A 136-room hotel with a rooftop garden will be built on top of a huge Nazi-era bunker in Germany.

Key points: The bunker was built in 1942 by forced labour to shelter Hamburg residents during air raids

The bunker was built in 1942 by forced labour to shelter Hamburg residents during air raids The walls of the bunker are 3.5 metres thick and it was too difficult to demolish after World War II

The walls of the bunker are 3.5 metres thick and it was too difficult to demolish after World War II The redeveloped bunker will also house a memorial to the victims of the Nazi regime

The NH Hotel Group announced plans to redevelop the St Pauli Bunker in Hamburg, creating a luxury hotel with a bar, cafe and restaurant.

The hotel will be built in a pyramid shape over five storeys, the developers said in a statement.

The developers have promised "fantastic panoramic views of Hamburg, in the line of sight of the Elbphilharmonie, and a leafy and meandering green walkway that will surround the bunker".

The bunker, which has 3.5-metre-thick walls, was built in 300 days in 1942 by 1,000 forced labourers.

It was designed to accommodate 18,000 people during air raids.

Authorities decided against demolishing it after World War II as the number of explosives required to demolish it would have also brought down the surrounding residential areas, according to the Hamburg local government.

The bunker has 3.5-metre-thick walls and was built in 300 days by forced labour. ( Supplied: Wikimedia Commons/Andreas Praefcke )

It currently functions as a live music venue, nightclub, music store, and a space for artists and musicians.

A spokesman for the bunker project told CNN the project to redevelop the building was part of a preference by residents to plant "a city garden for the neighbourhood".

"In addition to this new public natural oasis, the bunker will for the first time receive a memorial to the victims of the Nazi regime and the Second World War," Frank Schulze told CNN.

The hotel is set to open in mid-2021.