House building experiences

Hi,

Just this week I have received planning for a 3 bedroom semi-detached house onto my existing house. My plan is to save the money to build it which will be possible and go with a main contractor after I put my detailed drawings out for quotes.

As I have never undergone this process before could someone advise when I choose a main contractor what is left for me to do? I'm probably being naive that the whole thing is taken care of and all I need to do is pick a kitchen so what would be my main concerns or input I would need to provide going forward?

Would just like to know what I am getting myself into especially in reference to the fact I will not be sourcing individual trades myself.

Thanks
David