If you are planning to replace more than half of the internal plaster or external render of a wall, or if you are dry lining a wall, then you have to insulate to Building Regulations standards whether you had intended to or not.
The main condition to meet is the thermal performance of the insulated wall - if you live in England or Wales then it must have a u-value of no more than 0.30 W/m2K - as a rough guide you will need around 60mm to 120mm of insulation to achieve this, depending on the insulation material and your existing wall make-up.
Your installer should ensure that the insulation is up to standard and will arrange approval from the local Building Control Office for you, but check with them. If you are doing the work yourself, then contact Building Control to make sure you’re adhering to the standards.
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