Bio Retention Area - removing it?

I just purchased a new house in Howard County, MD. There is a bio-retention area approximately 12 feet from the house (the ground slopes away from the house in all directions).

I would really like to raise the cleanout pipes and fill in the giant cavity with dirt, but I can't find any information about that.. Everything I find is about installing one.

Can I legally fill in the bio-retention area (or rain garden)?

I'm planning to build a deck on the rear of the house that would extend into the bio-retention area so it would really help if I can fill it in or move it.

Would the county inspector say something if he came out to inspect the deck and saw that I filled in the bio-retention area?

Thanks.