Massive terrace partition wall, how wide?

Hello,

What would you say, how wide should we have the massive terrace partition wall built?

Our terrace will be 3m deep but we want to enlarge it to 4m later.

Should we also make the terrace partition wall 4m wide or keep it at 3m?
What is usual?
Is the terrace partition wall always as wide as the terrace is deep?

With a semi-detached house? I would adjust it to the depth of the terrace as long as the development plan allows it. Often only 3 meters depth and 2 meters height are permitted.