  We prove a Steiner formula for regular surfaces with no characteristic points
in 3D contact sub-Riemannian manifolds endowed with an arbitrary smooth volume.
The formula we obtain, which is equivalent to a half-tube formula, is of local
nature. It can thus be applied to any surface in a region not containing
characteristic points. We provide a geometrical interpretation of the
coefficients appearing in the expansion, and compute them on some relevant
examples in three-dimensional sub-Riemannian model spaces. These results
generalize those obtained in 10.1016/j.na.2015.05.006 and arXiv:1703.01592v3
for the Heisenberg group.
