You don't need planning permission providing that your 'permitted development' rights have not been removed by a condition on a previous planning permission.
Wildlife note: works to roofs and roof voids may have a negative impact on protected animals such as birds and bats, which often inhabit buildings. You've a legal obligation to ensure that your building work doesn't have an impact on protected animals and you may need to carry out a wildlife survey.
Bye-laws note: you'll need building bye-law permission if more than 50% of the surface area of the roof covering is to be replaced as part of the work to install roof insulation.
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