How to estimate the area of a polygon?

How roughly? If you increased every dimension by the same percentage, then the area would increase by the square of that percentage. That is, if all the distances increased by 10% so they are 1.10 of their previous value, the new area is (1.10)^2 = 1.21 of the previous area. If all the distances are increased by 50%, the new area is (1.50)^2 = 2.25 of the previous area (225%). So if you can approximate this increase as a percentage, you can use that squaring idea to estimate the effect on the area. I realize that 20 meters is a larger percentage of some lengths than of others, but if you can figure out an average percentage increase you won't be far off.

How roughly? If you increased every dimension by the same percentage, then the area would increase by the square of that percentage. That is, if all the distances increased by 10% so they are 1.10 of their previous value, the new area is (1.10)^2 = 1.21 of the previous area. If all the distances are increased by 50%, the new area is (1.50)^2 = 2.25 of the previous area (225%). So if you can approximate this increase as a percentage, you can use that squaring idea to estimate the effect on the area. I realize that 20 meters is a larger percentage of some lengths than of others, but if you can figure out an average percentage increase you won't be far off.

By buffer you mean moving,because it you move any object from is center and all vertices, you only are replaced it from it is original place the area still will be the same, but it increased in zize then you can use a ratio like 1:20.