"I've known that this is in the works for probably about a year," Sasha Pieterse tells PEOPLE of the newly announced Pretty Little Liars spin-off

She’s got a secret – but she no longer has to keep it!

Sasha Pieterse is so “happy” that she can “finally talk about” the Pretty Little Liars spin-off series, in which she’ll star alongside her former castmate Janel Parrish, the actress tells PEOPLE.

“I’ve known that this is in the works for probably about a year,” Pieterse revealed to PEOPLE after taping Dancing with the Stars Monday night. “I feel like PLL made me a good secret keeper, but I’m not a good liar. But I’m super excited about it, and I can’t wait to get started.”

The Freeform show – which will be called Pretty Little Liars: The Perfectionists – is based on another book series by PLL author Sara Shepard. PLL showrunner I. Marlene King is helming the adaptation, which sees Pieterse and Parrish reprising their roles as Alison DiLaurentis and Mona Vanderwaal, respectively.

ABC Family's "Pretty Little Liars" - Season Six
Sasha Pieterse on Pretty Little Liars
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The Perfectionists will be set in a town called Beacon Heights, where everything only “seems perfect,” according to Freeform. “But nothing in Beacon Heights is as it appears to be. The stress of needing to be perfect leads to the town’s first murder. Behind every Perfectionist is a secret, a lie and a needed alibi.”

“I get to work with all of my favorite people again,” gushed Pieterse to reporters on Monday, adding, “So it’s going to be one big happy family and I can’t wait to start. I think we start filming in January.”

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Pieterse also teased that she’d be more than happy if worlds collided and her DWTS partner Gleb Savchenko made a cameo on The Perfectionists.

“I would love that,” she told reporters. “Maybe we can work that in. I’ll speak to Marlene.”