Do I need Building Regs approval if I am only replacing the roof tiles?

Yes. Because of high number of relatively old houses in England and the slow rate of replacement new rules were introduced in 2008 to improve the thermal insulation of properties having building work done. This is part of the government’s drive to lower carbon emissions.

So, as a condition of granting approval, you will be required to increase your loft insulation to meet current legislation; ie a minimum 270mm thickness.