Southwest Airlines should pay for the costly improvements needed to accommodate the discount airline if it moves to Boeing Field, say some King County Council members.

Councilman Dwight Pelz, D-Seattle, said the legislation he will introduce today "isn't intended to stop Southwest Airlines. It's intended to set some ground rules.

"Southwest Airlines needs to know that if they are planning on operating commercial service out of Boeing Field, they're going to have to pay the costs of it. It is not going to be subsidized by the general taxpayers of King County," Pelz said.

Republican David Irons of Sammamish and Democrats Larry Phillips of Seattle and Julia Patterson of SeaTac co-sponsored the proposed ordinance.

The legislation would require any air carrier seeking to provide scheduled passenger service at county-run Boeing Field to pay all related operating costs and capital improvements on and off airport property.

Grants from the federal government or other noncounty sources would be exempt from that requirement.

King County Executive Ron Sims doesn't intend to use any tax money to operate the airport — it pays its own way — and has told Southwest the county doesn't have money for major investments, said Kurt Triplett, Sims' chief of staff. But, he added, "We have to see their proposal, then we have to evaluate it. ... It may be that for $50,000 in traffic-light synchronization we can help bring in a $100 million investment by Southwest Airlines. That would seem to me to be a great investment of county funds."

Southwest's possible move to King County International Airport — the official name of Boeing Field — has sparked debate since Sims revealed last month that discussions were under way between the airline and the county about a possible move.

Southwest has said rising fees at Seattle-Tacoma International Airport are making it difficult to continue providing discount airfares to passengers there.

The possible increase in activity at Boeing Field has drawn opposition from community activists concerned about noise and from Sea-Tac officials, who say Southwest is trying to evade its responsibility to help fund $4.2 billion of improvements it supported.

Triplett said it was "disappointing" that council members would introduce legislation before the county receives a specific proposal from Southwest Airlines.

"We have an opportunity here that needs to be explored, that could keep low-cost air fares in the region and benefit King County Airport, and why would you take options off the table before you know what they are?" Triplett said.

Estimates of what the airline would likely spend on a terminal, parking garage and other improvements generally range between $100 million and $200 million, Triplett said.

Southwest Airlines spokeswoman Marilee McInnis called the proposed ordinance "an interesting development," but declined to comment on it.

Until the airline develops a proposal for use of Boeing Field, McInnis said, she can't say whether it intends to pay for all improvements. "I think it's really too early to say on that," she said. "Until we've completed our investigation stage and looked at what our options are regarding King County International, we really couldn't say what the cost allocation would be."

If Southwest moves to Boeing Field, Irons said, he expects other airlines to follow suit. "The only way this pencils for Alaska and all the airlines is if we had King County residents step up and build all the infrastructure. I've never heard any of my colleagues propose a quarter-of-a-billion-dollar subsidy to make this pencil."

Such an investment at Boeing Field would undercut the massive investments being made by Sea-Tac, Irons said. Irons, who is running against Sims for county executive, said he is "basically opposed to the concept of corporate subsidy."

Keith Ervin: 206-464-2105 or kervin@seattletimes.com