  We show that a Lipschitz domain can be expanded solely near a part of its
boundary, assuming that the part is enclosed by a piecewise C1 curve. The
expanded domain as well as the extended part are both Lipschitz. We apply this
result to prove a regular decomposition of standard vector Sobolev spaces with
vanishing traces only on part of the boundary. Another application in the
construction of low-regularity projectors into finite element spaces with
partial boundary conditions is also indicated.
