  In this paper we study boundary value problems for higher order elliptic
differential operators in divergence form. We consider the two closely related
topics of inhomogeneous problems and problems with boundary data in fractional
smoothness spaces.
  We establish $L^\infty$ perturbative results concerning well posedness of
inhomogeneous problems with boundary data in fractional smoothness spaces.
  Combined with earlier known results, this allows us to establish new well
posedness results for second order operators whose coefficients are close to
being real and t-independent and for fourth-order operators close to the
biharmonic operator.
