Some 10,000 seats will be set aside on the second tier for premium ticket holders while the third tier will cater for around 1,300 corporate visitors with hospitality facilities.
An application for planning permission will be made in December and it could take a year to finalise. It is hoped work on the stadium will begin in early 2007 and that it could be ready to host major football and rugby matches in 2009. The existing stadium will host matches during 2006.
The architects added that space for the development has been created by realigning the pitch and creating new access over railway lines allowing spectators entry at premium seat level.
The ground will also provide better disabled access, press facilities and improved player facilities for both home and away teams.