Apart from the change of use RH, you need an architect's sign-off when you create more than 180 m2 of new living-space.
My renovation will be about 200 m2 but there was 45 m2 of old existing habitable-space so I didn't need to do that.
In any case my technician said that she regularly worked with an architect who would sign-off for E 250 or similar.
It's under 1,8 m that it doesn't count as habitable for all purposes inc tax and speaking of which, if that;s going to be a second-home, I would expect your charges to be stratospheric.
Have you been hit for the first or second installments of the one-off infrastructure charges - TLE, TSE and another one ? Normally 50% due 18 months after PP granted and last 50% after 36 months.
When are you coming back to re-start ?