NASHUA – Indoor skydiving has worked in south Nashua, so why not indoor surfing, too?

That’s the thinking behind a multimillion-dollar addition to SkyVenture, off Daniel Webster Highway, that by early next year will hold what its owners say will be the world’s largest indoor-surfing facility of its type.

“When we opened this, I said to my husband, ‘Now, we need the surf machine,’?” said Laurie Greer, co-owner of SkyVenture, which opened in 2006 as the only Northeast facility of its type. She was only half-joking.

The recession put those plans on hold for a few years, but recent city Planning Board approval of the 9,000-square-foot expansion, and the signing this week of a financing deal with Bank of New Hampshire means construction should start in a month or so.

“The financing was the hardest part,” said Rob Greer, Laurie’s husband and co-owner.

“We had several banks that said they were interested and then changed their mind,” Laurie Greer said. “Skydiving has become mainstream; surfing is cutting-edge, I guess.”

The expansion, using a technology called SurfStream by American Wave Machines, will provide three types of waves for different skill levels, in water up to a few feet deep, enough to allow the use of surfboards with fins, or even small kayaks. The Greers say this produces a more realistic surfing scenario than “sheet wave” machines, which use fast-moving sheets of water an inch or two deep.

American Wave Machine says two such facilities are already open – one indoors in Sweden, one outdoors in Peru – with another proposed for next year in Canada.

SkyVenture’s expansion, called Surf’s Up, will be 32 feet wide, twice the width of the one in Sweden and one-third bigger than the one in Peru, Laurie Greer said. It’s part of a large expansion into what is currently the parking lot, which will include a gift shop, cafe and places to lounge surfside.

SkyVenture’s massive vertical wind tunnel to replicate skydiving indoors draws a mix of curious newcomers and experienced jumpers who use the facility for training – a mix that is roughly 60/40 newcomers/experienced in summer, 50/50 in winter when outdoor diving goes on hold. SkyVenture has franchises around the world, but this is the only one in the Northeast.

The Greers hope Surf’s Up will draw a similar mix, casual water park fans as well as surfers anxious to hone their skills on waves warmer than you can find at Hampton Beach.

“People come from all over the world here,” Laurier Greer said. Many come to get coaching from general manager Joe Winters.

The couple estimated that their SkyVenture customers book at least 1,000 room nights a year at the nearby Radisson Hotel.

Surf’s Up will produce three types of waves: a training wave for newbies, a river standing wave that can be used by kayaks and a standing barrel wave for experienced surfers. Clients will buy time slots, during which 20-30 people at a time will have access to a certain type of wave.

Architects’ plans call for a glass room that can slide open in nice weather and is protective enough in winter to allow real palm trees to grow inside.

The Greers have some idea of how water and sky-diving mix. They have a stand-alone operation called Fishpipe that creates a sort of perpetual waterslide inside a gigantic rotating ball.

Fishpipe – a suitable name, since SkyVenture exists on a 1-acre lot on Poisson Avenue, which is French for “fish” – will operate inside the addition.

Construction should start in the parking lot by August at the latest and last about six months, they said.

David Brooks can be reached at 594-6531 or dbrooks@nashuatelegraph.com. Follow Brooks’ blog on Twitter (@GraniteGeek).