What are the Structural and Legal Implications for Building off Plan ?

I am involved in a project building a 2 story building 18x12M

The First Floor has changing rooms on each side 3m wide and 3 units in the middle, 4m wide. The second floor will be bar and restaurant.

The structural plans have received approval from Tessa Baan.

After flattening the land, we are considering building the 3 units one metre higher than the changing rooms on the side. The changing rooms will have higher walls so that the 2nd floor will be flush.

What are the technical, structural and legal implications of doing this ?

You'll need to talk to your architect about the structural implications of going 1m higher, you may (or may not) need bigger columns / more rebar depending upon how over-designed the initial structure is.

As to the legalities, we were told when we got planning approval for our house (in Pathum Thani) that we could change almost anything EXCEPT the external dimensions.

I suggest that you should talk to the planning authorities to discuss whether you need a new approval, a variation on the existing approval or nothing.