If I start the sentence like this, is that correct?

(a) Behind his house he has a plot of vacant land.

(b) Behind his house, he has a plot of vacant land.
The comma helps, but we would say, "a vacant plot of land."

I believe the comma is optional.
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so, If I start the sentence with "Behind his house..." is correct? comma is optional?
That's correct. I'd almost stake my life on it.

On top of the mountain is a beautiful house. A beautiful house is on top of the mountain. A vacant plot of land is behind his house. He has a vacant plot of land behind his house.

Behind his house is a vacant plot of land. (probably no comma in this case.)

Behind his house, inside the garage, he has a brand new Porsche.