I am cutting out a section of carpet as I expand my entryway, and I want to ensure it's held down properly.

I am putting a 4' high wall up where the double red line is, and cutting out that section of carpet (later I will probably tile). Right now, there are tack strips all around except for between where I drew the two arrows. If I cut out that section, I guess I need to put new tack strips down and attach the carpet somehow. Am I going to need to use one of those carpet stretching tools to do this? Should I put tack strips down before I put the new wall in place or after? Any other suggestions about the best way to cut this and avoid problems?

Update:

Just stumbled across this again, figured I'd add some post-photos (even though I finished this a couple years ago now).

I used some painter's tape to both mark the cut I wanted, and try to ensure the fibers weren't damaged/dislodged, then just cut it with a sharp utility knife and a level as a straight-edge.

The wall I built was just a 2×4 frame, and before I put it in place, I put a tack strip in along the edge of the carpet there, and pulled it as best I could to get it to attach.

Because I just cut a corner of the carpet out, and it was still attached on all 4 sides, it didn't really move any or need to be "streched" (which is what I was concerned about).

For the door opening, I used a carpet threshold tack strip (forget the name — it has a channel in it that you push a piece of PVC molding into to cover the seam up):

So after four years of living with a tiny entrance that only one person could be in at a time, we were totally happy with the final result… for the remaining two months we lived in that house. Such is life 🙂