Patients will be transferred from air ambulances to a lift and then a land ambulance, which will take them for treatment at Queen's Medical Centre.
The £3m helipad will reduce transfer time from around 20 minutes to two to three minutes, Nottinghamshire University Hospitals NHS Trust said.
A ground-level helipad would have been preferable but not possible, it said.
Helicopters currently land at a helipad at Highfields Park a mile away and are then transferred by road ambulance.
Under the plans approved by the city council earlier, patients will reach the East Midlands Major Trauma Centre on the QMC site within three minutes via a dedicated ambulance which will travel along the hospital's own service road.
"We're the biggest major trauma centre in the UK. We see an awful lot of patients," clinical lead Adam Brooks said.
"Our results are excellent but by minimising that time we can further improve the care we deliver."
The Air Ambulance Association's advice is that helipads should be at ground level but Mr Brooks said the layout of the QMC did not make that possible.
The County Air Ambulance Trust, which is giving £250,000 to the project, said every possible solution had been investigated.
"It's not the best solution but it's the only solution at the QMC," chief executive Robert Bertram said.
A fundraising campaign is underway to raise a total of £3m needed for the helipad.
The six-storey car park for more than 650 vehicles will be for staff and visitors and will cost the trust £1.5m. It is set to open in the Spring.
A plan for a helipad on top of a new multi-storey car park at a Nottingham hospital has been approved.