Unexpected growth will add $8.5 million to the sticker price of Gov. Butch Otter’s K-12 budget.

Here’s what happened.

When Otter finished up his 2017-18 K-12 spending proposal, his budget analysts figured the state would need to add 179 school “support units.” A support unit is a measure based on student attendance — and it equates roughly to a classroom.

Since then, budget analysts have revised that growth estimate. They expect the state to need an additional 99 support units, on top of that initial projection of 179 new support units.

For example, the new growth numbers add about $4 million to the price tag for the career ladder, the state’s five-year plan to boost teacher salaries. Otter had earmarked $58 million for year three of the career ladder.

To put the $8.5 million into perspective, Otter last week proposed a $101.3 million increase in K-12 spending. That number does not reflect the new growth projections. All told, Otter is seeking nearly $1.7 billion in general fund spending for K-12.