Add a column to a table, if it does not already exist

IF COL_LENGTH('table_name', 'column_name') IS NULL
BEGIN
    ALTER TABLE table_name
    ADD [column_name] INT
END

You can use a similar construct by using the sys.columns table io sys.objects.

IF NOT EXISTS (
  SELECT * 
  FROM   sys.columns 
  WHERE  object_id = OBJECT_ID(N'[dbo].[Person]') 
         AND name = 'ColumnName'
)