The architect has pioneered the art of converting the grim structures into bright living or working spaces, and his work is set to increase as Germany ramps up sales of the above-ground forts, originally designed as air-raid shelters.

"At the beginning, the authorities thought I was a bit daft," said Mielke, who spent six years in the 1990s begging the property office in the northern city of Bremen to let him buy the bunker he now lives in.

"They didn't think anyone would really want to live in a bunker."

Twice a year, he brought them elegant sketches to show how he could convert the grey box into a liveable space.

"At the time, the bunkers were still officially on standby use to protect civilians, and no one had done anything like this before," said the 56-year-old, sporting a waistcoat and cravat.

Mieke was eventually allowed to build on top of the bunker in one of Bremen's most chic districts on condition it could still be used if there were an attack.

It turns out Mielke was on to something. After early tentative sales efforts, Germany is now stepping up a campaign to sell the structures and this month launched a competition for conversion ideas.

But there's also a catch: nearly all were built with forced labour. And as bunkers become hot property, critics warn against treating them like any other real estate without acknowledging their past.

WINDOWLESS GREY BOXES

In 2007, Germany decided it was anachronistic to keep up the 2,000 above- and below-ground bunkers built before and during World War Two and in the Cold War.

The dangers the country faced were no longer conventional, involving bombings, artillery and tanks, but terrorist attacks and natural catastrophes - dangers against which the bunkers provided no protection.

Some 220 above-ground World War Two bunkers owned by the state were the first to be decommissioned en masse. Blowing up concrete fortifications often located in dense inner cities was not an option, and so they began to be sold. In addition to bringing in revenue, this ends maintenance costs that can reach hundreds of thousands of euros a year for bigger bunkers.

Only 50 of the bunkers have been sold so far, according to the BIMA Federal Agency for Real Estate.

The first conversions have shown that, with imagination and experimentation, these huge, windowless boxes with ceilings as thick as four metres can become striking living spaces.

"The advantage is that you can plan freely because there are no supporting walls," said Mielke, who converted the entirety of his first bunker once it was decommissioned. "The disadvantage is that it's quite difficult and requires a lot of special knowledge."

With his builder, the architect developed techniques to saw holes in the fortification and push the concrete blocks out.

Mielke has kept his first bunker, stuffing it with paintings and designer furniture, but seized upon the idea of bunker conversion as a hot business proposition. Since then, his architecture studio mielke+freudenberg has converted 13 others.

The architect says his first bunker was something of a bargain, but prices have risen tenfold since. Prices, all at auction, have ranged from 100,000 and 400,000 euros, BIMA says. The buyers are developers or individuals who hire architects to do the conversion, which can double the cost of the property.

HOT PROPERTY

The number of World War Two bunkers in Bremen, once a strategic industrial port city with munitions factories, is especially high at 107.

In two adjacent streets, mielke+freudenberg has purchased three bunkers for conversion. One is now an attractive block of flats, barely distinguishable from neighbouring residential buildings thanks to large windows and balconies.

Another two have been left in their raw state as unadorned concrete blocks, but have been divided up inside into rehearsal studios. The thick walls provide ideal sound isolation.

"Given that my instrument is really loud, it's difficult to find a location where you can rehearse properly - other than in a bunker," said 22-year-old German-Gambian Sascha Barasa Suso, after a drum rehearsal in his small bunker studio.

For art lovers, it's the security, wallspace and shelter from the outside world that proves the bunkers' appeal. Collector Christian Boros has filled a giant bunker in Berlin with more than 500 works by international artists.

Thick walls provide cool temperatures in the summer and store up warmth for the winter.

Yet the main draw of many bunkers is simply their location in the centre of town.

"There's a premium for bunkers in nice districts where you just don't have much choice otherwise," said BIMA's Gerd Oligschleger. He said Germany was stepping up its sales and hoped to auction off the remaining 170 in the next few years.

FORCED LABOUR

Oligschleger addressed the concerns about forced labour by pointing out that much of Germany's infrastructure, from roads to railways, was built by the Nazis in similar conditions. He said it wasn't an issue for today's buyers, who grew up in post-war Germany.

One of Bremen's bunkers is Germany's largest, "Valentin", looming on the outskirts of the city. More than 1,400 prisoners from across Europe died during its construction.

The bunker is not up for sale, but one section could be rented out to defray the maintenance costs, estimated at about 300,000 euros per year.

"It's morally and historically questionable," said historian Marcus Meyer, standing in the gloom of the gargantuan structure.

Meyer is creating a memorial exhibition for the bunker on behalf of Bremen's Centre for Political Education, a state institution that aims to increase awareness of all aspects of political life.

The bunker, 426 metres long and 33 metres high, was intended as a protected place to build submarines. Used for many years as a military depot, it now stands empty, though its damaged roof is home to at least eight species of bat.

Tens of thousands of people worked on it between 1943 and 1945 and a film from the period shows the haggard prisoners in striped uniforms lugging the steel used to reinforce the structure under the watch of Nazi soldiers brandishing guns.

"Valentin" was bombed in the final months of the war - before it could be completed - and there are gaping holes in the 4.5-metre thick ceiling, with the shell of an allied bomb still rusting on the floor.

Critics say renting out part of the bunker could destroy the sense of its immensity, which in turn conveys the enormity of the Nazis' military aspirations.

"Economic interests must be a second priority," said Georg Skalecki of the local State Office for Monument Preservation.

BIMA is in talks with a local citizens' group in Frankfurt that wants to make a museum out of a bunker built on the site of a synagogue demolished by the Nazis during the Kristallnacht pogrom in 1938. BIMA was anticipating a high price for the bunker due to its prime location.

"You just have to be aware of what you are doing, where you are living," said Meyer. "But at the end of the day it's up to each person to question if they are happy living, for example, in a bunker that has been built by forced labour."

(Additional reporting by Holger Koerner; Editing by Sonya Hepinstall)

• A Permanent Economic Emergency