  We propose a consistent set of boundary conditions for gravity in
asymptotically flat spacetime at spacelike infinity, which yields an
enhancement of the Bondi-Metzner-Sachs group with smooth superrotations and new
subleading symmetries. These boundary conditions are obtained by allowing
fluctuations of the boundary structure which are responsible for divergences in
the symplectic form, and a renormalisation procedure is required to obtain
finite canonical generators. The latter are then made integrable by
incorporating boundary terms into the symplectic structure, which naturally
derive from a linearised spin-two boundary field on a curved background with
positive cosmological constant. Finally, we show that the canonical generators
form a non-linear algebra under the Poisson bracket and verify the consistency
of this structure with the Jacobi identity.
