Add non-nullable columns to an existing table in SQL server?

Question

You just set a default value in the new columns and that will allow you to add them.

alter table table_name
    add column_name datetime not null
       constraint DF_Default_Object_Name default (getdate())

or this one for a varchar field.

alter table table_name
    add column_name varchar(10) not null
       constraint DF_Default_Object_Name default ('A')

You can also drop the default if you do not need it after you added the column.

alter table table_name
    drop constraint DF_Default_Object_Name