You can tune a guitar however far down you want, and the chords will still sound the same in relation to each other.

From the Wikipedia link I gave you previously:

Tunings as low as B♭ tuning may be practiced on an unmodified instrument provided that higher gauge strings are used, although an extended scale “baritone” guitar is better-suited to avoid warping, as its scale length and truss rod are designed for their tension. Many 8-string guitars feature a dual-action truss rod due to the sheer amount of tension from the strings. F and octave down tuning are impractical for a standard scale length guitar, since its scale length is too short for a clear sound and unmodified tuners are not built to admit bass guitar gauge strings.

I was going to say about the same thing that @SavoirFaire backed up with the Wikipedia link. Guitars, especially the neck, are specifically designed to withstand a certain amount of tension from the strings; and the strings are each designed for optimal tone within a certain range of tension. Tuning a guitar too low risks warping the neck.

I have played a 12-string for many years, and there seems to be a running controversy about whether to tune down a step and then capo up two frets. Those who favor such a tuning do so because it places the neck and the face of the guitar under much less tension. I feel that 12-string guitars are generally designed to withstand the tension related to the 12 strings and standard tuning; I also eschew the use of a capo. It’s just another thing to get lost while I’m setting up for the gig.