A fiscal watchdog group said Monday that Mayor Bill de Blasio’s executive budget underestimates future spending on police and other overtime, and on homeless services by hundreds of millions of dollars.

The Independent Budget Office said its spending projections generally are in line with de Blasio’s $82.2 billion blueprint for the upcoming fiscal year 2017.

But its figures for overtime of NYPD, FDNY and Department of Correction staff total $90 million more than the city is setting aside for 2017, and $115 million more each year from 2018 through 2020.

De Blasio budgeted $584 million for police overtime in 2017, but the IBO said the administration should plan for $50 million more.

IBO chief of staff Doug Turetsky said his group beefed up its estimates in response to recent agency overtime spending.

“Typically, they wind up spending more than budgeted,” he said.

The IBO in its analysis said that despite a bump in budgeted funds in 2017 for family and adult homeless shelters, spending will be $106 million greater than planned for 2018, $97 million more in 2019 and $79 million more in 2020.

More homeless single adults are expected to enter the shelter and the city has assigned all of its small allocation of rental subsidies, Turetsky said.

De Blasio spokeswoman Monica Klein in a statement said the city’s overtime estimates “are based on many factors, like snowfall, new hiring levels and our efforts to prudently reduce overtime.”

Shelter spending for the next fiscal year is based on numbers from the current year, when the city “halted shelter population growth for the first time in five years,” she said.

The Democratic mayor released his executive budget late last month. The City Council is reviewing and holding hearings on the proposal and a deal must be struck before the new fiscal year begins July 1.