A surprise ending can be a pleasant conclusion to a good book. A surprise ending to the city’s $18 million expansion of the Lawrence Public Library might not be so welcome.

City commissioners should remember that as they enter tricky territory on the library expansion.

City Manager David Corliss has recommended commissioners consider adding several new elements to the library project. The largest would be an additional level of parking to a new garage that will be built just south of the library. A price tag isn’t yet known, but it could cost about $1 million to add 50 to 70 parking spaces. Corliss said that money would be over and above the $18 million that voters approved for the library and parking garage in November.

Commissioners recently agreed to have the architects include the extra level in the design, but to do so in a way that it can be removed from the design if construction costs are deemed to be too high.

That’s reasonable enough. It is encouraging that the city manager is thinking long-term about downtown’s parking needs. But it also is important to recognize the potential pitfalls of expanding the library project.

Voters in November approved the library project — somewhat narrowly, about 55 percent to 45 percent — based on figures that showed the project would increase the city’s mill levy by 2 mills. If at the end of the day the city adds on several other elements and the mill levy is something greater than 2 mills, voters will have a reason to feel burned.

More than city dollars are at stake. The city’s reputation with taxpayers also is on the line. The Lawrence school district’s reputation has suffered because of its decision to use leftover bond money to build several athletic fields. That decision may make it more difficult for the school district to win future bond elections for needed projects. The city doesn’t want to find itself in that boat. City, county and school officials should be careful not to get the reputation of playing the “bait and switch” game to get undisclosed add-ons to costly projects.

There’s not much harm in considering the options for more downtown parking, but commissioners should go into this with the right mindset. First, they should not consider raising the mill levy to pay for any added extras. Second, they should not be hell-bent on spending the entire $18 million approved by voters.

The bond issue gives the city the authority to issue $18 million in bonds. The city can always choose to issue less. An $18 million price tag for a 20,000-square-foot library addition, a refurbishing of the existing 45,000 square foot library, and the construction of a 250-space parking garage is a substantial amount of money. City officials should be vigorous in questioning architects on how to accomplish all they promised voters but for less money. Some local residents still question just how careful city commissioners were in studying the costs and benefits of the library project.

Lawrence taxpayers will be counting on city commissioners to make this right. City commissioners hold the pen here. For good or bad, they’ll write the ending to this story.