The city has agreed to tweak how it calculates the amount of available space in school buildings, which are often shared by multiple schools, education department officials said Tuesday.

It has also agreed to consider bigger adjustments to the city’s school-space tally, known as the Blue Book, like factoring in how many special-needs students a school serves when calculating how much space it requires. The long-awaited changes stem from the recommendations of an advisory group formed last year to review how the city makes those space estimates, which guide decisions about whether buildings have enough room to house multiple schools and where new buildings are needed.

“These important recommendations will help us better use space in our schools,” Chancellor Carmen Fariña said in a statement.

But the city rejected one of the group’s key proposals: to lower class-size targets for every grade. Doing so would have altered the city’s available-space formula, which could have resulted in more schools being labeled overcrowded and the city appearing to need more school space.

The decision dismayed some advocates who consider the Blue Book’s calculations key to reducing class sizes and curbing co-locations, the contentious policy where multiple schools are housed in the same building.

“You have to incorporate smaller classes into the formula or else the city will continue to co-locate and cram more students into these buildings,” said Leonie Haimson, executive director of the advocacy group Class Size Matters, in an email. She called the decision “hugely disappointing.”

The matter of school space has bedeviled Mayor Bill de Blasio, who campaigned on smaller class sizes and a temporary halt to new co-locations, but as mayor has faced new demands for space caused by his pre-kindergarten expansion and a growing charter-school sector. When he decided it was necessary early last year to allow some new co-locations, he also assembled the Blue Book advisory group partly to appease critics of that space-sharing policy.

Those critics charge that the guide can underestimate the actual space schools require and overestimate how much they have available, paving the way for unworkable co-locations. The advisory group, which includes advocates, parents, principals, and the head of the city agency in charge of school construction, sought ways to make the Blue Book “a more accurate reflection of the current state of our school buildings,” the recommendations say.

The city already took some of the group’s initial suggestions. Last year, the city included students who attend class in outdoor trailers in the enrollment count of their main school buildings. That change was expected to highlight how crowded school buildings will become once students are moved from the trailers back into the main buildings, which the city has called a priority.

Now, the city says its space estimates will take into account whether certain schools have teacher workrooms, private rooms for students to meet with counselors, and at least two rooms for special classes like art and science in small elementary schools — spaces that the working group implied schools need, but are not always prioritized.

Principals will also now be asked to list how many students they serve who have disabilities or are still learning English on the annual space surveys they complete. The city said it will consider a related recommendation: using those enrollment figures to determine how much space for special-needs students each school requires.

Isaac Carmignani, a Queens parent and advisory group member, said some of the group’s suggestions arose from school visits. At one school, they found a large closet without windows or full ventilation being used for counseling; at another, teachers had installed a bench and lamp in the stairwell landing where they were forced to work with special-needs students. He said that collecting more information about schools’ needs is the first step in making sure they are met.

“The Blue Book is only good as the data that’s put into it,” said Carmignani, who is also a member of the city’s education policy board.

Group members said they had finalized their recommendations by March, which is when an education department official told City Council members that the final report would be released “soon.” Carmignani said the delay was partly because City Hall wanted to prepare different constituencies for the recommendations, but also because it did not want them to play a part in budget negotiations this spring or the debate over mayoral control of the school system.

“Timing was very important,” he said. “I think some of the dust needed to settle on those other issues first.”

The rejected recommendation would have reduced the standard class size for middle and high schools in the Blue Book’s formula for calculating a school’s capacity. For instance, the standard for middle schools would shrink from 28 to 23 students per class. Since the average middle-school class had nearly 27 students last year, such a change would have shown more schools to be over capacity.

The de Blasio administration has made plans to create 40,000 new school seats to help alleviate overcrowding, but critics say that is not nearly enough in a city where one-third of elementary schools were over capacity in 2014, according to a city report.

Officials said the advisory group will continue to meet. Education department spokesman Jason Fink said the city shares parents’ and advocates’ desire for smaller class sizes, and that the Blue Book changes will help with that.

“This is a step forward,” he said, “and we will continue to work towards this critical goal.”

Read the group’s full recommendations: