Building licenses

Hello and happy new year to you all.  I own a two bed apartment and there is space to put in a third in a largely unused open plan area of the apartment.  

We would be talking putting up two stud walls with a door.  The internal area already houses a window, sockets and switches etc.

basically we’re splitting a large room into two.   

Has anyone done this or similar and what legal process they followed in order to complete the work. I’ll add that I will be completing 90% of the work myself.  

Thanks in advance you helpful lot.  Here’s to a healthier and fully open 2022 sooner rather than later.  

Andrew