Last week, Mayor Sam Adams pitched his City Council colleagues on the latest downsized plans for an Oregon Sustainability Center—what he's touting as a green-business incubator in downtown Portland.

Those plans still call for the city to share the building space at Southwest 4th Avenue and Harrison Street with Portland State University. One element has changed. The proposal now calls for a significantly smaller building, one with 150,000 square feet and seven to 10 stories instead of 220,000 square feet and 13 stories. Cost considerations are driving his new push for a much smaller footprint on the "living" building, which uses design to reduce energy use.

The potential financial commitment from the city and Oregon University System remains considerable even at the smaller size: an estimated $75 million, with almost $6 million from the Portland Development Commission.

Yet the city's share of those costs for the proposed sustainability center is just one piece of a much larger real-estate puzzle City Council must consider this summer. In these tough budget times, commissioners also must weigh the financial impact of the proposal on the 540,000 square feet of office space the city already owns downtown.

Here are all the pieces they'll consider.

1. Oregon Sustainability Center, Southwest 4th Avenue and Harrison Street:
If the City of Portland and PSU succeed in their plans to build the sustainability center, Portland's Bureau of Planning and Sustainability would move its two current offices with 27,000 square feet and 114 employees to one office with 33,000 square feet in the new center. That move would cost Portland $33 million in rent over the first 25 years. The bureau would also occupy 2,000 to 3,000 square feet on the building's ground floor, where it would host exhibits on sustainable industries. At BPS's current location, the city would pay about $27 million for 25 years.

2. Park Avenue West, Southwest Yamhill Street and Park Avenue:
The mayor wanted to help developer Tom Moyer finish his massive private downtown project near the South Park Blocks. Now that the Portland Development Commission has turned down an offer to move its Old Town offices to Moyer's building, the project remains a giant hole in the ground. The city has an economic interest in getting the 26-floor building done; property taxes from a finished building would generate millions for Multnomah County and the city. That's why the mayor once considered moving bureaus other than PDC to Moyer's building. At one point, the mayor mulled moving big city bureaus like 320-employee Bureau of Transportation or 400-employee Environmental Services.

3. 1900 Building, 1900 SW 4th Ave.:
If the Bureau of Planning and Sustainability moved to the sustainability center, it would then leave two existing office buildings with empty space. One of those is the City of Portland's 1900 Building, which currently houses 83 employees from BPS. Without those employees, the 1900 Building would be underutilized and the city might consider selling it. Though PSU has expressed interest in buying the building, Portland still owes $16 million on the facility. Selling it eight years before the city is expected to pay off the debt in 2018 might generate some profits. But those profits could disappear if BPS moved to the more expensive sustainability center.

4. Ecotrust, 721 NW 9th Ave.:
Another 31 employees from the Bureau of Planning and Sustainability work in space the city rents from Ecotrust in the Pearl. That 4,700-square-foot space costs the city $120,000 a year—about $25 a square foot. Last year's estimate for the price of the sustainability center put the cost at $31 a square foot, although the center's planners say they intend to reduce the price by $6 a square foot, closer to market value.

5. The Portland Building, 1120 SW 5th Ave.:
It's no longer likely, but moving the bureaus of Transportation or Environmental Services from their current homes in the Portland Building would create vacancy in the space next door to City Hall. That would look odd right after the city had finished paying off the bonds for the Portland Building in April 2008, which means it's now left with only the building's operating costs.

6. City Hall, 1221 SW 4th Ave.:
In the meantime, City Hall itself could use some relief from overcrowding. The city attorney's office on the fourth floor of the building needs additional space for its employees, according to an outside review of the city's facilities. But the city's Audit Services division doesn't want to move from its City Hall home to make room.