The de Blasio administration on Friday proposed raising water and sewer rates 2.1 percent starting July 1 — the lowest hike in 16 years.

Under the plan, expected to be approved by the city’s Water Board next month, the bill for the average single-family home would go up by $23 to $1,078.

Mayor de Blasio has come under fire for not raising rates enough to complete a critical water tunnel serving Brooklyn and Queens.

But officials said the costs of completing the project by 2020 have been factored into the rate increase and that low interest rates and reduced operating costs ­allowed them to keep the hike so small.

In de Blasio’s first two years, rates went up 3.35 percent and 2.97 percent.

De Blasio had accused his predecessor, Michael Bloomberg, of jacking up the rates too high.