If you need an additional wire to separate the fan and light functions (is that what you're trying to accomplish) you will need to run a new cable. You can not make the "neutral hot" whatever that means most likely. I'd run a 14-3 (if this indeed is only a 15A circuit) from the fan to the box and not mess with anything else.

I'm going to guess that you will run into problems with Section 314.16 of the NEC. if you just add another 14-3 to the switch box.

To tell you the best approach, I need to know the cubic inch volume of the switch box, and whether it is a two gang or three gang box etc. Also, whether you have room for a bigger box for more individual switches. The volume of the junction box in the attic would also help.

I suspect he wasn't going to "add" another 14-3 but replace a 14-2 with 14-3.

Thanks Fred and Ron. To clarify, the junction box is a 4" wide (2.25" deep) box. The switch box is a 2 gang. Currently, that switch box has 1 "regular" switch and then 1 double horizontal switch. I believe it is NOT an extra deep box. I can replace the current 2 gang with a extra deep 2 gang if required, but there appears to be plenty of room in the existing box. The circuit is 15A. Appreciate the help and advice. Thanks

If I am understanding this correctly, you currently have a 14-3 going between the switch box and the junction box in the attic. And from the junction box, the red wire from the 14-3 feeds the fan/ light via a short piece of 14-2.

*There is no 14-2 in the switch box to just replace with a 14-3 for that fan/light.

*You can't just look at the box and see if there is room to cram in another cable. There is supposed to be lots of extra room in the box to dissipate heat, and your description as "not extra deep" doesn't tell me anything.

From your drawing, you show 4 cables entering the switch box. One 14-3 and three 14-2. In order to switch the fan/light separately, you either need to;

(1) Run another 14-2 directly to the fan/light, provided its junction box is listed and labeled for that many wires.

Needing a minimum 32 cubic inch switch box

OR

(2) Remove the 14-3 from between the junction box and switch box and the 14-2 from between the junction box and the fan/ light.

Then replace the 14-3 with a 14-2 and run a 14-3 directly to the fan/ light.

Needing a minimum 32 cubic inch switch box. ( Best option )

OR

(3) Run an additional 14-2 between the switch box and the junction box leaving the white wire unused. However, this cable needs to enter a steel junction box through the same cable clamp as the 14-3. Then you would replace the 14-2 between the junction box and the fan/ light with a 14-3. ( I don't suggest this option )

Still 32 Cu. In.

Now, since it a appears to have already been remodeled once, and someone else already went to the trouble of adding a junction box in the attic and replacing the original 14-2 with a 14-3 in order to add the fan, instead of just running an whole new additional cable. I have to assume it was due to the switch box not being big enough. An old works two gang only has around 25 cu. in. If it is a metal gangable box, it could even be smaller. You may get lucky with with some new construction two gangs being exactly 32 cu. in.

Thanks Fred for all the options. I'll ensure the boxes are the proper depth and size. I've attached a revised diagram based on Option 2. Could you let me know if this captures it? Line F and Line W are different/modified, as is the switch. Thanks again for your help.

Looks good.