Thx so much for the info!
I did see those links but was a bit confused with the options.
I did email a housing officer who mentioned Id need planning which was confusing as I assumed I just require build control.
Would I start with building control, once that was approved then landlords consent and finally lease change?
Also did you find the council quite responsive and agreeable to changes or was it a struggle to get cooperation. Is there a way I can find out what changes would be frowned upon in advance, so I dont waste time and money with building regs?
Assuming I do get building regs and the new design is an improvement on living space, can the freeholder still refuse?