Feya Buchwald points at a bike across the yard. At first, I'm not sure why she's picked it. There are hundreds of bikes in front of us, and at this distance the black one she's chosen seems a bit boring. Then we wander over and I realise we're looking at something special. It's got the frame of a road bike, the handlebars of a trick bike and the forks of a mountain bike. The back wheel is 28in in diameter, the front only 26in. If Frankenstein's monster were a bicycle, he might look like this.
Custom-built bikes are the latest trend in urban cycling. I'm at Brick Lane Bicycles (BLB), a hub for bike-building in east London, and the shop is crammed with adapted two-wheelers. In recent years the craze has been for riding fixed-gear bicycles; now people want to build their own, or pay others, such as BLB, to make one for them.
"Almost everyone is doing it," says David Kitchen, who runs the London Fixed-gear and Single-speed bike forum, an online gathering point for passionate cyclists. "Everyone" may be an exaggeration – this is still a niche interest – but it certainly includes novice cyclists. "We get a lot of people on the forum wanting to jump on the tail-end of the fixed-gear craze, wanting to know where to start," says Kitchen. "And unbelievably they start with trying to build a bike themselves."
It's hard to describe the trend in numbers: there aren't any nationwide industry sales figures. But most of the cyclists, bike-builders, and shop-owners I talk to agree that it is not restricted to east London hipsters. Bristol will hold the country's first bespoke bicycle fair this weekend. Bryan Jackson, who runs a bike-building course from his Canterbury shop, Downland Bicycles, says he has to turn would-be students away. At Brixton Bicycles in south London, sales of bike-making tools have doubled since 2006. Meanwhile, BLB's profits have trebled in the past three years, despite several rival bike-builders opening in the area.
Quite what defines a custom-built bike is a moot point. For Ed Tucker, a cameraman who built his first bicycle three years ago when he was living in Bristol, it meant gathering parts and mostly "cobbling it together out of old bits I found at scrap bike stores and on websites". A friend gave him a white Raleigh frame from the 80s, he bought wheels off eBay, and a chain set and saddle from a Bristol shop called Jake's Bikes. The front tyre came from a website, the back from another shop. It took a few months, but the whole thing cost less than £200 – about a third of what a similar bike would have cost ready-made.
For the author and cyclist Robert Penn, a handmade bike means something a bit different. In his book, It's All About The Bike, Penn describes how he built his own personalised steed. Like Tucker, he plucked parts from all over, including north America and Europe. But, crucially, Penn doesn't buy a frame secondhand: he has it built from scratch by artisan frame-maker Brian Rourke. It's a lengthy and meticulous process – the frame has to suit both Penn's body and his style of riding exactly. Before any work begins, Rourke tinkers with Penn's existing bike, and accompanies him on a long ride to assess his needs.
True aficionados also draw a line between handmade frames made by the likes of Rourke and those made by people known as boutique designers. "Someone making you a handmade frame will chat with you about what you want, gather lots of tubing, and make your perfect bike from scratch," says Phil Taylor, organiser of Bristol's inaugural UK Handmade and Boutique Bicycle Show. By contrast, he explains, "a boutique producer designs a frame, and then might send it off to Taiwan to be built in small batches. You'll buy a complete bicycle from them, but it won't be made to your specific requirements."
Either way, all the methods are increasingly popular. Rob Prentice, a bike-mad art history student from London, recently started assembling bikes in his flat for anyone who wants one. Orders are piling in. By the end of this month, he'll have put together 15 bicycles with secondhand frames. In July, he'll do 20 more. "I reckon we're coming up to a golden summer of bike-building," he says. "Having worked in bike shops over the past few years, it was usually just couriers coming in, or people who took their bikes to the velodrome. Now everyone's interested."
In Southampton, bespoke frame-maker Tom Warmerdam also reports an upsurge in demand. In 2009, the year he set up his firm, Demon Frameworks, Warmerdam made a dozen frames. In 2010, he made between 15 and 20. This year, he thinks he might top 30. Warmerdam is one of what Taylor calls "a new young blood of British frame-building", and he'll exhibit his work at Taylor's show this weekend, alongside roughly 35 other frame-builders (almost the entire British frame-building class). Taylor hopes their combined presence will attract 3,000-4,000 bike fans.
So why and how did all this happen? Cost is one answer. "It can be done extremely cheaply," says Kitchen. "At the budget end, cobbling things together from broken parts, it's possible to build a really high-quality bike for around £250. Most people might think that is a lot of money to spend on a bicycle. But if you're spending less than £300 in a shop, you won't get a great bike. Whereas if you get the parts off FreeCycle, Gumtree or eBay, you will." Even people buying a bespoke frame are, relatively speaking, getting a better deal. "If you want to replace your quite nice but battered old road bike," Penn tells me, "you might have to pay £2,500 to replace it with something on the same level. But if you go to a frame-builder and get a bespoke frame made to measure, it might only be about £750."
The internet has also played a big part, Penn adds. "You can be sitting at home, thinking, 'I've got this type of stem, I wonder what bar it would fit.' And you could Google it and have your question answered in no time." Bike parts are also much easier to track down, thanks to eBay and Wiggle. "I've a neighbour who goes around buying bike parts from jumble sales all over the country. But the internet's obviated the need for those."
Greater investment in cycling also helped: the introduction of schemes such as Cycle to Work, the creation of more cycle routes, and the formation of 17 "cycle towns" such as Derby and Blackpool, which saw a 27% rise in cycle use. And as cycling became more popular, people became more interested in how their bikes worked.
In London, where cycling has risen by 117% in the past decade, Prentice thinks the emergence of Boris bikes, a public cycle-sharing scheme, has also played a part. "We're riding the Boris bike wave," he says. "Everyone's tried one, cracked their fear of London cycling, but noticed those bikes don't perform that well. So the roll-on effect is that they now want to build their own." Some customers want a high-performance machine that will make their commute as fast as possible. "But there are people who just want the most beautiful thing you can make," he explains. They are in effect paying around £200 for the parts and about £40 for the labour, for a fashion accessory.
It was entirely apt then that the song Being a Dickhead's Cool, a viral send-up of the Shoreditch hipster, included the line: "Just one gear on my fixie-bike." Negative connotations aside, though, the fixie has a significant place in the bike-building story. Ten years ago, Kitchen explains, fixies "only existed as track bikes in velodromes. They weren't used on the street, and the only place in the world where you could buy them in bulk was Japan." So an increasing number of people started building them themselves – and a craze was born.
It was a reaction, Kitchen argues, against a homogenised bike industry. There are very few large British bike manufacturers left, and many of the brands sold here – Trek, Scott and Colnago, for instance – are built offshore by a single Taiwanese firm: Giant.
Companies such as Giant appeal to a mass market and are reluctant, Kitchen feels, to experiment with new designs. Ian Beasant, Giant UK's managing director, disagrees. "I think there's a huge choice of off-the-peg products. We have an expansive range, especially in the lifestyle and commuter section. And if you look at the mountain bike segment, we're all trying to drive that." Ironically, though, it's mountain bikers who are starting to embrace the handbuilt model, according to Warmerdam. People are taking traditional mountain bikes, giving them bigger wheels, smoother tyres "and running them as urban bikes". Now that the big firms have started mass-producing fixies, amateurs have had to find new ways to give their bikes personality.
And perhaps it's this opportunity for self-expression that best explains why unique, handbuilt bikes are so popular. "It's about imagining a bike that has not previously existed," says Kitchen. The process is practical, too: you learn how a bicycle works. "It's like Meccano," Tucker explains. "You experiment with what fits what. Sometimes that's quite frustrating, because you buy something that's not compatible with something else. But you end up with something very charming. You're riding something you've put together." Barny Stutter, co-owner of Brixton Cycles, puts it best. "What else is left like this in the modern world?" he asks. "You can't really build your own computer. You can't build your own car. But you can build your own bike."
• This article was amended on 10 June 2011. The original said: "If Frankenstein were a bicycle, he might look like this." This has been corrected.