The figures, published by the internet domain registrar GoDaddy, show that these so-called side hustles make between £500 and £5,000 extra cash for over half of those who set them up.
Camille Simpson, from Slough, is hoping to become one of them. At just 25 and as an expectant mum, she recently set up an arts events management business with two friends. Now she spends her evenings and weekends networking, making phone calls from her living room and building her website.
We started last year as a hobby, and we did our first event which was really successful, she says.
Now, she’s hoping to grow things further – but without giving up her product marketing manager’s job just yet.
At the moment I’ve got financial commitments and I need to have my full-time job. I’m not ready to take that step yet and go out there and do it full-time.
People like Camille are to be applauded, says Matthew Taylor, chief executive of the Royal Society of Arts, and author of a government-commissioned report into modern working practices.
But he’s also concerned that companies don’t use the concept of a side hustle as an excuse not to pay their workers properly.
He says the so-called gig economy , where people work for websites or apps like Deliveroo or Uber, is different from a side hustle, where someone develops their own business idea.
We don’t want a situation in which ordinary work is being turned into gigs for people who are then denied the kinds of protections and rights they ought to have.
And for him, there’s also an issue of well-being at stake.
I’d be concerned if people doing full-time jobs are then working lots of hours in the evenings and weekends in order to make ends meet because that’s ultimately not going to be good for them and they won’t be very productive either.
Some economists are also worried about the concept of a side hustle.
Annie Quick from the New Economics Foundation says: For many more people this is something they’re being forced to do.
We’ve had a decade now of stagnating wages, benefit cuts, and increasing prices so many people are finding that a full-time job is no longer enough to put food on the table and are often having to turn to often poorly paid, insecure employment to top up their income.
Camille’s side hustle may not be paying her much just yet, but she sees it as a potential route to her dream career.
For others, though, the concern is that side hustles don’t become an excuse for full-time stress and exploitation.