  We report the discovery of a peculiar L dwarf from the UKIDSS LAS, ULAS
J222711-004547. The very red infrared photometry (MKO J-K = 2.79$\pm$0.06, WISE
W1-W2 = 0.65$\pm$0.05) of ULAS J222711-004547 makes it one of the reddest brown
dwarfs discovered so far. We obtained a moderate resolution spectrum of this
target, and classify it as L7pec, confirming its very red nature. Comparison to
theoretical models suggests that the object could be a low-gravity L dwarf with
a solar or higher than solar metallicity. Nonetheless, the match of such fits
to the spectrum is rather poor and this and other peculiar red L dwarfs pose
new challenges for the modeling of ultracool atmospheres. We determined the
proper motion of ULAS J222711-004547 using the data available in the
literature, and we find that its kinematics do not suggest membership of any of
the known young associations. We show that applying a simple de-reddening curve
to its spectrum allows it to resemble the spectra of the L7 spectroscopic
standards. Given the negligible interstellar reddening of the field containing
our target, we conclude that the reddening of the spectrum is mostly due to an
excess of dust in the photosphere of the target. De-reddening the spectrum
using extinction curves for different dust species gives surprisingly good
results and suggests a characteristic grain size of $\sim$0.5 $\mu$m. We show
that by increasing the optical depth, the same extinction curves allow the
spectrum of ULAS J222711-004547 to resemble the spectra of unusually blue L
dwarfs and slightly metal-poor L dwarfs. Grains of similar size yield very good
fits when de-reddening other unusually red L dwarfs in the L5 to L7.5 range.
These results suggest that the diversity in near infrared colours and spectra
seen in late-L dwarfs could be due to differences in the optical thickness of
the dust cloud deck.
