In 1999, following the unexpected promotion of the team to the Bundesliga, the stadium was expanded and modified to meet the requirements of the German Football Association.

During first construction phase, prior to season 1999/2000, the capacity of the Nordtribüne – which contains the area for visiting supporters – was significantly increased.

As a result of the stadium's architecture, development of the Südtribüne (where the clubhouse is situated) and the Haupttribüne (which had an existing roof) was uneconomic.

Following the successful 1999/2000 season, a curve was constructed between the Südtribüne and Haupttribüne, the Osttribüne was further reinforced and a roof was added, and the Nordtribüne was raised.

In May 2000, Bayer 04 Leverkusen came to the Sportpark needing to win to clinch the Bundesliga championship on the final day of the season.

Unterhaching had a good home record and defeated both city rivals in 2000/01, along with victories that season against Borussia Dortmund and Hamburger SV.

Despite the upgrade, the stadium still has several charming idiosyncrasies, for example the two-storey clubhouse behind the Südtribüne and the players' tunnel that also comes out of the south stand.