You will still need to obtain permission to do this and would need to speak to your local council.
The main concern would be fire safety regulations.
You say that you can change from offices to facts under permitt3d development, but this is not quite correct as you would still need to submit an application and obtain permission.
I hope this answers your questions. If so, kindly rate the answer and provide feedback.
If you have any further questions, please do not hesitate to ask.
The same would apply. You would be best served speaking to your local council planning department and confirming whether the change of use would be permitted.