As far as I know you would need an architect to do the plans.
He will need a structural engeneer for the basement/extension plans.
Once you have the plans ready you can go to the council to get the permission.
Most builders won't give you a detailed quote until this is done.
You might find that an architect will do all this for you.
My husband is a builder doing similar projects so I get to hear/see a lot of this
Feel free to ask me if you need any more info
Good luck