Amazon is building out a forest within the three signature spheres at its new campus just north of downtown Seattle, and now the online retail giant is looking to create an “urban treehouse” in the neighborhood with a new office project.

The project team planning Amazon’s latest office building — a 17-story structure with 405,000 square feet at 2205 Seventh Ave. — will go in front of a Seattle design review board Tuesday, where a group of local architects, developers, residents and community representatives will weigh in on the proposal. Because of zoning restrictions, the newly proposed building is half the size of some of Amazon’s other office buildings in the neighborhood.

Project architect Graphite Design Group refers to the preferred “urban treehouse,” plan in project documents. Staircases in the building would be visible from the outside and represent a ladder one might climb to enter a treehouse.

Amazon affiliate Acorn Development in December paid more than $19 million for the site, a half-block along the west side of Seventh Avenue, between Blanchard and Bell streets. The property is home to a former Days Inn, which has been serving as student housing for Cornish College of the Arts, and an office for the city’s now-shuttered Pronto bike-sharing program.

The project represents the fifth block of office buildings for Amazon in the neighborhood north of downtown Seattle known as the Denny Triangle. As other neighborhoods around it bursted at the seams with development, Denny Triangle didn’t get as much attention, until Amazon planted its flag there.

Today, development is booming in the Denny Triangle as well as the neighboring area of South Lake Union, where Amazon’s massive expansion started back in 2007. Nowhere is this trend more obvious than along Denny Way, the street that separates the two neighborhoods.

Denny Triangle has 33 projects w/ almost 10,000 homes and 3.5M sq ft of office. (But will take years to build out and not all will happen.) pic.twitter.com/78fU3i5b2y — Seattle in Progress (@seattle_nprgres) March 31, 2017

Amazon’s first building in the Denny Triangle — the 36-story Doppler tower — opened in late 2015. That was followed late last year by the opening of the Day One tower, on the same block as the spheres. Construction is underway on the third block across from Day One and the Doppler tower. On another full-block site, formerly home to the Hurricane Cafe, Amazon plans to build a 23-story building and an 8-story structure in the future.