Hi Stephen,
As I understand it, Granny Flat Solutions build you a granny flat according to a template and your particular specifications on top of that? They will build this on your land. You will need to purchase the land yourself and gain council approval beforehand.
The website has a great explanation of the process:
Design Process
It appears to be a straight forward service (for design and building) contract. As long as there is a written contract, the contract has a detailed plan of what you want, specifications and plan, then you should be right. Make sure you talk about materials with them. As well as what would happen in the event of them making a mistake (e.g. using a wrong material, promising something to fit when it does't, mistakes in installing something) and who would bear the liability. Have all correspondences down in writing.
Is there anything in particular you don't understand about the process?