I have an existing floor that is already installed, and I have an addition off the back of my house that is getting the same wood. The addition is 7/16" higher than the existing room that is finished. What should I do to make this work?

I'd like to do something fancy with the ends of each board so it would mate up at an angle, but I don't think I should butt them up to the existing floor because I will run into problems if the floor expands/contracts. So what type of threshold should I use? I can fabricate one if I need to, however, 7/16" of an inch is quite a height difference.

I've attached a pic below. The board on the left is butted right up against the existing floor, and the one on the right is about a half inch away from it.

If it matters, the wood is custom milled 3.5" white birch planks. The existing floor has red rosin paper under it, which does cause it to expand and contract more than I'd like during humidity changes (I didn't install that one), and is nailed with staples. The new floor has Aquabar B underneath, and is nailed with Portanails barbed nail thingies.

This is my first floor, so I'm unsure what to do about this threshold, and I'm currently stuck because I don't want to go any further without finding out.

Use a reducer transition molding

DITTO! A reducer from the new floor to the existing is what you need.

Anyone have a good source for one? I want it to spread out the height difference as much as possible to make it less noticable.

I'll probably end up just making one so I can match the wood.

On a little different spin how much height difference do you pros feel is reasonable to just shim up with felt or other to avoid having to use a transition piece or reducer? 1/8 1/4 ? If it was an 1/8 inch difference could you just use say double or triple felt under the whole lower floor to bump it up? thanks Bruce

So should I use an overlap style reducer, or a one-sided reducer? I like the look of the one sided better, and it's much easier for me to fabricate a one sided one.

Use a reducer, not an end cap.