Hot water baseboard heat return ques

I'm having an addition put onto my split level home. It adjoins the back wall of the kitchen. The kitchen sits over the basement, while the new addition is 4 feet or so above the outside ground. The outside underneath area will eventually be finished with cement board with a stucco coat of some kind.
My baseboard heat in the kitchen runs along the left wall as you look into the kitchen. The heat can just continue in a straight line into the new room.
The question is how to handle the return. My father mentioned that we shouldn't run the return under the new floor because it will be exposed (even though it will be an enclosed area). He suggested we raise the baseboard on the wall a bit and run a 1/2 return pipe underneath the fins. The old & new baseboards won't line up in height where they join but we could always put something in front of it.
I haven't had a chance to talk to the contracters yet, but I was wondering what people thought of that idea and whether having a section of the pipe go under the floor could cause problems with the cold weather (Eastern PA region). I think their plan is to run the baseboard all the way around the room & go under the floor to get around the exterior door.
thanks.
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saag wrote:

If you are using SLANT FIN baseboards they are designed to allow a return pipe of the same size inside the baseboard. Don't downsize the return pipe it will reduce your flow and efficiency. The company likely makes a trim piece to handle the change in height of the baseboards. I agree with your older/wiser father, don't run the pipes in the unheated area. See page six on the left side to plumb a return: http://www.slantfin.com/pdf/fl30rr.pdf
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I replaced the in-slab radiant heating with a (3/4") SlantFin loop around the outside wall of this ranch house. The return is within the SlantFin units. It works well. I added bleeder valves at the ends of the dips under doors.
RayV wrote:

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