Internal solid wall insulation
If your home was built before 1930 it will most likely have solid external walls with no cavity. Up to 45% of the heat from your home can escape through the walls if they are not insulated.
Internal solid wall insulation works by adding a layer of thermal material to the inside surface of the external walls of the property. Installing this type of insulation is usually more expensive and more disruptive than having external wall insulation applied, however, it may be particularly appropriate where you are not able to change the external appearance of the property e.g. in cases where the external character needs to be maintained.
Planning permission is not needed for internal wall insulation, however, if the property is listed, it is likely to require listed building consent. More information about planning and listed building consent can be found here.
A fact sheet about internal solid wall insulation from Centre for Sustainable Energy is available here