Re-decorating the surface of an existing wall would not normally require approval.
External walls are considered to be thermal elements (defined in Regulation 2a of the Building Regulations 2000 (as amended).
It is likely that a renovation of a thermal element will trigger a requirement to upgrade the thermal insulation of that element at the same time.
Disclaimer - Wales
This is an introductory guide and is not a definitive source of legal information. This guidance relates to the planning regime for Wales. Policy in England may differ. If in doubt contact your Local Planning Authority.