You need enough space around the stadium for the entire capacity to be evacuated into a controlled space whilst still allowing space for emergency vehicles.
You also need to plan for 20000 odd people suddenly packing into an area with infrastructure not currently designed to accommodate it. Road, pavement and junction improvements, traffic management Utility upgrades etc
Football stadiums dont get 'compulsary purchase orders.
One reason Arsenal's stadium was so expensive was not only did they have to buy the surrounding buildings, they also had to buy replacement houses/ commercial properties for the occupants.
Stamford Bridge's redevelopment was effectively cancelled due to the objections of one lone householder.
Carparking under a very large structure with significant weight fluctuations on former victorian industrial site with an underground stream on one side and a railway tunnel on the other?
Network Rail held up a housing project near Edinburgh because 4 houses were too close to the roof of a tunnel. When asked what they think about having a stadium built close if not on top of their tunnel the short answer will ne 'No' and the long one 'FUCK no'
Finally Emergency Access 'under a building'?! Yeah you really want to be parking the fire engine under the raging inferno.
Gonna throw this out there.
Whatever you may think or Architects, Stadium Designers, Engineers, Town Planners, and the Officials at the Club, do you think if Broadford works was a feasible they wouldnt have considered it?