That can be found where sod had been previously laid.

And I am confused about the landscape designer.
It is (or was) a pretty common practice in sod farming.
A pain to get rid of..

There's no reason to use plastic in the garden. Just dig it out. Since you're not going to have a lawn, do it as the dog uncovers it.

Not that it is critical for your question, but if you want specific help regarding your lawn in "LA," we will need to know specifically where you are. Almost nobody really lives in LA. Everyone lives in the suburbs where climates range from El Segundo fog to back up to the drier hills in West Covina.

Thanks so much!
Forboystoo, The landscape designer is young and maybe showing her inexperience with this. :)
Dchall, We are just off Mulholland in Sherman Oaks. Lol, I don't even know those areas you mentioned! Still think of myself as a Pennsylvanian.... The variety of climates here is interesting though.
Back to snipping plastic.