Squeezed for space on Longwood Avenue, Boston Children’s Hospital explored moving to another part of the city — even the suburbs — before deciding on a $1 billion upgrade of its existing campus.

“We looked at it: Can we find 50 acres?’’ said Dr. Kevin Churchwell, the hospital’s chief operating officer.

About three years ago, Children’s executives considered relocating to the Seaport District, Allston, and as far away as Interstate 495. The hospital’s design firm, FKP Architects, studied the idea.

But Churchwell, at a meeting with the Globe’s editorial board on Thursday, said hospital leaders decided they wanted to stay in Longwood so they could keep institutions like Harvard Medical School, Brigham and Women’s Hospital, and Beth Israel Deaconess Medical Center as neighbors.

Moving the entire hospital would have been a complicated undertaking with a huge price tag: as much as $6 billion, executives said. That figure helped them decide against the idea, but the fact that it was discussed at all reflects how many different scenarios were under consideration before executives settled on their current plan.

“We tried to put all ideas on the table,’’ Churchwell said.

Children’s Hospital, the dominant pediatric medical center in Massachusetts, is seeking state approval to add a new 11-story tower to its Longwood campus. The hospital also plans to renovate existing buildings, construct an outpatient building in Brookline, and add inpatient hospital beds to a facility in Waltham. Together, these projects are expected to cost $1.5 billion. The hospital says it needs to expand to keep up with patient demand.

But Children’s faces opposition from supporters of the Prouty Garden, a healing garden on the hospital’s Longwood campus that would be demolished to make room for the new tower.

A group of garden supporters is suing the hospital to stop the project, but a judge issued a preliminary ruling last month in the hospital’s favor. The green space holds sentimental value for many families whose children found solace there when they were sick.

The state Department of Public Health, which must approve large hospital construction projects, has asked Children’s to submit a report that demonstrates its expansion plans won’t undercut the state’s goals of containing health care costs. Children’s is among the highest-priced health care providers in the state.

The hospital has hired Navigant Consulting to do the analysis and expects to file it this summer, Churchwell said.

Sandra L. Fenwick, chief executive at Children’s, said she’s confident the hospital will gain state approval for its project, after years of planning.

“We are now six years behind schedule,’’ she said.

Priyanka Dayal McCluskey can be reached at priyanka.mccluskey @globe.com. Follow her on Twitter @priyanka_dayal.