Why can't the database owner create schemas and how can I enable that?

I am setting up a database for an app. So I create a user without superuser
but with login priveleges

I then create a database and set it's owner to that user like this...

dropdb --if-exists api_development
dropuser --if-exists api_user

createuser api_user -P -d
createdb api_development -O api_user

The app can now connect to the database but it can't create any tables,
schemas or anything else unless I give it superuser privileges.

Is there any way I can make this user a superuser for this database without
making it a superuser on other databases?