to two years, and then developed in the following ten years.
The only illustration is a rendered view of the new province hall, and a map of the central part of the complex, containing the district office (bottom right), the PAO office (middle right) and the province hall (upper middle). The buildings left and right to it are an auditorium and a cultural center. Another very low scale map shows this is only the central part, so the other offices to be moved here will be located around. The forum will probably be the first place where one can find any news in English on this project.
To further illustrate I have created a Google Maps, showing the locations of those offices I know or found on maps like this and the location of this new site. Additions or corrections to that map are of course welcome - I haven't made it into Samut Prakan town yet to check out the current locations of the offices.
View Samut Prakan administration in a larger map