  We develop a systematic classical framework to accommodate canonical
quantization of geometric and matter perturbations on a quantum homogeneous
isotropic flat spacetime. The existing approach of standard cosmological
perturbations is indeed proved to be good only up to first order in the
inhomogeneities, and only if the background is treated classically. To
consistently quantize the perturbations \emph{and} the background, a new set of
classical phase space variables is required. We show that, in a natural gauge,
a set of such Dirac observables exists, and their algebra is of the canonical
form. Finally, we compute the physical Hamiltonian that generates the dynamics
of such observables with respect to the homogeneous part of a K-G "clock" field
$T$. The results of this work provide a good starting point to understanding
and calculating effects that quantum cosmological spacetime in the background
has on the quantum perturbations of the metric tensor and of matter fields.
