The under construction Burj Dubai in Dubai, United Arab Emirates is the world’s current tallest freestanding structure on land, rising 604.9 m (1,985 ft). When completed, it is planned to rise over 800 m (2,625 ft). This building estimated completion will be around December 2008. Current skyscrapers (five tallest buildings in the world) compared with notable under construction skyscraper can be seen below.

Council on Tall Buildings and Urban Habitat (CTBUH) defined four categories in which the world’s tallest building can be measured:

1. Height to the structural or architectural top (including spires and pinnacles, but not antennas, masts or flagpoles)
2. Height to the highest occupied floor
3. Height to the top of the roof
4. Height to the top of antenna

Source : wikipedia