How do you spell new adding er?

The word "new" does not change when you add "-er" to it.

Here's why:

* "-er" is used to form the comparative adjective. This means it's used to compare two things. For example, "newer" means "more new" than something else.

* "New" is already a comparative adjective. It means "more recent" than something else.

So, the correct spelling is "newer".