downer

The word you want is "lower."
Elevators can go higher or lower; they can also go up or down. They cannot go upper or downer.

Clear?

AnonymousIs "downer" a word?

Yes, but it's a noun. Your example is incorrect.

The elevator can't go any lower once at ground level.

http://www.merriam-webster.com/dictionary/downer

Yes, "downer" is a noun when used as a slang expression meaning "depressing event."

However, it is not a noun in the sentence he gave.

Hi Anonymous!
Downer--- A depressant or sedative drug
Downer----Someone or something that brings the mood down
But if the word refers to the elevator you can tell in this way
For ex:The elevator go down the hole.
I take an elevator from the ground floor to the top of the building.

"The elevator can't go any downer once at ground level."
The elevator can't go down any further once at the ground.

Doctor D

is this an option
The elevator can't go down any further once at the ground.

AnonymousDoctor Dis this an optionThe elevator can't go down any further once at the ground.

AnonymousDoctor Dis this an optionThe elevator can't go down any further once at the ground.

Yes, it's another option.

Yes, you could say:
The elevator can't go down any further once at ground level.
Of course, this assumes that there isn't a basement level!