Looking on the internet deeply has found these results:

on the ground floor is the most popular phrase on the web.

at the ground floor 15,380,000 results on the web	on the ground floor 110,100,000 results on the web More popular!
Some examples and use cases from the internet:	Some examples and use cases from the internet:
So, if there's going to be a "three tree in a year" market, I'll get in there at the ground floor and go to the top. the facades at the ground floor level, as well as of the wing buildings, were built in contemporary brick. You could had me at the ground floor, dad. There's commercial at the ground floor.	I know where to find him... he lives on the ground floor. Molly was a fanatic about locking them because iris slept on the ground floor. I was happy living on the ground floor. So with characteristic independence she had established herself on the ground floor of her house. Vicky's business is on the ground floor of our brownstone. The staff entrance is on the ground floor. Meet on the ground floor in 30. The Hesses and Paws on the ground floor.

at the ground floor

15,380,000 results on the web

on the ground floor

110,100,000 results on the web

More popular!

So, if there's going to be a "three tree in a year" market, I'll get in there at the ground floor and go to the top.
the facades at the ground floor level, as well as of the wing buildings, were built in contemporary brick.
You could had me at the ground floor, dad.
There's commercial at the ground floor.

I know where to find him... he lives on the ground floor.
Molly was a fanatic about locking them because iris slept on the ground floor.
I was happy living on the ground floor.
So with characteristic independence she had established herself on the ground floor of her house.
Vicky's business is on the ground floor of our brownstone.
The staff entrance is on the ground floor.
Meet on the ground floor in 30.
The Hesses and Paws on the ground floor.