Chambers of secrets

What people really get up to behind the steel shutters of Britain's private lock-ups

'We like the spit-and-sawdust': Philip and Cat Else (far left) in their martial arts school – a storage unit in Reading. Photograph: Suki Dhanda

"I've wanted to run my own gym since I was 12," says Else, now 35 and a professional mixed martial arts trainer and fighter. It was an impossible ambition while he was working as a bouncer and a security guard, briefly as a shelf stacker and a nude model, trying to support a secondary career in the ring. But Else is among a growing number of people who saw glittering opportunity in the bare concrete and steel of a storage unit. Rent was cheaper than most business premises; lighting, security and insurance were covered, the terms and conditions flexible, generally decided on the whim of a site manager. Couldn't a unit be used for more than just offloading bubble-wrapped lamps and surplus kitchen equipment?

Living space has shrunk, recession-struck businesses have downsized, but we are still incorrigible land grabbers at heart, comforted by the idea of a room of one's own, and willing to pay for it. The self-storage industry has grown rich on the impulse, and, in America, where self-storage first emerged in the 1960s, the industry now out-earns the music business. In Britain, growth has been steady since an injection of investment from property moguls at the end of the last decade, and companies like Safestore, Access Self-Storage and Storage King pull in a combined £360m a year.

In the middle of a house move last year, Else and his wife, Cat, mentioned to the manager of their unit that, cleared of boxed possessions, it would make a decent martial-arts training gym. They were half-joking, but the manager thought it over and liked the idea, suggesting a more appropriate space and even offering to arrange a new lick of paint – moody grey, to better suit the fighting aesthetic. Their gym has been open since the summer.

A little creative blinkering when it comes to the cost of self-storage is not uncommon – the rolling rent to harbour, say, an unwanted sofa will quickly top out the expense of junking the sofa and buying anew. "The vast majority of people have a need for storage temporarily, at odd times in their life," says Paul Glenister of the Self Storage Association UK (SSAUK), a group set up to protect the interests of this quickly fattening industry. "But, increasingly these days, we're getting long-term, continuous users. We call them lifestyle customers."

Unit 3602 is deep in the interior of Access Self-Storage in north London, well away from the noise of the industrial shredder that whirrs constantly in the forecourt. Aideen Donoghue needs to concentrate, having splayed herself on a portable ballet barre in a shape that should not be possible. Legs at right angles, body doubled, arms extended behind her back – "I shouldn't really do this," says the 23-year-old dancer from County Tyrone, "it's not an official stretch." But she suffers from scoliosis, or curvature of the spine, a condition that doesn't mesh particularly well with six hours of dancing every day. She has to do a lot of stretching.

When she first moved to London, it raised a difficulty: in an overcrowded city, living in an overcrowded flat, where to stretch? She found the answer in a quiet 12x12 opposite King's Cross station, and has come here to limber up for more than a year. "I didn't think I was going to stay; it was a quick-fix solution. But it's so much cheaper than anywhere else – £150 a month, and to have the same space in a studio would be £30 an hour."

As well as stretching, she comes here to plot steps for dance classes she teaches at a local college, and to practise routines for occasional jobs in musicals. It is handy training, getting used to performing pliés and frappés and jazzy pick-up steps in a confined space. West End chorus girls are no less forgiving than metal walls should you tumble into them.

The location of 3602 on the outer rim of the building means Donoghue gets the use of a knee-high ledge along one wall, and in the spartan world of storage units, a small ledge earns the status of high luxury. She cherishes it. "I don't get a lot of privacy; my life is centred around students or castings and auditions. The only time I get by myself to collect my thoughts is in my unit. I come here to sit on my little ledge and read my book, or phone my family back home. It's like a mini-flat for me. I might bring a bed in here, get an hour's sleep."

The bed bit cannot be mentioned within earshot of anybody in charge, as sleeping in storage units is strictly forbidden. "We call them hotels for your stuff," says SSAUK's Glenister, "but not for you." Still, the industry has its horror stories. In January 2002, Wanda Hudson of Mobile, Alabama was let out of her storage unit suffering from advanced starvation after two months stuck inside. She had survived on tinned vegetables and boxes of juice after an employee had mistakenly padlocked her in; lawyers acting for the storage warehouse, which eventually paid Hudson $100,000 in damages, suggested she had been sleeping in the unit at the time.

It is a problem that has troubled the storage industry – that by providing people a private space, they can get up to things that really need to remain private. The list of crimes committed or concealed behind storage shutters must make surrounding police forces itch to conduct pot-luck raids. The murdered bodies of secretary Kathryn Chappell and teacher Jane Longhurst were both found in storage, Chappell in Manchester in 1993 and Longhurst in Brighton in 2003. A record cocaine seizure was made from a Buckinghamshire lock-up in 2004. A year earlier, 600kg of ammonium nitrate fertiliser was found in a makeshift bomb factory at an Access Self-Storage warehouse in Hanwell, west London; seven men with links to al-Qaeda were convicted following a police operation that involved replacing the warehouse receptionist with an undercover agent.

The receptionist at Access Self-Storage in King's Cross admits to being less clued up than that ("I've worked here for three years, and last week I found a new area I'd never even seen before"), but a neighbouring shop owner remembers talk of an IRA bomb plot foiled on this site by police. Donoghue is untroubled by the thought. Several hours in a locked box every week teaches more localised thinking: one of her students has an impending tap-dancing exam, there have been struggles with a complex step, and Donoghue is considering bringing her into unit 3602 for extra rehearsals.

Unit FE07, on the upper deck of a Storage King in Reading, accommodates rehearsals of a noisier nature. "We were desperate for a place to practise," shrugs David Viner, a guitarist with heavy metal group Seven, formed last year from the scraps of various dissolved bands in the Kent area. "The idea came to me and the guys when we were travelling back from an Iron Maiden gig in London. We kept passing storage places in the coach. More and more and more." (Storage warehouses, as anyone will have noticed if they've looked out of the window during train journeys or motorway drives, are everywhere. There are currently 750 in the UK, enough for every citizen to have just under half a square foot to themselves if they wanted.)

Storage units have littered popular culture, exciting novelists as diverse as Alan Bennett (his 1998 novella The Clothes They Stood Up In hinged on a storage mix-up) and Thomas Harris, whose Hannibal Lecter self-stored a severed head; a 2004 episode of forensics show CSI centred on gender-reassignment surgery performed in one of these anonymous units. Popular culture, likewise, has littered storage units – 32 previously undiscovered Jackson Pollocks, for instance, were found in a Long Island depository in 2005, and a set of previously unseen Stanley Kubrick photographs in Ohio the following year.

A report by the Commission for Architecture and the Built Environment released in August this year warned that new-build British houses often don't have enough space for storage, to entertain friends or for children to play. An alarming 47% of those surveyed said their homes couldn't house all their furniture, and 35% said their kitchens couldn't even accommodate appliances such as toasters. If home is no longer a castle, then at least a storage unit allows some room for manoeuvre outside the ramparts. They realise a self through storage, these renters – as ballerina, budgie rearer and repairman, martial artist, musician and merchant prince – and a few hundred a month seems no great cost.

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• This article was amended on Monday 30 November 2009. We incorrectly described Access Self-Storage as Access Storage Solutions. This has been corrected.