So when you want to set the default value when you are adding a new column in the databse table, you can use the following example. I have created a column named 'tempTest' and its datatype is 'bit' and default value is '0' i.e. false.
ALTER TABLE tblTable
ADD isTempTest bit NOT NULL
CONSTRAINT [DF_tblTable_isTempTest] DEFAULT ((0))
I have added CONSTRAINT 'DF_tblTable_isTempTest' to set the default value 0.