On Monday, March 9, 2015 at 2:45:02 PM UTC-4, dpb wrote:
On 03/09/2015 1:32 PM, trader_4 wrote:
...

I don't know where the reusing is coming from. All I saw was that he's
putting in a new bath in the basement.

His second posting (followup to his own first) says "Still going to use
the 1/2 feed from regulator and hot water tank." Seems pretty clear to
me his plan was to simply tap onto the existing 1/2" feed for the second
by cutting in a tee. I'm saying run new 3/4" to that point and reducing
tee there; can reuse that 1/2" run he replaces of whatever length out of
the 20-ft it happens to be. He didn't say about the cold supply but
same idea applies.

--

OK, I'm with you now. I agree if it's all 1/2" cold and hot water
to supply the whole house, then I'd do 3/4" for the portion that will
serve everything up to where the new bath will split off.