Our sources tell us The Oppenheim Group's flagship office in West Hollywood -- where the streaming series shoots all the office banter -- is expanding, and construction is already underway.

As you can see, scaffolding is up in front of the office building adjacent to the current "Selling Sunset" HQ ... and we're told the Oppenheim bros are taking over the property and knocking down a wall to make the current office bigger.

Tourists and locals driving past the office, which sits on the Sunset Strip, might have noticed a ton of activity there this week ... with movers and construction crews hard at work.

Office supplies are being boxed up and carried out. but we're told it's just to clear the space for the renovations and expansion.

It appears new signage may be part of the office facelift ... the current sign has vanished, at least for now.

We also got "Selling Sunset" star Romain Bonnet at Sunset Plaza and asked him about the expansion .... and he told us he's helping oversee construction on the new and improved office.

Remember, Romain's a project manager on the show and is also married to another cast member, Mary Fitzgerald.

The Oppenheim Group office has been the backdrop of the hit Netflix series for all of its 5 seasons ... and it's since become a tourist attraction.