Soft shadow quality

How can I improve this to make shadows softer, with no artifacts? It was rendered at max lighting quality. Light source is a Spot Light with non-zero radius. Looks like there are insufficient fake duplicates of this source are placed.

Try increasing the lightmap size of the floor

I had those artifacts when building lighting in Preview, but it fixed itself after building using production quality. As Beef doctor said, increase the lightmap texture size of the floor, that ought to do it!