My issue lies with when the sprinklers come on, there is a significant drop in water pressure in the sink, shower, etc. I checked the sprinkler lines and there are no obvious leaks (ie holes in the ground). Part II of my pressure problems is that there is no pressure in sprinkler zone 1 (sprinkler heads do not come up but dripple water), 1/2 pressure in sprinkler zone 2 (sprinkler head come up but are at 1/2 flow) and sprinkler zone 3 has full pressure. The sprinkler repair company wants to add a pressure regular to the line to the sprinklers (runs at over 100 psi when s/b be 75 psi) and split sprinker zone 1 into two zones due to the # of sprinkler heads on this zone). However this does not answer why sprinkler zone 2 is at 1/2 pressure. Ironically, the house was built in 2009 and for the past 5 years, including last year, the sprinklers ran fine with no water pressure drop problems. Any suggestions?

Sprinkler line should Tee off from main line before main water line enters house. It appears to me that your sprinkler line is connected somewhere to a pipe that supplies house with water. Sprinklers have high water flow and they will drain all pressure from the house plumbing when turned ON. I suggest that you separate house and irrigation pipes.

Sprinklers should have water pressure not higher than 95 psi. If higher, install high-flow ( or high velocity ) pressure regulator. ( FYI: regular pressure regulators adjust water up to 75 psi, high velocity pressure regulators go higher... you want to have that option available if needed to adjust for higher pressure )

If your sprinklers don't have enough spray and you have 100 psi of pressure - than sprinkler pipes are most likely either undersized, sprinkler heads screens dirty, or anti-siphon valve is partially closed. But since you say you used to have good pressure until recently than undersized pipe should not apply.

Hope that helps

Milo