I have ordered 1/2 engineered maple flooring for my living room and bedrooms. My subfloor is 3/4 plywood on 16" on center 2x10 floor joists. My question is I want my new flooring height to be 3/4" above the plywood subfloor height to be level with tile installed in other parts of the house. Can I install 1/4" plywood screwed and glued over my existing 3/4" subfloor to bring my 1/2" engineered height to 3/4" above existing subfloor, or will this cause issues with the hardwood? I dont see any problems, but I would like others opinions. Or should I just install the 1/2" engineered flooring over my existing subfloor and live with the 1/4" floor height difference? Thanks

There is no problem installing the 1/4 you suggested. It will give you much better results than having a height difference everywhere else.

Like Mickey says,it's no problem installing the 1/4" underlayment. Are you looking at a lot of footage? Just a thought ... if your just worried about where the hardwood transitions to the tile, 1/4" isn't much. You can install a reducer on top of the wood, ripped to 1/4" thickness at the tile, and tapering at the wood side, it works out pretty descent. At the tile, grout the reducer in with siliconized grout.

Its about 550sq ft, not worried about the cost, having it even is more important.

yup not a problem. This is pretty common for people concerned with matching the height of other existing flooring.