In SQL Server 2008, while you try adding a new column, even though ‘is null is true’, or any change which needs a table drop and recreate, you might get this error message :
Saving changes is not permitted. The changes you have made require the following tables to be dropped and re-created. You have either made changes to a table that can’t be recreated or enabled the option Prevent saving changes that require the table to be re-created.
Basically, there is a small setting that you need to do before this, to have it work.
Go to Tools -> Options -> Designers -> Uncheck the option “Prevent Saving Changes that require table re-creation”
This will rebuild the table , and generally it isn’t worth the hassle if the table is at all large and will make deployment to production trickier.