Building Sukkah over a lawn:[1]
One who waters seeds in order so they grow is liable for the plowing and planting prohibition.[2] Therefore, it is forbidden to pour liquid over grass, plants, [trees[3]] and the like. Due to this reason, when eating outside over a lawn which contains grass, plants, or trees, one must beware not to spill or pour water over it. [Hence, one should not build his Sukkah over a lawn, as it is exceedingly difficult to avoid any spillage. If necessary, one should cover the lawn with a makeshift floor.]
[1] Admur 336:9
[2] The reason: As the water makes the soil soft for planting, as is done by plowing, as well as the water is placed so the seeds grow, which is like planting. [Admur ibid]
[3] Ketzos Hashulchan 142 footnote 18, Upashut.