Consider it a plot twist for the nearly $20 million expansion and parking garage project for the Lawrence Public Library.

City commissioners at their meeting on Tuesday will consider approving a $1.2 million design contract with Lawrence-based Gould Evans Architects, but some city leaders now want to consider enlarging the proposed 250-space parking garage.

“We have one opportunity to do this,” said City Commissioner Aron Cromwell. “I think it is at least worth having the option and a price tag presented to us.”

City Manager David Corliss is proposing that the architects put together a bid option that would add another level to the garage, which will be built on the existing parking lot that is between the library and the senior center. The new level would add another 50 to 75 spaces to the 250 spaces that are currently proposed.

But it also would add to the costs of the $19 million library project — $18 million from a bond issue approved by voters in November and $1 million from private fundraising done by the Lawrence Public Library Foundation. Both Corliss and Cromwell said the money for additional parking would not come from the bond issue. Instead, Corliss said it could come from the city’s parking fund, capital reserve funds or other sources.

Cromwell said if the city moves ahead with the extra parking that it will be important to do so without increasing the mill levy.

“I think there could be a tendency for people to worry about a bait-and-switch going on here,” Cromwell said. “That is not at all what we’re trying to do. I think because of that, though, it is imperative that we not raise the mill levy to do this.”

A major selling point in the library bond campaign was that the project would not increase the mill levy by more than two mills — 1.5 mills for the construction and 0.5 mill for increased operations.

If commissioners approve the design contract on Tuesday, bids for the project could be received by this fall. Construction on a new library is expected to take about 20 months.

Several other design issues also are scheduled to be discussed Tuesday. They include:

• Designing the parking garage in a way that would allow it to also function as a public transit hub. Currently, the city’s main transfer point for the transit system is at Ninth and New Hampshire streets. The transfer station today is essentially just an outdoor bus stop. A facility in the parking garage would include restrooms and indoor waiting area.

• Consider how a public plaza area in front of the new library could be utilized to host the Downtown Lawrence Farmers’ Market. The market currently operates in the 800 block of New Hampshire and the 1000 block of Vermont streets. But leaders of the market previously have expressed interest in having more space for vendors, access to restroom facilities, and perhaps other amenities such as a portable kitchen.

• Cromwell said architects will be told to continue working on exterior design options for the library building. Exterior renderings were presented during the November bond issue, but Cromwell said he wants more options to consider.

“We’re really focusing on trying to get the best building we can,” Cromwell said. “We’re not at all going to be limited by those previous renderings. I think everybody agrees there is significant work still to be done on the exterior.”

• Corliss also plans to begin studying the feasibility of a special bus route that may help people get to the library and the senior center during the 20-month construction project. Parking in the area will be limited during construction. But Corliss said a special transit route that travels between key parking lots in downtown and then drops people off at the library area could be a potential solution.

Commissioners meet at 6:35 p.m. on Tuesday at City Hall.