Harry Hughes, 16, and a friend cycled to another pal’s home on Elizabeth Drive in the New Mill area of Tring late on Wednesday night.
Harry said: “We were only in there for 15 minutes, but when we came back our bikes had gone. My friend isn’t bothered about his, because it was only an old one he got from the tip.
“We were only about 15 to 20 metres away from the bikes the whole time.”
The teen, who is studying visual arts at Aylesbury College, knocked on neighbours’ doors to find out more information but his efforts proved fruitless.
He said: “The bike’s worth about £900, so that’s why I’m pretty hacked off.”
The theft occurred between 12.30am and 1am on Thursday. The bike has shiny silver chrome handlebars, a black frame and black forks.
Harry, of Wigginton, saved up his money during the summer to pay for the bike parts before building it himself from scratch. He’s been BMX-ing for more than five years, and is a frequent user of Tring’s skate park.
He said: “If anyone gets a close up look at it, there’s some pink on the front rim where the paint’s chipped away.”
A police spokesperson said: “If anyone has any information then they should call the non-emergency number 101. For more information on securing your bike safely, visit www.herts.police.uk.”