There is no requirement for sound insulation to an external wall.
The work is classed as renovation of a thermal element.
Reg
23.—(1) Where a person intends to renovate a thermal element, such work shall be carried out as is necessary to ensure that the whole thermal element complies with the requirements of paragraph L1(a)(i) of Schedule 1.
There is some flexibility allowed for in the guidance of the approved document, but this would have to be on a case by case basis. If its a listed building then planninf legislation will take precedance.
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