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.

Experts say tiger numbers are the most reliable indicators of the health of the population. But they also warn that it is more important to monitor individual tiger populations every year to really get a handle on their health. "Once-in-four-years country-wide estimates do not have much  practical use. But everyone, including politicians and conservationists, seems to set much in store by these numbers," says Dr K Ulhas Karanth, one of India's top conservation experts.
According to the latest tiger census, the tiger population rose from 1,706 in 2011 to 2,226 in 2014. The latest tiger estimation identified 1,540 tigers through images collected from 9,735 camera trap locations in 18 states. "Because of the extensive survey effort and camera trap results, which identified nearly 70% of the estimated tiger number; these figures are the most accurate ever," claims WWF India, one of the country's top conservation organisations.
Sure, tiger numbers have definitely increased since 2006 when India upped investments under pressure from global and international conservationists in hiring more guards, protecting reserves and promoting voluntary village relocation. All this helped, say experts, in many parts of India, although over large swathes, tigers have been wiped out or are in low numbers.
But many questions remain. What is the state of availability of prey in India's tiger reserves? Every tiger requires a breeding prey population of 500 animals in its territory to ensure a "food bank". Tiger populations thrive on abundant prey - a breeding female tiger produces a litter of three cubs every third year. Mortality rates can be high: Dr Karanth's studies show 20% or more higher mortality rates in a thriving tiger habitat in Nagarhole in southern India.
"If we have 600 breeding tigresses in India, we will be adding 600 cubs to the population every year. So we should not be obsessing over individual tiger deaths and focus on population recovery," says Dr Karanth.
India has some 200,000 sq km (77,220 sq miles) of tiger habitat. Well-managed habitats with abundant prey can support anything between 5,000 to 10,000 tigers in the long run. "We have a long way to go, but it is doable if we get our act together. This is no time to be complacent," cautions Dr Karanth.
A plan for a helipad on top of a new multi-storey car park at a Nottingham hospital has been approved.
[NEXT_CONCEPT]
How good is the news that India has almost a third more tigers than it did four years ago?