I specialize in building nylon string classical guitars. My father, John Gilbert, built 6-string instruments and added 8-string guitars to his repertoire. I have followed suit, and also build 6 and 8-string guitars.
I have also added to the lineup two other types, a 7-string instrument and a 6-string cutaway. Both of these instruments feature extended fretboards. The 7-string extended fretboard is extended two ways, an extension for the 7th string behind the nut (for extra bass range), and an extended area for the 1st and 2nd strings above the 19th fret to extend the range upward (see the photos here on the site). The 6-string cutaway also features an extension of the fingerboard to allow for up to a 24th fret (double octave) for the 1st string with the 22nd fret for the 2nd string.
All of these instruments are available in two string lengths, 654 mm (the Gilbert standard) and 649 mm.