I've already seen a few posts around if Chorus will install a new ONT into a granny flat and the short answer appears to be no unless there is a seperate address. 

I've got the interesting situation where I've just purchased a house that has a fully consented "minor dwelling". I emailed the council and they quickly issued me a new secondary address and told me to put another letterbox out if I want mail. The power and water meter remains the same though.  

Chorus has already installed Fibre to the main property, actually running the cable past the front door of the granny flat in 2019. 

On the Chorus website, I can see the new address appearing, and it even says fibre available, and asks me for contact details.

In this situation, do you think I could get a free install of a second ONT for the granny flat? Or would I need to go through the full infill process, which sounds expensive?

Basically, the two houses existed before Chorus showed up, Chorus only put one ONT because there was only one address at the time, and I've recently got the council to give me a second address.