Let me also clarify, they seem to be "house flies" ...

Any dog poop out there? Or other animal manure? If they are, indeed, houseflies, you'll need to figure out what they're breeding in and feeding on.

Manure, rotting garbage, decaying vegetable matter including grass clippings, and similar substances are what houseflies require.

There are thousands of species of flies and most look very similar to what we call House Flies. Why some congregate in one area, or another, may be related to a food source or maybe to mating, or maybe just a recent hatch of them.

kimmq is kimmsr

Perhaps we are just recent interlopers to the area. This is new construction; two years old. I am also noticing that they are congregating on the opposite side of the house (when hit with the sun and warmth). They also did this last year. We built on formerly agricultural acerage - so manure amendments in the past would make sense. What I am a little concerned about is that this particular area (where they are congregating in the grass) is adjacent to the newly constructed septic field ...

any chance of posting a picture of the site where you see them, also another of the critters?