BOSTON (AP) A sweeping series of reforms to how state and local governments manage public construction projects was signed into law yesterday by Lt. Gov. Kerry Healey.

The reform package represents the most significant changes in public construction reform in nearly a quarter century, said Healey, who was acting as governor while Gov. Mitt Romney was in Seattle for a National Governors Association meeting.

The legislation was based on suggestions made by a special commission that included representatives from all aspects of public construction.

The new law mandates the appointment of a professional project manager for all projects over $1.5 million. Also under the new law, all public construction projects over $5 million will be eligible to use a so-called “construction manager at-risk” method. This ensures cost overruns will be paid by the contractor, not taxpayers.

The law also allows Massachusetts to join 45 other states in a “design-build” system, which allows for hiring one team to design and build a project.

Healey said the series of reforms contained in the laws will save as much as 10 percent on building costs across Massachusetts, which spends $3 billion annually on public construction.

Sen. Dianne Wilkerson, the Senate chairman of the commission, said the new law was “incredible, progressive, aggressive.”