Hi all, and thanks in advance for any insight you offer.

I live in portland, Oregon. The house is currently 2550 sq feet. Has one unit to do the whole house.
Trouble is, the upstairs is always significantly hotter than the downstairs.
We will call this problem 1.
Problem 2 is that I would like to do an addition of about 500 sq ffet, mainly upstairs. But at the end of the house furthest from the unit, and I would like it heated and cooled.

I have talked to at least three hvac guys (each from a different contractor bidding the job). And they don't know what each other has suggested.

current setup... only one return, in the stairwell that goes upstairs at about the floor level for upstairs. Only one down stairs room is vaulted, and that vault doesn't open to the upstairs (there is actually a wall with windows from an upstairs room looking into the vaulted downstairs room) Temperature differential from the thermostat (downstairs) and the master bed room upstairs in the summer is about 5 degrees.

Guy 1 says he can just add vents to the existing system for the new rooms, and make the system a two zone system if I want to make it so they balance better.

Guy 2 says he can do the same as guy 1, but we need to add a return upstairs to get the balance we are looking for with the 2 zone system.

Guy 3 says he would have to add a unit in the attic to handle the addition, and unless we want to mess with the existing upstairs rooms so that the attic unit can manage them as well, we will need to add a return for the existing system upstairs and two zone that existing system.

needless to say it gets more expensive from guy 1 to guy 3.
But the apparent experience level of the guy increases as you go from 1 to 3.
And the amount of time they actually looked at what was there increased from guy 1 to 3. As well as time spent brainstorming about a solution.

So normally I would think guy 3 was trying to sell me more than I need. But the time and experience factors seem to imply that the other guys were just trying to get their contractor the job, and might not be concerned about the end product.

Any tips on how I can figure out what is really needed here?